calciobetafite (new mineral of the pyrochlore group) and related … · 2007. 8. 26. · dovkina et...

American Mineralogist, Volume 6E, pages 262-276, 19E3

Calciobetafite (new mineral of the pyrochlore group) and related minerals from CampiFlegrei, Italy; crystal structures of polymignyte and zirkelite: comparison with

pyrochlore and zirconolite

FronBNzo MnzztC.N.R. Cento di Studio per la Cristallografia Strutturale

Istituto di Mineralogia dell'UniversitdVia A. Bassi 4. Pavia, Italy

aNo Roselse MUNNoIstituto di Mineralogia dell'Universitd

Via Mezzocannone 8, Naples, Italy

Abstract

Polymignyte, zirkelite and zirconolite have often been considered the same mineral.Their occurrence, with calciobetafite (a new member of the pyroclrlore group), in a"sanidinite" from Campi Flegrei has allowed their crystal-chemical study and identifica-tion as three distinct phases. The three minerals are polymorphs of the compound: (Ca,Na,REE,Th . . )yrrtzrytr(Ti,Nb, . )Y\Fe,Ti)v,Ivo1a. The crystal structure of zirconolite(space group C2lc)has been previously determined (Gatehouse et al., l98l) on syntheticcrystals; those of polymignyte (e.g., Acam) and zirkelite (e.9., P3Q) are described in thepresent paper. The crystal structures of polymignyte, zirkelite and zirconolite may bederived from that of pyrochlore. Chains, formed by distorted (Ca,REE . . ) cubesalternating with (Ti,Nb . . ) octahedra in pyrochlore, are replaced in the remainingminerals by either chains of Zr polyhedra with seven vertices or chains in which(Ti,Nb . . ) octahedra alternate with distorted (Fe,Ti . . ) tetrahedra or trigonal bipyra-mids. The different arrangement of these chains gives rise to the diferent symmetries of thethree phases. The crystal structures of zirkelite and zirconolite are very similar, as theyditrer only in the stacking of identical pairs of layers of polyhedra. The difficulties indistinguishing the three polymorphs by X-ray powder diagrams are discussed.

IntroductionCalciobetafite, a new species of the pyrochlore

group, and related minerals have been found atMonte di Procida (Campi Flegrei, Campania, Italy)in a rock known as "sanidinite". This subvolcanicrock is present in a phreatomagmatic explosionbreccia. The age of this pyroclastic formation,determined on an alkali-trachitic obsidian by the IUAr method (Gillot, pers. comm.) is 0.035 m.y.,whereas the absolute age of sanidinite, determinedwith the same method on the enriched feldspathicfraction, is 0.084 (i0.008) m.y. (Civetta, pers.comm.). The sanidinite is composed of 75% sanidine(Oros), l6Vo plagioclase (An35), Mg-hastingsitic am-phibole (occasionally with a core of clinopyroxene),biotite, magnetite, apatite and sphene. Minor inter-stitial glass is also present. Occasional small col-ored crystals were found scattered throughout therock: three possible species were tentatively recog-

m03-004>u83/0102-0262$02.00 262

nized on the basis of their morphology and someoptical features: (1) small (0.1-0.2 mm) octahedralcrystals, reddish brown in color and isotropic; (2)elongated prisms (maximum 0.4 mm) enclosed insanidine and sometimes in the interstitial glassbordering the K-feldspar, dark red in color, show-ing parallel extinction and very weak pleochroism;(3) platy crystals with roughly hexagonal outline;luster resinous, brittle with splintery fracture; blackin color and reddish brown in very thin splinters,showing birefringence with extinction parallel to anedge.

By X-ray analyses the three species were identi-fied as a member of the pyrochlore group, polymig-nyte and zirkelite respectively; the last mineral isalways the dominant phase in intergrowths of zirke-lite, zirconolite and pyrochlore. Conflicting datahave been reported in the literature concerningpolymignyte, zirkelite and zirconolite, which have

often been considered the same mineral. As it willbe seen later, distinguishing among these threeminerals is very dfficult: fortunately their occur-rence in the sanidinite, as well-developed crystals(not in their usual metamict form), allowed theircrystal-chemical study and consequently their iden-tification as three distinct phases.

Previous work and identification of the specimens

Polymignyte, found in a pegmatite from Fre-dricksviirn (Norway) was first described by Belzeli-us (1824); more detailed studies of its morphologi-cal, physical and chemical properties were reportedby Br

264 MAZZI AND MUNNO: PYROCHLORE AND RELATED MINERALS

Table l. Calciobetafite, polymignyte, zirkelite and zirconolite: chemical analyses and formula units

Ca-betaf i te Polymignyte Z i r k e l i t e m d Z i r c o n o l i t e

C a O

Na2O

La203

CeO2

Pr2O3

N d z O o

Yzo:

ThO2

uo^

ZrOz

MnOMgoFe0Fe203A12O3

Ti02Nb2o5Ta2O5

FR e m .

Total

Fr

6 . 9 8o . 5 9

5 . 1 36 . 2 0

2 , 2 6

3 , 9 2

29.71-

I . 3 2o . 1 62 . O A7 . 6 6o . 1 9

1 8 . 9 0l -1 .99

1 . 3 5

2 , O 4

8 . 6

o . 72 . 6n . a .

2 . r2 . 4

n . a

n . a

2 3 . 7

4 , 1

1 4 . 1

1 0 . 5

n . a

n . a

o . 2 313.77

35.27

1 . 9 63 . 7 3

1 6 . 0 1 6 . 0

o . 7 0 . 7

4 . O 4 . 0

n . a 0 . 6

r . 2 \ , 40 . 6 0 . 6

n . a 4 . 5

n . a 4 . 8

1 . 1 1 . O

n . a

n . a

1 . 9 2 . O

n . a

1 4 . 3 1 5 . 0

2 7 . 5 3 2 . 9

n . a 2 . 4

1 . 3 n . a

a b

7 . 6 8 . 6

o . 4

n ' a o . a

n . a 3 . 7

n . a 0 . 6

n . a 2 . 3

n ' a 2 , 5

n . a 3 . 5

n . a o . 7

2 5 . 4 2 5 . 4

O . 9 n . a

O . 2 n . a

8 . 7 8 . 0

O . 4 n . a

1 8 . 0 1 9 , 5

n . a 1 2 . 7

n . a 0 . 6

n . a n . a

Bzk Czk Czk*

1 0 . 7 9 6 , 4 7 8 . 5 5

t l2 . 6 4 I I

t 2 . 6 4 l O - 3 2t lt t

0 . 2 L 1 . O 8

7 . 3 1 - 2 0 . 4 4 8 . 3 31 . 4 0 r . o 2 4 . 4 8

5 2 . 4 9 3 0 . 7 3 3 4 . 1 9

0 . 0 3o . 2 2 2 . 3 4 1 . 3 37 . 7 2 4 . O 7 4 . 7 2

c z k * * z i z i *

8 . 1 8 1 1 . 0 5 1 1 . 0 O

o . 3 7 1 . 4 0

6 . 5 2 4 . 1 9

5 . 4 9

1 . 0 3

1 4 . 9 5 2 9 . 5 0 3 6 . 2 6 3 4 , 8 7 3 1 . . 6 9

3 . 2 6

o . 5 8 2 . 9 0r . 4 7 0 . 3 4

3 2 , 4 4 2 5 , 0 0

0 . 0 6 0 . 3 a0 . 4 5

6 . O 0

1 . 1 1

1 4 , 1 9

2 4 . A 4

2 , O O

0 , 6 0

2 . 4 81 - . O 2 0 . 4 4 1 . 7 0 2 . 1 - 2 5 . 4 0

6 9 . 8 s a 7 . 6 6 1 - , 2 8 9 . 7 1 0 0 . 4 A 7 6 . A 9 9 . 1 5 9 9 . 6 0 9 9 . 8 8 1 0 0 . 1 3 1 0 0 . 2 r - 1 0 0 . 2 2 s

Formula unit based on 14 oxygens:

C a , N a 3 . 0 5

R E E O . 4 O

T h 0 . 1 5

u 0 . 1 6

T o t a l ( C a , N a , R E E , T h , U ) 3 . ? 6

1 . 5 5 1 , 6 5 1 . O 2o . 5 4 0 . ) - 4 0 . 2 1

o . 2 4 0 . 6 4o . 0 4 0 . o 3

2 , 0 9 2 , O 7 1 . 9 0

I . 3 - 4 0 . 9 7 0 . 9 52 , 2 4 1 . 6 0 3 . O 70 . 8 0

4 . 2 2 2 . 5 7 4 . O 2

t . I a 1 . 1 4 1 , 6 0 1 . 9 1o . 0 1 0 . 2 9 0 , 1 9o . 2 4 0 . 0 1 0 . 0 2 0 . 0 9o . 1 3 0 . 4 0 0 . 0 4 0 . o 1

1 . 5 6 1 . 5 5 1 . 9 5 2 . 2 0

0 . 7 6 0 . 7 4 O . 7 7 0 . 8 I

3 . 5 0 3 . 4 0 3 . O 4 1 . A O

o . 1 9 1 . 5 5

4 . 2 6 4 . 1 4 4 . 0 0 4 . 1 6

o . o 7

t . 1 7o.71-o . L 2

2 . O 0

1 . 4 9o . 5 9o . r 2o . 0 2

2 . 2 2

1 . 4 7

1 , O O I . 2 42 . 1 8 7 . 9 4o . 8 a o . 7 9

4 , 0 6 3 , 9 7

3 . 6 7 z . O a 2 . ' , 1 - 4 2 . 2 3 2 . O 4 1 . 6 1

F e , M n , M g , A l 0 . 2 5

T i 1 . 6 8

N b , T a 2 . 3 2

T o t a l ( F e , . . T i , N b , . , ) 4 . 2 5

Total cat ions a. oa 4 . 1 5 7 . 9 4 4 . 2 5 4 . 3 1 4 . 0 0 7 . 9 6 7 . 9 6 7 . 9 9 7 . 9 7

F r : p o l y m i g n y t e f r o m F . e d r i c k s v 8 r n . R e m . i s S i 0 2 0 . 4 5 , P b O 0 . 3 9 , S n O 2 0 . 1 5 , K 2 O O . 7 7 , H 2 O O . 2 a ( B r d g g e r , 1 4 9 0 ) .

a x : i n t e r g r o w t h o f z i r k e l i t e , z i . c o n o t i t e a d p y r o c h t o r e . a , a * : i n c o m p l e t e d a l y s e s .B z k : z i r k e l i t e f r o m B r a z i l R e m . i s H 2 0 ( P r i o r , L a 9 7 ) .

C z k : t h r e e z i r k e l i t e s f r o m C e y f o n w i t h d i f f e r e n t m o m t s o f T h a n d U . R e m . i s : C z k P b O 0 . 3 8 , H 2 O 0 . 4 6 ; C z k * H 2 O 1 - . 7 O i

C z k * * P b O O . 4 4 , H 2 O I . 6 8 ( B l a k e a n d S m i t h , l - 9 1 3 ) .

z i : z i r c o n o l - i t e . R e m . i s S i 0 2 2 . 0 5 , H 2 O 3 . 3 5 ; z i * : N b - r i c h z i r c o n o l i t e . R e n , i s H 2 O ( B o r o d i n e t a ] . , 1 9 6 1 )

S a f t e r s u b t r a c t i o n o f 0 = F . n . a i n o t a n a l v z e d .

oxides is much less than l00Vo even though it couldbe increased (roughly l%) to account for those rareearths, which were not analyzed. Nevertheless, theformula units derived from the chemical analysesappear fairly reliable and in good agreement withthe results of the crystal structure determinations.

Sets of analyses were carried out by us in the

Institute of Mineralogy of Modena University(marked with a in Table 1) and by W. L. Griffin atthe Mineralogisk-Geologisk Museum of Oslo, Nor-way (marked with b in the same table). Apparatusand techniques used for the chemical analyses wereas follows: (a) enr-seue electron migroprobe oper-ated in the wavelength dispersive mode; (b) enl-

MAZZI AND MUNNO: PYROCHLORE AND RELATED MINERALS

EMX microprobe: the major elements were mea-sured by energy-dispersive techniques and the rareearths by WDS (Amli and Griffin, 1975).

Table I summarizes the results obtained on thesamples from Campi Flegrei together with someother analyses of polymignyte, zirkelites and zir-conolites taken from the literature. The formulaunits were calculated for 14 oxygens (includingfluorine) and only those obtained from the analyseswith the highest oxide sum are given in Table 1.

According to the chemical classification of pyr-ochlores: A2B206 (O,F) (Hogarth, 1977), "pyro-chlore" from Campi Flegrei may be considered as anew species, since it fills a void (Ca dominant in A)in the list of the minerals of the betafite subgroup(2Ti > Nb+Ta in B). After the resolution of theIMA Subcommittee on Nomenclature of the Pyr-ochlore Group, its name is calciobetafite.

Nearly all the calculations and the results of thecrystal structure determinations, which will be seenin a following section, suggest the same crystal-chemical formula unit for all three minerals, poly-mignyte, zirkelite and zirconolite:

(ca,Na,REE,Th)yIIZryrI(Ti,Nb,Ta)yr(Fe,Ti)v'rvOr4;

only the analysis of the original zirkelite by Prior(1897) shows a Zr amount nearly double thoseobtained from the remaining analyses.

The following points are emphasized: (1) Zirconi-um, almost absent in calciobetafite from CampiFlegrei, is an essential component of the remainingminerals; it apparently replaces (Ca,REE, .) (Table1), but, as will be seen in a following section,(Ti,Nb, . . ) are also involved in the replacement.(2) Very likely, iron is also an essential componentof natural polymignyte, zirkelite and zirconolite,although the synthetic zirconolites studied by Pya-tenko and Pudovkina (1964) and by Gatehouse el a/.(1981) contain no iron. (3) The above formula unit isto some extent a theoretical one: Zr-contents some-what different from two, and consequent variationsin the amounts of the remaining cations, result bothfrom the data of Table 1 and from those of syntheticzirconolites. (4) There is strong evidence that poly-mignyte, zirkelite and zirconolite are polymorphs ofthe same chemical compound.

Experimental

General remarks

All X-ray determinations were carried out with anautomatic single crystal Philips PW 1100 diffrac-

tometer and with MoKa radiation. The lattice pa-rameters were obtained using a least squares tech-nique applied to the diffraction angles of 25 strongreflections, measured at the center of gravity of theprofile of each reflection. Diffraction intensitieswere measured in the

26

terminated when the changes in the variables werelower than the corresponding standard deviations.The coherency between the chemical and site occu-pancy results was finally verified by comparing thetotal number of electrons in the cationic positions,derived from the site occupancy refinement, withthat obtained from the stoichiometric units of Table1 (Tables 4,7 and 1l). Table 2 gives a summary ofthe most relevant conditions used in the data collec-tions and in the refinements. Lattice parametersand space groups are reported in Table 3. Thedimensions of C-centered "orthorhombic" multiplecells, obtained from the lattice parameters of thefour minerals (calciobetafite, polymignyte, zirkeliteand zirconolite) are given in the same table, togeth-er with the corresponding transformation matricesof the axes. As zirkelite and zirconolite are presentin the same intergrowth, their lattice parameterswere obtained from the measurements of the corre-sponding difraction angles for non-superposed re-flections.

Calciobetafite

The crys-tal structure of pyrochlore, (Ca,REE,Th, U, . . ))'ru(Nb,Ti, . . )ylo6(o,F), is well known(Gaertner, 1930): however, as good intensities ofthe reflections were obtainable, we decided to col-lect them from a single crystal ofcalciobetafite fromCampi Flegrei and to refine its crystal structuremainly for two reasons: (l) as far as we know, thestructural studies on natural crystals were madestarting from powder difraction intensities (e.g.Perrault, 1968), and no detailed information hasbeen given until now about the anisotropic tempera-

Table 2. Calciobetafite, polymignyte and zirkelite: experimentalconditions in data collection and crystal structure refinement

Ca-beta f i te po lyn igny te

D i n e n s i o n s o f t h e O , 1 3 x O . 1 3 O . O 4 X O . O 4

crys ta l (nm) xO.13 xO.4O

Max inm 2O ( " ) MoKa 60 60

S c o y i d t h ( o ) 1 . 5 1 . a

S c e s p e e d ( o / s e c ) O . O 5 O . O 9

Measred re f lec t ions I57L I67L

Independent re f lec t ions 99 826

"Observed" re f lec t ions :

Fz / o G2) >- L 75 ?43

L 4 l

Vd iabLes in re f inenent 13 ? l

Ex t inc t ion coef f i c ien t

( x 1 0 6 ) 4 s ( 5 ) 4 8 ( 2 )

R ( " o b s e r v e d " r e f t e c t i o n s ) O . O t g O . 0 2 5

R ( independent re f fec t ions) O.027 O.O31

Z i r k e l i t e

O . 0 5 x0 . O g x 0 , 2 0

602 . 5

o . 0 5

3605

907

659


Table 3. Calciobetafite, polymignyte, zirkelite and zirconolite:lattice parameters

Ca-betafi te

S D a c e' F d 3 n

soup

Polymigny te Zirkel i te Zi .rconol i te

P312 C 2 / c

Uni t ce l l s :

_ . t l l 10 .2s7a(5)b ro .297a( 5 )9 10 .2974(5)a ( ' ) 9 0P s o7 9 0

a ; b : c 1 : 1 : 1

v ( 8 3 ) 1 0 9 2z a *v ( 1 4 ) ( 8 3 ) 2 7 3

1 0 . 1 4 8 ( 4 )

1 4 . 7 4 7 \ 5 )

7 . 2 7 4 t 3 )

90

9090

O . 7 1 7 : 1 : 0 . 5 1 5

1045

426I

7 . 2 A 7 \ 2 ) 1 2 . 6 1 1 ( 5 )7 . 2 A 7 ( 2 ) 7 . 3 r r ( 2 1

1 6 . 8 8 6 ( 9 ) 1 , 1 , . 4 4 4 1 A 190 909 0 1 O O . 5 2 ( 5 )

120 90

L t I t 2 . 3 I 7 1 . 7 2 5 : 1 : 1 . 5 6 5

7 7 6 . 5 1 0 3 73 4

t i p f e c e l l s :

t i l 12 .6r7 , 2 4

L7 .44

909090

1 . 5 V

7 . 2 4r 7 . 4 t

9090.77

90

3 V

72.6\7 . 3 I

4 x l 6 . a g

904 9 . 9 490

6 V

7 . 2 91 6 . 4 9

9090

9q

2 V

Trans foanat ion mat r ices i

- 1 0 . 5 0 . 5

o - o . 5 0 , 5

1 1 1

2 I O 2 1 0 1 0 0o o I 0 1 0 0 - 1 0

- 1 1 0 0 0 1 1 0 6

Est imated 6 tdd i ld dev ia t ions ( in p i len theses) re fe r to the

I a 6 t d i g i t .

V = vo lwe o f the un i t ce f l ; V(14) = vo lme based on the fo rnu la mi t

u i th 14 oxygens; Vn = vo lme o f the mul t ip te ce l l .

Es t ina ted s tmdrd dev ia t ions ( in paentheses) re fe . to the las t d ig i t+ ca lcu la ted f rom the usua- l fo rmula un i t w i th 7 (O,F) .

ture factors of the atoms; (2) the population refine-ments of the cationic sites in this particular crystalwould have been a good test for the reliability of thecorresponding results obtained from the remainingminerals from Campi Flegrei, which have compara-ble chemical composition. In this respect the con-cordance between the total number of electrons incations calculated form the site population and thatobtained from the chemical analysis of calciobeta-fite is surprisingly very good (Table 4). The resultsof the refinement are given in Table 4 (atomiccoordinate and temperature parameters, and inter-atomic distances and angles), and Table 5r (thermalvibration ellipsoid parameters and electrostaticcharge balance). The list ofobserved and calculatedstructure factors is given in Table 6r

Polymignyte

The results of the crystal structure determinationare given in Table 7 (atomic coordinate and tem-

I To obtain a copy ofTables 5, 6, E,9, 10, 12, and 13, orderDocument AM-E3-215 from the Mineralogical Society of Ameri-ca, Business Office, 2000 Florida Avenue, N.W., WashingtonD.C. 2fiXD. Please remit $1.ffi in advance for the microfiche.

2

7 5

0

o . 0 6 3o . 1 . 3 0


Table 4. Calciobetafite: atomic coordinate and temperature parameters; bond distances (A) and angles (')

267

D ^ i h +

A t o m M u I t i p , ' " - " -sym.

I' 1 1

u 8. , F* P. .^ N o o fE .'

23 electrons !

M e B 1 6 3 m o o o 0 . 7 7 ( 1 ) 1 8 1 ( 7 ) 1 8 1 1 8 1 - 1 3 ( 4 ) - 1 3

Me6 16 5 . o '5 o .5 o .5 0 .94( 1 ) 222(71 222 222 -58( 4 ) -sa

o 4 a m 4 . L 7 ? I ( 4 ) 0 . 1 2 5 o ' 1 2 s o . s 7 ( s ) 2 5 6 ( 3 0 ) 1 7 9 ( 1 9 ) 1 7 9 O O

( o , F ) e i s m 0 . 1 2 5 o . L 2 5 o . r 2 5 1 . 7 7 ( 6 ) 4 1 6 ( 4 0 ) 4 1 6 4 r o o o

s i t e p o p u f a t i o n : M e 8 0 . 7 6 1 ( 2 ) ( C a , N a , 1 9 . 1 e l e c t r o n s ) , 0 . 2 3 9 ( R E E , T h , U , 6 8 . 4 e l e c t r o n s ) ;

M e 6 0 . 5 3 5 ( 6 ) ( T i , 2 2 e l e c t r o n s ) , 0 . 4 6 5 ( N b ' T a ' 4 2 ' 3 e l e c t r o n s )

- L3 123 . s (4 )-5a 125 .8 ( 5 )

e3( 2s)o

249 .3 +

250 **

M e 8 - OM e B - ( o , F )

o ^ oo ^ ( o , F )

l 8 O x 3

M e 6 - O

o ^ o

MeB polyhedron

2 . 5 7 7 ( 3 ) x 62.2295 x2

6 2 . e ( 1 ) x 6 r I 7 . 2 ( r ) x 6

8 0 . 3 ( 1 ) x 6 9 9 . 7 ( 1 ) x 6

Me6 polyhedron

1 . 9 6 9 ( 3 ) x 6

85,9( 1 ) x6 94 .1 ( 1 ) x6 180 x3

$ based on 8 cat ions: * f rom si te populat ion, ** f ron chemical analysis '

k'"li"lil.iil ';:il:,'::*:i:.:":::'::ff.:T;:''.." -{n'p,.*r.2 e,,*r2 F,,*znxg,,+2h1 p,,+2kr 8,.) " ro-5Estimated standard deviat ions ( in parentheses) refer to the last digi t '

perature parameters), Table 8t (parameters for theellipsoids of vibration), Table 9r (electrostaticcharge balance) and Table 101 (comparison betweenobserved and calculated structure factors).

Zirkelite and zirconolite

Since we did not find true single crystals ofzirkelite, we decided to collect X-ray intensitiesfrom a zirkelite-zirconolite-pyrochlore intergrowthand to use them for the determination of the crystal

structure of zirkelite. A simple explanation of theepitaxy of pyrochlore, zirconolite and zirkelite fol-lows from the observation of the parameters of themultiple cells (Table 3) derived from the lattices ofthe three minerals: a- and b-edges are practically

the same and the corresponding axes are parallel inthe crystalline intergrowth. Furthermore, the c-axisof zirconolite is just four times that of zirkelite,whereas the c-axis of pyrochlore is somewhat largerthan that of zirkelite. Because of the similarities of

Table 7. Polymignyte: atomic coordinate and temperature parameters

Aton B 8' 2 2

' 3 31 3

No of electronss231 1

Me8

Me7

M e 6 ( I )

M e 6 ( 2 )

Me4

M e 5

o ( 1 )

o ( 2 )o ( 3 )o ( 4 )o ( 5 )

7 2 . 7 1 5 )8 0 . o3 1 . 6 ( 3 )4 8 . s ( 5 )2 4 . O ( 3 )

1 . 5 ( 2 )

a

b

a

1 6

I

I

a

2

m

2

2

m

1

1

m

m

m

o . 7 5 0 . 1 1 s 9 4 ( 4 ) 0 . 2 5 0 . 6 3 ( 1 )o . 0 1 5 o 5 ( 6 ) o . 2 3 3 1 s ( 4 ) O . s o , s s ( 1 )

o o o 0 . 7 9 ( 2 )

0 . 2 5 0 . 1 3 3 2 3 ( 7 ) 0 . 2 5 0 . 5 6 ( 2 )o o 0 . 4 3 0 6 ( 6 ) 1 . s 7 ( 1 o )

0 . 0 3 9 ( 5 ) O . O l s ( 2 ) 0 . 5 o . 3 ( e )

o . 1 2 5 s ( 3 ) 0 . 0 3 3 3 ( 3 ) o . 1 e o 3 ( 5 ) O . s 6 ( 7 )0 . 1 1 9 7 ( 3 ) 0 . 2 3 3 1 ( 2 ) o . 2 r . o 4 ( 5 ) O . s 2 ( 7 )

- o . 1 o o 7 ( 5 ) 0 . 1 0 8 9 ( 4 ) o , 5 O , s 7 ( 1 1 )- o . o 9 o 4 ( 5 ) 0 . 1 2 9 s ( 4 ) o 0 . 8 4 ( 1 0 )

0 . 1 7 9 9 ( 5 ) 0 . 1 3 9 6 ( 4 ) 0 . 5 O . 9 4 ( 1 0 )

1 4 ( 1 ) e ( O ) 2 9 ( 1 ) O 2 ( 1 ) o

1 3 ( 1 ) 5 ( o ) 3 3 ( 1 ) - 1 ( o ) o o

1 5 ( 1 ) e ( o ) 4 s ( 2 \ - 1 ( 1 ) o o

1 3 ( 1 ) 8 ( o ) 2 6 ( 2 1 o 2 ( 1 ) o

5 1 ( 4 ) 1 6 ( 1 ) 1 2 0 ( 1 0 ) 2 L ( 2 ) O o

1 6 ( 3 ) L 1 . ( 2 , 6 3 ( 6 ) 2 ( 2 ) - 1 1 ( 4 ) - 1 ( 3 )

1 7 ( 3 ) s ( 2 ) 6 4 ( 7 \ s ( 2 ) 4 ( 4 \ 3 ( 3 )

2 L ( 4 ) 8 ( 2 ) 6 6 ( 1 0 ) - 2 ( 3 ) o o

2 5 ( 4 ) e ( 2 ) 3 s ( s ) - 2 ( 3 ) O O

2 4 ( 4 ) 1 6 ( 2 ) 2 7 ( 8 ) 6 ( 3 ) O o258.7 +

2 5 3 . 1 + rs i t e p o p u l a t i o n : M e 8 0 . 5 7 4 ( 7 ) ( C a , N a , 1 9 . 3 e l e c t r o n s ) , 0 . 4 2 6 ( R E E , T h , 5 9 . 3 e l e c t r o n s ) ; M e 7 ( Z r , 4 0 e l e c t r o n a ) ;

M e 6 ( 1 ) 0 . 5 1 7 ( 8 ) ( T i , 2 2 e l e c t r o n 6 ) , 0 . 4 8 3 ( N b , T a , 4 1 . 9 e l e c t r o n s ) ;

M e 6 ( 2 ) O . 8 7 8 ( 1 1 ' ) ( T i , 2 2 e l e c t r o n 6 ) , 0 . 1 2 2 ( N b , T a , 4 1 . 9 e l e c t r o n s ) ;

M e 4 o . 4 6 2 ( 6 ) ( F e , 2 6 e l e c t r o n s ) ; M e 5 0 . 0 2 9 ( a ) ( F e ) .

M = mult ipl ic i ty; PS = point symmetry. Beq is the equivalent isotropic temperatEe factor.

$ based on 8 cat ions: * f rom si te populat ion, ** f rom chemical dalysis.

E s t i m a t e d s t m d d d d e v i a t i o n s ( i n p d e n t h e s e s ) r e f e r t o t h e l a s t d i g i t .

The disotropical thermal pdmeters ae def ined by: exp -(h29 11+u2 p22+12 p33+2hk 912+2hl F1,3+2kLp2i) x l }a

26E MAZZI AND MUNNO: PYROCHLORE AND RELATED MINERALS

the lattice parameters, several superpositions of theX-ray reflections of the three phases occurred. Itwas necessary to take this into account to obtain"clean" intensities of the zirkelite reflections. Thesubtraction of the contributions of pyrochlore fromthe observed reflections was fairly easy: total orpartial superpositions were limited to the followingreflections (zirkelite: hkl,pyrochlore: HKL): -H +K + L : 3n (the only ones possible for pyrochlorein the trigonal indexing) and h : H, k : K,0 < I :L < 5 or 13 < I : L * | < 22. Because of the highmultiplicities of the pyrochlore reflections, it waspossible to measure the intensities of a complete setof independent reflections of this mineral at pointsofthe reciprocal lattice free ofsuperpositions, thento subtract them from the intensities of the super-posed reflections. The correction for the super-posed reflections from zirconolite was more diffi-cult because of the greater similarity of the twolattices. Zirkelite reflections with2h I k * 4l : 6nwere perfectly superposed and those with 2h + k +4l : 6n -r I partly superimposed with those ofzirconolite. However, having collected the intensi-ties of six (sometimes three) equivalents for eachreflection, it was possible to find one, two, or fourof them without any superposition for all diffractionefrects, except those with -h + k * I : 3n and h,k : 2n, which are unfortunately the strongestobserved reflections. An approximate correctionwas possible after the calculation of the theoretical

intensities of zirconolite by using the parametersgiven by Gatehouse et al. (1981). Even though thefinal intensities obtained this way were adequatefor the determination of the crystal structure ofzirkelite, they were too imprecise for a satisfactoryrefinement: for instance the temperature factors ofthe oxygens were treated as isotropic because anyattempt to consider them anisotropic producedthermal vibration ellipsoids with negative axes. Inaddition the anisotropic temperature factors of cat-ions are not completely satisfactory. The results ofthe crystal structure determination of zirkelite aregiven in Table 11 (atomic coordinate and tempera-ture parameters), Table 12t (electrostatic chargebalance) and Table l3r (comparison between ob-served and calculated structure factors).

Structural relationships among pyrochlore,polymignyte, zirkelite and zirconolite

The crystal structures of polymignyte, zirkeliteand zirconolite are based on the same kinds ofpolyhedra: distorted cubes (P8), octahedra (P6), aswell as polyhedra with seven (P7), five (P5) or four(P4) vertices. The cubes are always centered byMe8 cations (Ca,REE,Th. .), and octahedra by Me6cations (Ti,Nb,Ta). Me7 cations (Zr) are at thecenters of P7 polyhedra and Me5 or Me4 cations (Fein natural samples and Ti in synthetic zirconolites)at those of P5 or P4 polyhedra. Only P8 and P6polyhedra (symmetry 3m) are present in pyroch-

Table ll. Zirkelite: atomic coordinate and temperature parameters

Aton M P S B ( B e q ) t s 1 1 Fr :9 r ,pF,, Fzz No of electrons$

M e a ( 1 ) 3 2

M e 8 ( 2 ) 3 2

l4e7 6 1

M e 6 ( 1 ) 3 2

M e 6 ( 2 ) 6 1

M e 5 6 1

o ( 1 ) 6 1

0 ( 2 ) 6 1

o ( 3 ) 6 1

0 ( 4 ) 6 1

o ( s ) 6 1

0 ( 6 ) 6 1

o \ 7 ) 6 1

0 . 4 3 2 4 ( 7 ) 00 , 3 2 9 5 ( 7 ) Oo . 1 6 s 3 ( 6 ) o . 6 6 7 6 ( 8 )0 . 3 3 3 ( 2 ) Oo . 4 9 7 ( 1 ) o . 3 3 4 ( 2 )0 . o 4 4 ( 3 ) 0 . 8 9 4 ( 2 )

0 .3333o.3333o.0166( 1 )0 .43330.1640 ( 1 )0 . 1 6 9 ( 1 )

3 3 ( 1 )3 s ( 1 )a026\L')s 6 ( 1 )2 6

1 . O ( 1 ) 3 8 ( 7 ) 3 0 ( 1 1 ) 1 7 ( 1 )

0 . 9 ( 1 ) 1 6 ( 5 ) 6 4 ( 1 0 ) 1 6 ( 1 )

o . 7 ( 1 ) 3 s ( 7 ) 4 2 ( 7 ' 1 0 ( 1 )

1 , 1 ( 2 ) 7 6 ( 1 1 ) 8 6 ( 2 1 ) 6 ( 1 )

o . 7 ( t ) 4 8 ( 8 ) 4 5 ( 8 ) 5 ( 1 )

3 . o ( 6 ) 2 6 7 ( 6 4 ) 3 3 4 ( 3 4 ) 1 ( 1 )

1 . 0 ( 2 )

o . ? \ 2 )1 . s ( 3 )

7 . 4 ( 2 )

r . 2 \ 2 )o .7 12 )7 . 2 \ 2 )

1s -3 ( 2 ) -6

3 2 2 \ 2 1 a2 0 ( 5 ) - 6 ( 2 ) 4 ( 2 )4 3 - 1 1 ( 2 ) 2 22 r . ( e ) e ( 2 ) 3 ( 2 )

r . 7 9 ( 4 0 ) - 1 6 ( 1 2 ) - L 7 ( 1 1 )

0 . 5 9 5 ( 2 ) o . 6 3 4 ( 3 ) o . r 4 2 a ( 7 )0 . 0 1 o ( / ) o . B 2 o ( 2 ) O . O s 7 s ( s )o . 5 2 7 ( 2 ) 0 . 3 1 4 ( 2 ) o . o 4 g 8 ( 7 )o . 2 0 2 ( 3 ) o . 2 2 7 ( z l o . 1 4 4 2 ( 7 )0 . 5 3 2 ( 2 ) 0 . 8 9 o ( 2 ) o , o s 3 6 ( 6 )o . 9 a 7 ( 2 ) 0 , 3 1 2 ( 2 ) O . o 5 4 O ( 6 )o . 2 o 7 \ 2 ) O . 6 1 6 ( 2 ) o . L 4 2 5 \ 7 )

s i t e p o p u l a t i o n : M e g ( 1 ) O . 6 4 ( 1 ) ( C a , N a , 1 9 . 3 e l e c t r o n s ) , O . 3 6 ( R E E , T h , 5 9 . 3 e f e c t r o n s ) ;M e 8 ( 2 ) 0 . 5 0 ( 1 ) ( C a , N a ) , o . 5 0 ( R E E , T h ) i M e 7 ( Z t , 4 0 e t e c t r o n s ) ;M e 6 ( 1 ) 0 . 7 8 ( 1 ) ( T i , 2 2 e l e c t r o n s , , o . 2 z ( N b , T a , 4 1 . 9 e t e c t r o n s ) ;M e 6 ( 2 ) o , 6 9 ( r ) ( T i ) , 0 , 3 1 ( N b , T a ) ; M e 5 o . 5 ( F e , 2 6 e l e c t r o n s ) ,

260 *

24L **

M = mult ipl ic i ty; PS = point synnetry. Beq is the equivalent isotropic temperatue factor.$ based on 8 cat ions: * f rom si te populat ion, ** f rom chemical analysis.E s t i m a t e d s t m d d d d e v i a t i o n s ( i n p o e n t h e s e s ) r e f e r t o t h e l a s t d i g i t .The misotropical thernal prdeters oe def ined by: exp - ln2 p1*U2 022+t2 p3g+znU p12+2h1 p13+2k1 Fz;) x to4

lore, where each cube shares an edge with six othercubes and with six octahedra, and each octahedronis connected by an edge to six cubes and by a vertexto six other octahedra.

The bond distances and angles in polymignyte,zirkelite and zirconolite (Tables 14, 15 and 16),

269

show that the shape of each kind of polyhedra ispractically the same in all these minerals.

As in pyrochlore, the P8 polyhedron is still adistorted cube, but, whereas in the former structuretwo Me8-(O,F) distances are 0.354 shorter than theremaining six Me8-O distances, the bond lengths


Table 14. Polymignyte, zirkelite and zirconolite: bond distances (A) and angles (") in P8 cubes and P7 polyhedra

P8 -polyhedra P7 -polyhedra

Polymignyte

Me- Me8-

o ( i ) - o ( a ) 2 . 4 4 3 ( 4 )

o ( i i ) - o ( 3 ) 2 . 3 7 0 ( 4 )0 ( i i i ) - o ( 3 ) 2 . 3 7 o ( 4 )0 ( i v ) - o ( 4 ) 2 . 4 4 3 ( 4 \

0 ( v ) - o ( 2 ) 2 . 5 2 e ( 4 )o ( v i ) - 0 ( 1 ) 2 . 4 s 8 ( 4 )

o ( v i i ) - o ( 2 ) z . 5 z s ( 4 )

o ( v i i i ) - 0 ( 1 ) 2 . 4 9 8 ( 4 )

n e a n 2 . 4 6 4

0 ( i ) ^ o ( i i ) 8 1 . 7 ( 1 ) *

o ( i i i ) s B . 7 ( 1 )0 ( i v ) 1 7 1 . o ( 3 )

o ( v ) 1 o 1 . 3 ( 2 )

0 ( v i ) 6 6 . 6 ( 2 ) + * *

0 ( v i i ) 7 a . 9 ( 2 ) * *

o ( v i i i ) 1 2 2 . o ( 2 )

0 ( i i ) ^ 0 ( i i i ) 1 7 5 . 2 ( 3 )

o ( i v ) s 8 . 7 ( 1 )0 ( v ) 6 7 , 3 ( 2 ) * *

o ( v i ) 9 8 . 8 ( 2 )

O ( v i . r l 1 1 7 . 2 ( 2 )

0 ( v i i i ) 7 7 . o ( 2 ) * * * *

0 ( i i i ) ^ o ( i v ) 8 1 . 7 ( 1 ) *

o ( v - ) r I 7 . 2 ( 2 )

o ( v i ) 7 7 . o ( 2 ) * + * x

o ( v i i ) 6 7 . 3 ( 2 ) * *

o ( v i i i ) 9 8 . 8 ( 2 )

0 ( i v ) ^ 0 ( v ) 7 o . e ( 2 ) * *

0 ( v i ) I 2 2 . o ( 2 )

0 ( v i i ) 1 o 1 . 3 ( 2 )

o ( v i i i ) 6 6 . 6 ( 2 ) * * *

o ( v ) ^ o ( v i ) 1 6 3 . 4 ( 1 )

0 ( v i i ) 6 4 . 7 ( 2 r * * +

o ( v i i i ) 1 1 8 . 1 ( 1 )

o ( v i ) ^ o ( v i i ) 1 1 8 . 1 ( 1 )

o ( v i i i ) 6 4 . 5 ( 2 ) * * *

o ( v i i ) ^ o ( v i i i ) 1 6 3 . 4 ( 1 )

M e a ( 2 ) - C a -

Zirkel i te Zirconol i te$

Me7- Zr- ( a, l ( b)

- o ( 6 ) 2 . 3 5 ( 3 ) - O ( 6 ) 2 . 3 6 0 2 . 3 9 0- 0 ( 5 ) 2 . 4 2 ( 3 ) - o ( 5 ) 2 . 3 1 0 2 . 3 2 1- o ( 5 ) 2 . 1 0 ( 1 ) - o ( 6 ) 2 . 1 0 5 2 . o 8 2- o ( 6 ) 2 . 1 3 ( 1 ) - O ( 5 ) 2 . 1 1 7 2 . O 9 1- 0 ( 3 ) 2 . 1 4 ( 1 ) - O ( 3 ) 2 . O 3 9 2 . 0 1 1- o ( 2 ) 2 . 0 6 ( 1 ) - 0 ( 2 ) 2 . 1 8 1 2 . 1 9 6- o ( 7 ) 2 . 2 r ( r ) - o ( 7 ) 2 . 1 4 2 2 . L O s

2 . 2 0 2 . I 7 9 2 . I 7 7

M e 8 ( 1 ) -

Polymignyte

Me7-

- o ( 2 ) 2 . 3 6 0 ( 4 )- o ( 2 ) 2 . 3 6 0 ( 4 )- 0 ( 2 ) 2 . 1 o 8 ( 4 )- o ( 2 ) 2 . 7 o a ( 4 )- o ( 4 ) 2 . o 8 8 ( s )- o ( 3 ) 2 . 1 1 5 ( 5 )- o ( 5 ) 2 . 1 3 3 ( s )

2 . 1 4 2

126.5\ 2 )

6 9 . 1 ( 1 ) * r

1 6 0 . 2 ( I )

8 0 . 6 ( 1 ) *

1 0 4 . 4 ( 1 )A a 1 l 1 t + * +

1 6 0 . 2 ( 1 )6 9 . 1 ( 1 ) * *

8 0 . 6 ( 1 ) r

1 0 4 . 4 ( 1 )

6 e . 3 ( 1 ) + * *

e 3 . 2 ( 2 )

9 1 . 5 ( 1 )

8 0 . 1 ( 1 ) *

1 3 0 . s ( 1 )

9 1 . 5 ( 1 )

8 0 . 1 ( 1 ) *

1 3 0 . 5 ( 1 )

1 6 7 . 4 \ 2 )

1 0 6 . 9 ( 2 )

a 5 . 4 ( 2 ) * * * *

( a ) ( b )

2 . 4 O 4 2 . 4 2 4

2 . 3 9 6 2 , 3 9 3

2 . 4 5 7 2 . 4 6 7

2 . 4 0 r 2 . 4 0 0

2 . 4 5 2 2 . 4 2 0

2 . 5 3 3 2 . 5 0 8

2 . 5 1 - 7 2 . 5 0 2

2 . 5 4 6 2 . 5 0 7

2 . 4 6 3 2 . 4 5 2

4 3 . 9 4 4 . 5 *

9 5 . 9 9 5 , 7

1 7 2 . 4 L 7 r . O

1 0 2 . 6 1 0 1 . O

6 6 . a 6 7 . 3 r * +

7 2 . O 7 1 . 2 * +

1 1 4 . 4 1 1 9 . 9

1 6 9 . 3 1 6 7 . 9

9 8 . 6 9 8 . 7

6 8 , 5 6 a . 3 * + *

1 0 1 . 6 1 0 1 . 2

1 2 1 . 7 1 2 2 . 7

a 2 . a 4 3 . 0 *

r z L . 7 L 2 3 . 3

6 4 . a 6 7 . 9 * * * r

6 a . o 6 4 . 4 * *

9 9 . 5 9 4 . 3

7 2 . 3 7 2 . 6 * * * *

1 1 4 . 9 1 1 9 , 9

1 0 1 . 0 1 0 0 . 1

6 a . a 6 9 . 1 * *

1 6 6 . 8 1 6 5 . 6

6 6 . 5 6 6 . 9 * * *

I I 7 . 7 I 7 B . O

r 1 5 . 0 1 1 4 . 5

6 4 . 6 6 4 . 9 r r *

1 6 5 . 4 1 6 4 . 3

1 s 9 . 5 ( 5 )

6 9 . 1 ( 6 ) * *

7 4 . 4 \ 7 ) *r o 5 . 8 ( 6 )

7 o . 4 \ a ) * * '

9 2 . 2 \ 5 )9 4 . 5 ( 6 )

7 8 . O ( 8 ) *

r 3 0 . 1 ( 9 )

9 2 . O ( 6 )

7 9 . 9 ( 7 ) +

1 3 1 . 1 ( 9 )

1 6 8 . 6 ( s )r 0 5 . 6 ( 5 )

8 5 . 8 ( 5 )

1 5 9 . 6 1 5 9 . 5

7 0 . r 7 0 . 7 + *

4 3 . 2 4 2 . 5 *

1 0 1 . 2 1 0 1 . 8

6 9 . 6 7 0 . 0 * * *

9 0 . 7 9 0 . 6

9 1 . 3 9 2 . 0

8 1 . 5 8 1 . 2 +

1 3 0 . 6 1 3 0 . 3

9 2 . 4 9 3 . O

B O . 7 a l . 2 *

r 3 2 . 3 1 3 2 . 4

1 6 9 . 9 1 7 1 . O1 0 7 . O 1 0 6 . 4

8 3 . 1 8 2 . 6

- o ( 3 ) 2 . 4 2 ( 3 ) - o ( 3 ) 2 . d 1 ( 3 ) - o ( 3 )- 0 ( 3 ) 2 . 4 2 ( 3 ) - o ( s ) 2 . 4 1 ( 3 ) - o ( 3 )- o ( 2 1 2 . 4 1 , ( 2 ) - o \ 2 ) 2 . 3 6 ( 2 ) - O ( 2 )- o \ 2 ) 2 . 4 r ( 2 \ - o ( 2 ) 2 . 3 6 \ 2 ) - o ( 2 )- 0 ( 4 ) 2 . 4 6 ( 1 ) - o ( 1 ) 2 . 4 6 ( 1 ) - 0 ( 1 )- o ( 4 ) 2 . 4 6 ( 1 ) - O ( 1 ) 2 . 4 6 ( 1 ) - o ( 4 )- 0 ( 6 ) 2 . 5 0 ( 2 ) - o ( 5 ) 2 . 4 5 ( 3 ) - 0 ( s )- o ( 6 ) 2 . s o ( 2 ) - o ( 5 ) 2 . 4 5 ( 3 ) - o ( 6 )

2 . 4 5 2 , 4 2

8 1 ( 1 ) + 8 2 ( 1 ) *

s s . 6 ( 7 ) s 8 . 4 ( 6 )1 7 s . 1 ( 5 ) 1 7 5 . O ( 4 )

1 0 1 . 2 ( 5 ) s 8 . s ( 5 )

6 6 . 1 ( 8 ) * * + 6 6 . o ( 7 ) ' + *

7 2 . 4 1 9 ) * * 7 2 . 8 ( 8 ) * *

1 1 8 . 1 ( 4 ) 1 1 8 . 6 ( 5 )

1 7 5 . 1 ( 5 ) 1 7 5 . o ( 4 )

9 9 . 6 ( 7 ) 9 8 . 4 ( 6 )

6 6 . 1 ( 8 ) * * * 6 6 . O ( 7 ) * * *

1 O 1 . 2 ( 5 ) 9 8 . 9 ( s )1 1 8 . 1 ( 4 ) 1 1 8 . 6 ( 5 )

7 2 . 4 \ 9 ) a * 7 2 . 8 ( B ) * *

8 0 . o ( 8 ) * 8 2 . o ( 7 ) *

7 r a . 4 ( 7 1 1 1 8 . 8 ( 7 )7 4 . a | 4 ) * * * * 7 6 . 7 \ a ) * * * ,

6 6 . 6 ( 5 ) * * 6 6 . o ( 5 ) * r1 O 3 . 9 ( 6 ) 1 O 2 . 9 ( 6 )

7 a . 8 ( a ) * * * * 7 6 . 7 ( 4 J , + + *

1 1 8 . 4 ( 7 ) 1 1 8 . 8 ( 7 )

1 0 3 . s ( 6 ) 1 O 2 . e ( 7 )

6 6 . 6 ( 5 ) * + 6 6 . o ( s ) * *

1 6 4 . O ( 9 ) 1 6 0 . 9 ( 9 )

6 6 . o ( 7 ) r r * 6 4 . 3 ( 7 ) + { *1 1 6 . 0 ( 7 ) 1 1 8 . 3 ( 7 )

1 1 6 . O ( 7 ) 1 1 8 . 3 ( 7 )

6 6 . o ( 7 ) * * * 6 4 . 3 ( 7 ) * + *

1 6 8 ( 1 ) 1 6 6 ( t )

1 2 5 . 7 \ 4 ) 1 2 4 . 5 1 2 8 . 2

7 0 . 9 1 7 ) * * 6 9 . 5 6 9 . 6 * *

1 6 0 . 8 ( 5 ) 1 s 9 . 1 1 5 8 . 9

8 0 . 6 ( 7 ) + a l . 7 8 0 . 7 *

1 O 4 . 5 ( 6 ) 1 0 2 . 1 1 0 2 . 3

6 8 . r ( 8 ) * * * 6 8 . 5 6 a . 6 * * *

Equivalent posi t ions (referred

M e x ! z

o ( i ) ( 1 + x ) y z

0 ( i i ) ( 1 / 2 - x ) y ( 1 / 2 - z )

o ( i i i ) ( 1 + x ) y z

o ( i v ) l 1 / 2 - x \ y ( r / 2 - z )

to the tab les o f the a ton ic coord ina te odeeters ) :

( y - x + 1 ) ( 1 - x ) ( 2 , / 3 + z )

x y z x y zy x - z y x - z

x ( y - 1 ) z ( x + 1 ) y z( y - l ) x - z y ( x + I ) - z

x y z

x y z( r / 2 - x t ( r / 2 - y ) ( r - z l

x - y ( I / 2 + z J

\ r - x ) y ( 1 . / 2 - z )

x y z

x y z

x y ( 1 - z )

- x ( I / 2 - y ) ( I / 2 - 2 ,

- x l 1 / 2 - y ) l L / 2 + z )

x y z( x - 1 ) y z

x y z( y - 1 ) x - z

v x - z

( I -x ) y ( I /2 -z )- x y \ L / 2 - z )I x - t / z ) ( t / z -v )

\ z - r /2 )x - r (z -L /z )(r / 2-x) (r/ z-y) -zx y \ z - I l

0 ( v ) ( L / 2 + x ) ( l / 2 - y ) z y x - z y x - z

o ( v i ) ( 1 - x ) - y z x y z x y zo ( v i i ) ( r - x ) \ t / 2 - y l ( 1 / 2 - z ) y ( x - r l - z y x - z

o ( v i i i ) ( r / 2 + x ) - y ( r / 2 - z ) ( x - 1 ) y z x y z

x y z - x \ L / 2 - y ) \ 1 - / 2 - z ) y x - z

x y z x y z x y z( 1 - x ) - y ( 1 - z ) x y z x y z(1, / 2-x) () , / 2-v) (1-z)

S a f t e r c a t e h o u s e e t a l . ( 1 9 4 1 ) : ( a ) C a g . r r 3 Z r r . S O a T i t . Z O O S

S h d e d e d g e s : * v i t h P 8 , * * w i t h P 7 , * * I w i t h P 6 , * * + * r 1 a n

; (b ) cao .961zro .gsoT iz , rog9

P4/5 . Es t imated s tadad er ro rs ( in paentheses) re fe r to the las t d ig i t .


Table 15. Polymignyte, zirkelite and zirconolite: bond distances (A) and angles (') in P6(l) and P6(2) octahedra

P 6 - p o l y h e d r a

o ( i ) ^ o ( i i ) 8 e . o ( 2 ) * * * * 8 3 ( 1 ) * * * i

o ( i i i ) 1 8 0 1 7 5 . 1 ( e )

o ( i v ) e r . o ( 2 ) s 2 . 2 ( 6 )

o ( v ) s 4 . 4 ( 2 ) 9 5 . 4 ( s )

o ( v i ) 8 s . 6 ( 2 ) * 8 6 . 7 ( 8 ) *

Polymignyte zirkel i te

M e - M e 6 ( 1 ) - M e 6 ( 1 ) -

zirconol i teS

r i ( 1 ) - ( a ) ( b )

- o \ 7 ) 1 . 9 4 2 1 . 9 3 3- o ( 1 ) 1 . 9 4 0 1 . 9 3 3- o ( 4 ) 1 . 9 5 9 1 . 9 7 5- o ( 1 ) 2 . o o o 1 . 9 6 9- 0 ( 3 ) 1 . 9 9 7 1 . 9 8 6- o ( s ) 1 . 9 7 1 1 . 9 4 8

1 . 9 6 8 1 . 9 5 7

8 9 . 3 8 9 . 5

1 6 9 . 1 1 7 0 . 4

4 9 . 5 9 0 . 1

9 4 . 5 9 4 . 4

8 1 . 1 8 2 . 1 * *

1 0 0 . 4 9 a . o

r 7 7 . 4 r ? 7 . 9

a 7 . a a 7 . t +

9 0 . 9 9 0 . 3

8 0 . 5 8 2 . 2 * * | *

8 6 . a 4 7 . 3 r

93.7 92.6

9 4 . 6 9 4 . 9

a 6 . 6 8 7 . 7 *

1 7 8 . 7 I 7 7 . 4

x y z

x y z( r / 2 - x l ( r / 2 - y ) ( r - z l

x y zx - y ( r / 2 + z \

x y z

( r - x ) y ( 3 / 2 - z )

o ( i )

o ( i i )

0 ( i i i )

o ( i v )

0 ( v )

0 ( v i )

m e o

o ( i v ) ^ 0 ( v )

o ( v i )

- o ( r )-o ( 1 )- o ( r )-o ( 1 )-0 ( 4 )-o (4)

x y 2

x y z- x - y z

-x -y -z

x y - z

x y z' x - y z

9 2 . 2 \ 6 )1 7 5 . 1 ( 9 )

4 6 . 7 ( 8 ) r

9 5 . 4 ( 9 )

9 3 . 5 9 3 . 9

t 7 r . 6 7 7 r . 4

4 7 . 9 4 7 . 8 *

9 4 . 7 9 3 . 5

9 4 . 7 9 4 . O

4 2 . 6 8 3 . 3 r *

9 5 . 1 9 5 . 5

9 5 . 1 9 5 . 5

4 2 . 6 4 3 . 3 ' +

1 7 6 . 6 1 7 4 . 3

9 4 . 3 ( 2 )

7 7 4 . 7 \ 3 )

8 1 ' a ( z ) * *

9 3 . 5 ( 2 )

8 1 ' a ( z ) * *

8 7 . 6 ( 2 ) *

9 3 . 1 ( 1 )

9 4 . 3 ( 2 )

9 0 . a 1 2 1 + + r *

1 7 5 . 3 ( 2 )

r . 9 4 2 \ 4 ) - O ( 4 ) 2 . 0 1 ( 3 )

1 . 9 4 2 ( 4 ) - 0 ( 4 ) 2 . 0 1 ( 3 )

r . 9 4 2 ( 4 ) - O ( 7 ) 1 . 8 9 ( 3 )

r . 9 4 2 ( 4 1 - o ( 7 ) 1 . 8 s ( 3 )

2 . o 4 e ( 5 ) - o ( 6 ) 1 . 9 3 ( 1 )

2 . o 4 9 ( s ) - 0 ( 6 ) 1 . 9 3 ( 1 )

1 . 9 7 8 1 . 9 4

- o l 4 ) 7 , 9 7 7 7 . 9 6 4 - O ( 1 ) 1 . 9 4 2 ( 4 )- o ( 4 ) r . 9 7 7 r . 9 6 4 - O ( 5 ) 1 . 9 5 6 ( 2 )- o ( 7 ) 1 . 9 1 3 1 . 9 1 o - o ( 2 ) 1 . e 5 6 ( 3 )- o ( 7 ) 1 . 9 1 3 1 . 9 1 O - O ( 5 ) 1 . 9 5 6 ( 2 )- o ( 6 ) 1 . 9 3 3 1 . 9 1 6 - o ( 2 ) 1 . s s 6 ( 3 )- o ( 6 ) 1 . 9 3 3 1 . 9 1 6 - O ( 1 ) 1 . 9 4 2 ( 4 )

1 . 9 4 1 1 . 9 3 0 1 . 9 5 1

7 8 . 4 7 8 . 4 x x * * 9 0 . 4 ( 2 ) * * * *

1 7 1 . 6 1 7 t . 8 1 , 7 5 . 3 \ 2 )

9 3 . 5 9 3 . 9 9 3 . 5 ( 2 )

9 4 . 7 9 3 . 5 9 3 . 1 ( 1 )

8 7 . 9 A 7 . A ' 4 6 . 7 \ 2 ) +

zirconol i te S

r i ( 3 ) - ( a ) ( b )

Poljmignyte

M e 6 ( 2 ) -

Z i r k e l i t e

M e 6 ( 2 ) -

- o ( 7 ) 2 . o o ( 3 )- 0 ( 1 ) 1 . e 6 ( 3 )- 0 ( 4 ) 1 . 9 2 ( 3 )- o ( 1 ) 1 . s 6 ( 3 )- o ( 3 ) 1 . s 7 ( 1 )- 0 ( 5 ) 1 . 9 8 ( 1 )

I . 9 6

8 9 . 7 ( 8 )

1 - 7 2 . 6 1 9 )

8 8 . 7 ( 5 )

9 5 . a ( 8 )

4 4 . 3 ( 8 ) ' r

9 7 . 6 ( 6 )

1 7 7 . I \ 3 )

8 4 ' 8 ( 9 ) r

9 4 . 5 ( 9 )

8 4 . o ( 8 ) + * + *

8 6 . 6 ( s ) *

9 3 . 4 ( 9 )

9 7 . 7 \ 9 )8 3 . 0 ( 9 ) *

L 7 S \ 2 )

x y z( 1 - x ) ( y - x ) ( 1 / 3 - z )

x y zx y z( r-x) ( v-x) ( r /s-z )

x y z

( l - x ) ( y - x ) ( 1 / 3 - z )

o ( i i ) ^ 0 ( i i i ) e l . 0 ( 2 )

o ( i v ) 1 8 0

o ( v ) 8 s . 6 ( 2 ) r

o ( v i ) 9 4 . 4 ( 2 \

0 ( i i i ) ^ 0 ( i v ) 8 e . o ( 2 ) * * * r s 2 ( 1 )

0 ( v ) 8 5 . 6 ( 2 ) * 8 3 . 8 ( s ) * *

o ( v i ) 9 4 . 4 ( 2 \ 9 4 . 4 ( 8 )

9 4 . 4 \ 2 )

8 5 . 6 ( 2 ) *9 4 . 4 ( 8 )

a 3 . a ( 9 ) * *

o ( v ) ^ o ( v i ) 1 8 0 r 7 7 ( z )

Equiva len t pos i t ions ( re femed to the tab les o f the a tomic coord ina te pareeters ) :

Me

o ( i )o ( i i )o ( i i i )0 ( i v )

o (v )o ( v i )

- v ( x - v \ ( r / 3 + z lx y z-x\y-x) (1/3-z)-x(y-x\ ( t ' /3-zl

x y z

( 1-x) ( y-x+1 ) ( 1/3-z )

( x - l ) y z

x y z x y z( x - \ / 2 ) l r / 2 - y \ ( z - 1 / 2 ) x y z( I / 2 - x ) ( r / 2 - y ) ( 7 - z ) x y zx - y ( z - r / 2 ) l 7 / 2 - x \ y ( 7 / 2 - z \- x - y ( a - z ) ( r / z - x ) y ( r / 2 - z \

x y z x y z

- x y ( I / 2 - z ) ( r / 2 - x ) y ( L / 2 - z )

S a f t e r G a t e h o u s e e t a l . ( 1 9 8 1 ) : ( a ) C f u . 9 9 g Z " t . S O a T i l . 7 O O O 7 ; ( b ) C a O . 9 6 t Z r O . 8 5 O T i 2 . 1 6 9 0 7

S h a e d e d g e s : * w i t h P 8 , * * w i t h P 7 , * * * * w i t h P 4 l 5 . E s t i n a t e d s t a n d d d e r r o r s ( i n p a e n t h e s e s ) r e f e r t o t h e l a s t d i g i t .

are somewhat more uniform in the other minerals,the largest difference being about 0.164. All cubeshave the same type of distortion, as may be seenfrom the bond angles (Table 14), and nearly 222symmetry: one of these twofold axes is real inpolymignyte and zirkelite, but none of them areretained in the crystal structure of zirconolite. Twoindependent P8 polyhedra are present in zirkelite,and only one in the remaining minerals. The meanvalue of the eight Me8-O distances in each polyhe-dron ranges from 2.42 and 2.464.

The P7 polyhedra have almost rn symmetry: themirror plane is real only in the crystal structure ofpolymignyte. Three oxygens lie in the plane, andone of them (O(v) or O(vi) in Table 14) shows theshortest distance from the Me7 cation. On the otherhand, two oxygens (O(i) and O(ii)), lying at o,ppositesides of the plane, have bond lengths 0.30A largerthan the mean of the other distances. This polyhe-

dron can be described alternatively as a distortedcube without one vertex. or a distorted octahedronwith a centered face: actually the P7 polyhedronreplaces both P8 and P6 polyhedra present in thecrystal structure of pyrochlore. The mean value ofthe seven Me7-O distances in each polyhedronranges from 2.17 and 2.204.

All the crystal structures show two independentoctahedra (P6(1) and P6(2)) having different symme-try, respectively 2lm and 2 in polymignyte , 2 and 1in zirkelite or zirconolite. In general thp Ti contentis diferent in the two independent octahedra, butthere is no evidence for a correlation between thesite population and the symmetry of the polyhedra.The mean value of the six Me6-O distances in eachoctahedron ranges from 1.93 and 1.98A.

Finally P4 and P5 polyhedra are formed around acation, which does not center a cavity among eightoxygens, but is shifted toward four or five of them.


Table 16. Polymignytq, zirkelite and zirconolite: bond distances (A) and angles (") in P4 tetrahedra and P5 trigonal bipyramids

P4,/5 - polyhedra

271

Me-

o ( i )o ( i i )o ( i i i )o ( i v )o ( v )

nean

o ( i ) ^ 0 ( i i )o ( i i i )

o ( i v )o ( v )

o ( i i ) ^ o ( i i i )

o ( i v )o ( v )

o ( i i j . ) ^ o ( i v )o ( v )

o ( i v ) ^ o ( v )

Me

o ( i )o ( i i )o ( i i i )o ( i v )o ( v )

x y zx y z

' x - y z- x - y z

x y z

x y z

x y z

- x - y z

x y z

x y z

x y ( 1 - z )

E q u i v a l e n t p o s i t i o n s ( r e f e r r e d t o t h e t a b l e s o f t h e a t o n i c c o o r d i n a t e p a r i l e t e r s ) :

Polynignyte

Me4-

- o ( 3 ) 1 . s 1 6 ( 5 )

- o ( 3 ) 1 . e 1 6 ( 5 )

- o ( 1 ) 2 . 2 r 7 ( 5 )- o ( 1 ) 2 . 2 1 - 7 ( 5 )

2 .067

1 4 s . s ( 3 )9 4 . 1 ( 2 ) *

1 0 1 . 1 ( 2 )

1 1 0 . 1 ( 2 )9 4 . r 1 2 ) +

7 5 . 4 ( 2 ) x * *

Me5-

- o ( 3 ) 1 . e s ( 3 )- o ( 3 ) 1 . 8 6 ( 3 )- o ( 5 ) 2 . 2 6 ( 4 )- o ( 1 ) 2 . 4 3 ( 2 \- o ( 1 ) 2 . 4 3 ( 2 )

2 , 7 9

1 5 3 ( 3 )

8 6 ( 1 ) x +

1 0 1 ( 1 )

1 0 1 ( 1 )

1 2 2 ( 2 \

8 9 ( 1 ) *

8 9 ( 1 ) *

7 2 ( t ) x * x

7 2 \ r ) + + *

1 3 6 ( 2 )

Z i rke l l te

Me5-

- o ( 2 ) 1 . e 3 ( 2 )- o ( 2 ) 1 . 8 e ( 2 )- o ( 4 ) 2 . r 4 ( 2 )- o ( 4 ) 2 . 4 4 ( 2 )- o ( 1 ) 2 . 3 2 ( 2 ' '

2 . 1 4

149 U- )e 3 ( 1 )

1 0 3 ( 1 )

1 O 4 ( 1 )

1 1 7 ( 1 )

8 5 ( 1 ) *

e o ( 1 ) *

7 O l 2 ) * * *7 1 ( r ) * * *

1 3 3 ( 1 )

Z i rcono l i te $

r i ( 2 ) - ( a ) ( b )

- o ( 2 ) 1 . a 4 5 1 . 8 r . 5- o ( 2 ) 1 . 8 3 2 L . 7 9 2- o ( 4 ) 1 . 8 9 0 r . 7 9 2- o ( 4 ) 2 . 2 3 4 2 . 2 5 3- o ( 1 ) 2 . 2 2 A 2 . 1 9 4

, . 0 0 6 1 . 9 6 ,

1 3 8 . 4 1 3 2 . 1

9 a . 1 1 0 0 . 4

1 0 6 . 6 1 0 3 . 4

1 0 1 , 1 1 0 3 . 3

123,5 L26,7

8 7 , 2 a 6 . O *a9.2 90 .O*

7 4 . r 7 4 . 8 * + *

7 6 . 4 8 0 . 5 * * +

t4 t .7 146.3

\ y z

x y z( r -x \ y ( ! /2 -z \

x -y (z -L /2 )

( 1 - x ) - y ( 1 - z )

x y z

x y z

x y z-x (y -x \ (1 , /3 -z l

x ( y + 1 ) z- x ( y - x + 1 ) ( 1 / 3 - z )

( 1 - x ) ( y - x + l ) ( 1 / 3 - z )

S a f t e r G a t e h o u s e e t a l . ( 1 9 8 1 ) : ( a )

S h r e d e d g e s : * w i t h P a ' r + w i t h P 7 ' * * r

c .o . ggsZ" t . 3o4T i l . zoooz(b) c .o .9612"0 .85oT iz . tagoz

w i t h P 6 . E s t i m a t e d s t a n d a r d e r r o r s ( i n p i l e n t h e s e s ) r e f e r t o t h e l a s t d i g i t '

Me4 and Me5 sites, whose multiplicity is doublethat of the point at the center of the cavity, areincompletely and statistically occupied. Both Me4and, to much lesser extent, Me5 positions areoccupied in polymignyte. Apparently only Me5sites are occupied in zirconolite and zirkelite, evenif in the latter mineral Me5-O(iv) distance is largerthan in the former (Table 16). In fact, only a slightshift of the position of the cation is able to changeone coordination into the other, and very likely thelocal charge balance is responsible for the choice ofeither cationic position Me4 or Me5. The P4 polyhe-dron is a very distorted tetrahedron and P5 adistorted trigonal bipyramid. The mean of the fouror five distances in these polyhedra ranges between1.97 and 2.19L: the lower values are in syntheticzirconolites. where the central cation is titaniumrather than iron as in the crystals from CampiFlegrei.

A comparison of the crystal structures of pyro-chlore, polymignyte, zirkelite and zirconolite can

be easily made by an examination of the arrange-ment of the atoms in the multiple cells reported inTable 3. All these crystal structures are obtained byrepeating along the c-axis pairs of adjacent layers ofpolyhedra (say M- and N-layers). Figures 1,2 and3show the positions of the atoms in these two layersrespectively for pyrochlore, for zirkelite-zirconoliteand for polymignyte: the crystal structures of zirke-lite and zirconolite are practically based on thesame pair of layers. All figures have been drawn insuch a way that one Me6(1) cation is always at thepoint 000 of the M-layer on a twofold axis parallel tothe b-edges of the multiple cells.-The thickness ofeach layer is slightly less than 3 A: twelve pairs ofM- N-layers are contained in the multiple cell ofzirconolite and only three in those of the remainingminerals. The most regular atomic arrangement isobviously in the cubic pyrochlore: the M-layershows P6 octahedra and distorted P8 cubes in afrequency ratio 3: l; the same thing occurs in the N-layer, but with an inversion of the frequency ratio.


Table 17. Calciobetafite, polymignyte, zirkelite and zirconolite: X-ray powder diffraction data.

ca-betaf l te Polymignyte Z i r k e l i t e Zirconof i tes

( a ) $

h k l d

2 2 2 2 . 9 7 3 1 0 0

4 0 0 2 . 5 7 4

3 3 1 2 . s 6 3

3 3 3 )

u r , j 1 ' n a 2

4 4 4 1 . 4 8 6 1 0

( a ) $

h k f d I

1 3 1 3 . 6 8 7 7 2

2 0 2 2 . 9 5 7 9 3

2 4 0 2 . 9 0 1 1 0 0

o 4 2 l ^ - ^ ^

a o o J ' ' " " " 5 u

3 2 2 2 . 3 3 8 1 0

1 6 0 2 . 2 9 6 5

o o 4 1 . 8 1 9 1 5

4 4 2 I . 7 9 4 6 8

o 8 0 1 . 7 6 8 1 4

2 4 4 r . 5 4 ) , 2 4

6 0 2 1 . 5 3 4 1 3

6 4 0 1 . 5 2 6 1 5

2 A 2 1 , 5 1 A 2 2

4 0 4 I . 4 7 9 1 04 B O 1 . 4 5 1 7

o a 4 t

" o o J t ' ' 6 8

9

2 0 6 1 . 1 A O 5

6 4 4 1 . 1 6 9 9

0 4 6 1 - _ ^z r z o J t ' t u o

5

8 a 2 \ , , . o4 a 4 t - ' ' " '

4 4 6 1 . 0 4 5 7

4 I 2 2 r . 0 2 6 4

( a ) S

h k I d

( a ) '

d r d

5 . 2 4 1

3 . 9 7 6

3 . 3 3 3

) ^ ̂ . ^> 5 . 2 4 V)

( b ) * * * ( c )

t . a 2

2 0 2 2 . 9 5 6

0 0 6 2 , a \ 4o 2 4 2 , 5 2 7

2 0 5 2 . 3 0 6

1 0 t d

B

1 1

1 1

1 0

r 0 0 2 . 9 2 9 1 0 0 2 . 9 4

5 4 2 . 9 0 3 7 A

3 8 2 . 7 4 9 5 6 2 . a 7

1 9 2 . 5 0 3 5 4 2 . 5 2

1 1 2 . 4 9 0 1 9

8

4

5

100

27

2 6

5

1 2

h k 1

1 1 1r L 21 1 33 1 13 l - 22 2 74 0 20 0 42 2 34 0 22 2 34 0 43 3 13 3 2o 2 52 2 5

> 2 . 9 5 4

2 . 8 1 3

2 . 5 3 0

2 . 5 2 5

2 . 3 0 4

2 . 3 0 2

2 . 9 3 51 0 0

2 . 9 1 6

2 9 2 . 7 4 7

1 9 2 . 5 0 9

I 2 . 4 9 4

3 2 . 2 4 7

2 2 . 2 7 4

\ , . o t ,t

] t . n o o)

34

4 4 0 1 . 8 2 0 5 3

6 2 2 L . 5 5 2 4 5

2 2 A 1 . 8 2 2

2 0 a r . 7 5 a

o 4 2 1 , 5 5 1

2 2 6 1 . 5 2 9

0 2 1 0 1 . 4 8 9

A O 4 I . 4 7 A

o 4 8 r . 2 6 4

4 2 2 1 . 1 a 1

2 4 4 1 . 1 4 8

2 2 1 2 t . I 7 4

6 0 0 1 . 0 5 2

4 2 B 1 . 0 3 a

1 . a 2 8 1 t -

7 . 4 2 L 1 a

1 . 7 5 5 1 6

1 . 7 5 2 7

1 . 5 5 4 8

1 . 5 4 9 3

1 . 5 3 3 6

] r . s za rot

) t . ouu 7

1 . 4 8 1 4r . 4 7 5 2

J r . 2 6 4 5

1 . 1 8 5 3

I r . r " t 4)1 . 1 5 1 2

\ t . r o , 3t

)> 1 . 1 1 4 3)

1 . O 5 4 2

1 . O 5 0 2

)> 1 . 0 3 9 at

1 5 1 . 4 1 5 2 7

3 2 1 . 7 9 4 6 7

2 6 1 . 7 4 0 6 6

1 - 7 r . 7 3 7 5 A

1 0 1 . 5 4 1 3 2

5 1 . 5 2 8 1 2

7 7 . 5 2 2 2 0

t : ] r . u ' a 2)

a o o 1 . 2 4 7

6 6 2 1 . 1 a 1

8 4 0 1 . 1 5 1

o 4 0

6 2 1

2 2 5

4 0 6

4 4 2

a o o

o 4 4

6 2 3

2 2 7

4 0 6

4 4 2a o 4

o 0 8

4 4 6

8 0 4

2 6 7

4 4 0

1 0 2 3

2 6 3

8 4 4

1 0 2 1

0 4 4

6 2 7

6 2 9

6 6 1

1 2 0 2

2 6 5

4 4 4

r o 2 7

1a

a

7

5

8

1 . 8 1 5

1 . 8 0 0

7 . 7 3 6

1 . 5 4 1

1 . 5 2 9

r . 5 2 1 -

) . - . ^| ' ' " '

r . 4 7 6

r . 4 7 2r . 4 6 71 . 4 5 8

7 . 2 5 4

7 . 2 4 7

1 . 1 - 7 6

) - _ - ^J

1 . r b 6

1 . 1 4 3

I . I 3 7

I . I 3 2

) I - 1 0 : lt

1 . 0 4 5

r .o37

6 1 - . 4 7 9 7 7 1 . 4 a

4 L . 4 7 5 2 45 1 . 4 6 9 7 44 1 . 4 5 7 1 5

1 . 3 9 4 2

4 1 . 2 6

2

3 1- . J-79

I

2 L . 1 4 9

2

2

1 3

4 4 4 1 . 0 5 1 t 2

1 . 1 1 5 1

3

3 1 . 0 5 0 2

2

1 . O 3 8 3

(a) ca-1culated with the experimental lat t ice pi l i leters of the minerals fron Cmpi Flegrei ;( b ) s y n t h e t i c c r y s t a l s ;( c ) h e a t e d p o l y n i g n y t e f r o m F r e d r i c k s v H r n ( L l n a - d e - F r i a , 1 9 5 8 ) ;

S calculated from the experimental s ingle crystal intensi t ies;* cal-culated from the theoret ica-I intensi t ies obtained for a crystal structwe md a chenicaf composit ion aalogous to

those of z irkef i te from Cmpi FLegrei ;** calculated from the theoret ical intensi t ies obtained fron the data by catehouse et aI . (19a1)i*** af ter Pyatenko and pudovkina (1964).

Both layers are formed by chains of polyhedra,which develop in three directions at 120'to eachother. Chains of octahedra sharing vertices (P6chains) alternate with mixed chains of octahedraand cubes sharing edges (P68 chains) in each ofthethree directions at 120'in the MJayer. Conversely

chains of cubes sharing edges (P8 chains) alternatewith P68 chains in the N-layer.

The crystal structure of pyrochlore is modified inthe remaining minerals by the introduction of zirco-nium, which centers P7 polyhedra. Half of the P68chains, pointing in one of the three directions

MAZZI AND MUNNO: PYROCHLORE AND RELATED MINERALS n3

MNFig. l .Pyrochlore:adjacentM-andN-layers(seetext)ofthecrystalstructureprojectedalongIl l ] ;a-edgecorrespondsto[-10.5

0.51 and D-edge to t0 -0.5 0.51. Threefold and twofold axes, and minor planes determined by the arrangement of the atoms in eachlayer have been traced.

described above for pyrochlore, are replaced by P7chains: pairs ofFT polyhedra replace couples ofP6-P8 polyhedra. In order to achieve this, one oxygenis moved away from each P8 polyhedron and at thesame time another oxygen approaches each P6. Theremaining oxygens show minor shifts. The P7chains are present only in the NJayers of thezirkelite-zirconolite structures, where they havereplaced all P68 chains pointing in one direction.When such chains are formed, the consequent shiftsof the oxygens give rise to a reduction of the volumeof the P8 cavities of the P68 chains in the adjacentM-layer. In particular, two oxygens approach eachother, leaving no space for a (Ca,REE . . ) atom atthe center of the cavity: then a smaller cation (Fe inour minerals, Ti in synthetic zirconolites) locatesitself statistically on one or two nearby sites, whichhave multiplicities double that of the central posi-tion and are surrounded by four or five oxygens.The passage from the pyrochlore structure to thoseof zirkelite or zirconolite accordingly involves

changes in one of the three sets of parallel P68chains: all ofthese are changed into P7 chains in theN-layer and into P64l5 chains in the M-layer. Thesame transformations also take place in polymig-nyte, but here P7 chains, and consequently alsoP64l5 chains, are formed in both M- and N-layers,where they alternately replace P68 chains of pyro-chlore. Consequently, the positions of P6 and P4l5polyhedra are interchanged in alternate P64l5chains, with respect to the original structure ofpyrochlore, and the a-edge of the multiple cell ofpolymignyte is double those of the remaining miner-als.

The arrangement of the atoms gives rise to sym-metry elements parallel (twofold axes) or perpen-dicular (threefold axes and symmetry planes) to theM- and N-layers.

The highest symmetry G3m) is obviously pre-sented by both layers of pyrochlore. The symmetryis nearly the same also for the M-layer of zirkelite-zirconolite, if one ignores the positions statistically


Fig. 2. Zirkelite-zirconolite: M- and N-layers of the crystal structure projected along [001] (zirkelite) or [06j (zirconolite); a-edge is[201] of zirkelite or [l([] of zirconolite, b-edge is [010] of zirkelite or [0-10] of zirconolite. Twofold axes (real in the M-layer of bothminerals and in the N-layer of zirkelite) and symmetry centers (real only in zirconolite) determined by the arrangement of the atoms ineach layer have been traced. Numbering of oxygen atoms corresponds to that reported in Table I I (zirkelite) or by Gatehouse el a/.(1981) (zirconolite); the same figures generally hold for both minerals: when different, those in parentheses refer to zirconolite.Me8(l) and Me8(2) are equivalent in zirconolite. Bonds in Yt andP64l5 chains are identified by solid lines.

occupied by Me4 and Me5 atoms. The epitaxy andthe consequent intergrowths among pyrochlore,zirkelite and zirconolite most likely originate fromthe similarities of the M-layers of the three miner-als. The C3m symmetry is reduced to that of itssubgroup C2lm in the layers where the W chainsreplace P68 chains, that is in the N-layer of zirke-lite-zirconolite and in both layers of polymignyte.All the symmetry elements present in both layers ofpyrochlore and polymignyte are retained in thewhole crystal structure, obtained by the repeat MNMN MN of the layers along the c-axis. However,only part of the symmetry of the layers is retained inthe crystal structures of zirkelite and zirconolite:the mirror planes disappear, and either one twofoldaxis or the symmetry centers present in each layerbecome real elements of symmetry for the wholecrystal. The twofold axis is maintained in the M-

layer for both minerals, whereas the N-layer ofzirkelite still shows a twofold axis (rotated 120" withrespect to that of the MJayer) and that of zircono-lite shows only symmetry centers. In this way oneobtains the centrosymmetric space group C2lc inzirconolite and the acentric one P3Q for zirkelite,even though slabs of the crystal structures arepractically the same.

Conclusions

The same types of polyhedral chains are presentin all three minerals. P6 and P8 chains are always inalternate layers (M and N respectively); the sameholds for P6415 and P7 chains in zirkelite andzirconolite. whereas the latter chains are in alllayers of polymignyte. As the P7 and P8 chains areparallel to the twofold axis, they point only in onedirection [001] in the structure of polymignyte;


Fig. 3. Polymignyte: M- and N-layers of the crystal structure projected along [-ll0]; a-edge corresponds to [210] and D-edge to[001]. Twofold axes and mirror planes determined by the arrangement of the atoms in each layer have been traced. Numbering ofoxygen atoms refers to Table 7. Bonds in W andP64l5 chains are identified by solid lines.

275

however, as the same chains make an angle of 60'with the twofold axis in the M-layer, they arealternately in two directions at 120" ([110] and tl101)in successive N-layers of zirconolite; finally, be-cause of the presence of the additional twofold axisrotated 120'in the NJayer, the W and P8 chainsdevelop in three directions at 120" (U001, [010] and[110]) in successive N-layers of zirkelite. The re-sults of the crystal structure determinations there-fore confirm that polymignyte, zirkelite and zircon-olite are polymorphs of the same compound.

AcknowledgmentsWe are greatly indebted to Dr. W. L. Grifrn for the micro-

probe analyses made at the Mineralogisk-Geologisk Museum ofOslo. The Consiglio Nazionale delle Ricerche is acknowledgedfor financing the electron microprobe laboratory at the Istituto diMineralogia e Petrologia of the University of Modena. Manysincere thanks also to Dr. A. Kato, Chairman of the Commissionon New Minerals and Mineral Names of IMA, who, in readingthe manuscript, first noticed that the Campi Flegrei pyrochlore isa new member ofthe betafite subgroup. Thanks are extended toMr. G. Germani for his valuable technical assistance in thecomputational aspect of this research.

References

Am[, Reidar and Griffn, W. L. (1975) Microprobe analysis ofREE minerals using empirical correction factors. AmericanMineralogist, 60, 59-606.

Berzelius (1824). Svenska Vetenskapsakademien, Handlingar,338 (not seen; extracted from Palache, C., Berman, H., andFrondel, C. (1944) Dana's System of Mineralogy, 7th ed., vol'I, Wyley, New York).

Blake, G. S. and Herbert Smith, G. F. (1913) On varieties ofzirkelite from Ceylon. Mineralogical Magazine, 16, 309-316.

Borodin, L. S., Bykova, A. V., Kapitonova, T. A., and Pya-tenko, Yu. A. (1%l) New data on zirconolite and its niobiumvariety. Doklady Akademiya Nauk SSSR, 134, 1022-1024 (not

seen; extracted from Mineralogical Abstracts, 15,538).Borodin, L. S., Nazarenko, I. I., and Rikhter, T. L. (1956) On a

new mineral zirconolite---a complex oxide of ABrOz type.Doklady Akademiya Nauk SSSR, 110, 845-848 (not seen;extracted from Mineralogical Abstracts, 13, 3E3).

Brrigger, W. C. (1E90) Die Mineralien der Syenitpegmatitgangeder sudnorwegischen Augit- und Nephelinsyenite. Zeitschriftftir Kristallographie, 16, 387 -j96.

Busing, W. R., Martin, K. O., and Levy, H. A. (l%2) oRFLs, aFortran crystallographic least-squares refinement progf,am.U.S. National Technical Informaticin Service omtl-rru-305.

Donnay, Gabrielle and Allmann, Rudolf (1970) How to recognize

276

O2-, OH- and H2O in crystal structures determined by X-rays. American Mineralogist, 55, 1003-1015.

Gaertner, H. R. (1930) Die Kristallstrukturen von Loparit undPyrochlor. Neues Jahrbuch fiir Mineralogie, 61, l-30.

Gatehouse, B. M., Grey, I. E., Hill, R. J., and Rossell, H. J.(1981) Zirconolite, CaZr*Ti3-xO7; structure refinements fornear-end-member compositions with x=0.85 and 1.30. ActaCrystallographica, B37, 306-312.

Hogarth, D. D. (1977) Classification and nomenclature of thepyrochlore group. American Mineralogist, 62, 403-410.

Hussak, Eugen and Prior, G. T. (1895) Lewisite and zirkelite,two new Brazilian minerals. Mineralogical Magazine, ll, 80-88.

Lima-de-Faria, J. (1958) Heat treatment of metamict euxenites,polymignites, yttrotantalites, samarskites, pyrochlores, andallanites. Mineralogical Magazine, 31, 937 -942.

Lima-de Faria, J. (1964) Identification of metamict minerals byX-ray powder photographs. Junta de Invest. do Ultramar,Portugal; Estud., Ensaios & Documentos, ll2 (not seen;extracted from Mineralogical Abstracts, 17, 243).

North, A. C., Phillips, D. C., and Mathews, F. S. (1968) A semi-empirical method of absorption correction. Acta Crystallogra-phica, A24,351-359.

Perrault, Guy (1968) La composition chimique et la structurecristalline du pyrochlore d'Oka,PQ. Canadian Mineralogist, 9,3E3-,102.

Prior, G. T. (1897) On the chemical composition of zirkelite.Mineralogical Magazine, I I, 180-183.

Pudovkina, Z. V., Chernitzova, N. M. and Pyatenko, Yu. A.(1969). Crystallographic study of polymignyte (in Russian).Zapiski Vsesoiuznogo Mineralogicheskogo Obshchestva, 98,193-199.

Pyatenko, Yu. A. and Pudovkina, Z. V. (196./.) The lattice metricof CaZrTizOt crystals. Kristallografiya, 9, 98-100 (transl.Soviet Physics-Crystallography , 9, 76J8).

Sinclair, William and Eggleton, R. A. (1982) Structure refine-ment of zirkelite from Kaiserstuhl, West Germany. AmericanMineralogist, 67, 615-620.

Manuscript received, May 11, 1982;acceptedfor publication, August 23, 19E2.


Appendix: The identification of polymignyte,zirkelite and zirconolite

It is very likely that at other localities where only one of thepolymorphs (polymignyte, zirkelite and zirconolite) has beenrecorded, all three minerals may actually occur together, asdescribed here for the Campi Flegrei rock. We suspect that theproperties of each of these minerals reported in the literature areoften an "averbge" of the properties of all three minerals; forinstance both polymignyte and zirkelite were described asprismatic or platy crystals: as for the Campi Flegrei material, it islikely that the former crystals are polymignyte and the latterzirkelite-zirconolite. Also, discovery of pure zirkelite orzirconolite should be very rare: as they differ only in the stackingof some layers, the epitaxy of the two species should be nearlyalways present.

Apart from the above possible distinction based on themorphological features, the identification of the three mineralsby X-ray powder patterns is clearly not straightforward. Table 17gives the spacings and the intensities for the strongest reflectionsof calciobetafite, polymignyte, zirkelite and zirconolite,calculated from the lattice parameters and the intensitiescollected in the single crystal studies of the Campi Flegreimaterial. Analogous data are reported for the syntheticzirconolite studied by Gatehouse et al. (1981). Table 17 also liststhe experimental X-ray powder data of synthetic zirconolite(Pyatenko and Pudovkina, 1964) as well as those reported byLima-de-Faria (1958) for the metamict polymignyte fromFredricksvirn heated to 1000" C. The differences in the spacingsof the three minerals are very small: the calculated and observeddifiraction patterns of synthetic zirconolite are comparable, butboth are more similar to the calculated pattern of polymignyte,than that of zirconolite from Campi Flegrei. On the other hand,the powder difraction data for heated polymignyte are moresimilar to those of zirkelite and zirconolite from Campi Flegrei,the latter ones being practically indistinguishable from eachother. Surely the differences, which derive from the variations ofthe lattice parameters caused by the isomorphous replacements,overwhelm those due to the different crystal structures. Onlysingle crystal X-ray studies, made on non-metamict materials,allow a clear distinction between these minerals.

calciobetafite (new mineral of the pyrochlore group) and related … · 2007. 8. 26. · dovkina et...

Documents