CLINOPYROXENES OF ALKALI OLIVINE-BASALT N,IAGI,IA

J. F. G. WrrrrNSoN, The Uniaersity of New England,Armid.ale, ILS.W ., Australia.

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In representatives of alkali olivine-basalt magma, as contrasted to tholeiitic rocks,only one common pyroxene, a clinopyroxene close to diopside or saiite, appears. Theclinopyroxene crystallization trend is parallel to the Di-He join, principally in the fieldsof diopside or salite. Analytical data indicates that alkali olivine-basalt clinopyroxeneshaving compositions more iron-rich than CarsMg:;F€:o fiust be comparatively rare.

The contrasting behavior of the pyroxene assemblage of alkali olivine-basalt andtholeiitic magma depends principally on the undersaturated or saturated nature of therespective residual magmatic solutions, which determine whether or not orthopyroxene asa product of magmatic reaction with early-formed olivine will appear. The higher modalpvroxene content of tholeiites, their more varied pyroxene assembiage, and the greatervariation in clinopyroxene composition with differentiation compared with alkali oliyine-basalt clinopl'roxenes, are important consequences of this reaction.

Iwrnonucrrox

While much has been written on the crystallization trends and mutualrelationships of clinopyroxenes from tholeiitic rocks, clinopyroxenes ofunder-saturated basic alkaline rocks have been comparatively neglected.Except for Murray's (1954) investigation of the Garbh Eilean clino-pyroxenes, little attention has been directed to crystallization trends ofclinopyroxenes from a single intrusion of alkali olivine-basalt type.

Several of the concepts in the present paper result from the investiga-tion of the Black Jack teschenite sill, near Gunnedah, New South Wales.This differentiated five hundred foot thick intrusion of Tertiary age is atypical representative of alkali olivine-basalt magma. The principalmineralogical (modal) variations in the sill are expressed in a progressivedecrease in olivine with increasing height above the lower contacts,while clinopyroxene and iron ore are slightly more abundant in the upperlevels of the intrusion. During differentiation, the olivine becomes morefayalitic (Fart to Faoo) while the plagioclase feldspar is enriched in albite(An72 to An +s) (Table 1). The homogeneous iron-titanium ores also revealconsiderable compositional changes. fn strong contrast, the associatedbulk clinopyroxene (usually strongly zoned) undergoes little change incomposition during differentiation (Caasl{grzFers to Caa71\'fg3aFe1e). Thelimits of clinopyroxene composition are close to the range, CaaeMg3zFeuto Ca+zMgsrFe2s (Table 2), based on analyses of the Mg- and Fe-richfractions of the analyzed bulk clinopyroxene.

The writer follows Poidervaart and Hess (1951, Fig. 1-B) on clino-pyroxene nomenclature. The clinopyroxene compositions, except in cer-tain instances where they have been derived from a previous source, are

724

C],INOPYROXEN ES OII ALKALI OL]V IN]]-BASALT MAGMA

expressed in atomic percentages of Ca, I{g and Fe (following Hess, 1949) .The following concepts are fundamental to any discussion dealing with

the nalure and crystallization histories of the pyroxenes:(i) The recognition of certain magma types.

(i i) Clinopyroxene nomenclature ha,sed on optical properties, all iecllvith the use of oplics for compositional purposes.

(i) l 'he concept of magma types, proposed by the authors of the trIullmemoir (Bailey et al., 1924) is now well known. It is "a permanent con-tribution to petrologic thought" (Tii ley, 1950, p.38) .In their definit ionof the Plateau and Non-Porphyritic Central magma types, the Mullauthors (1924, p.14) state that "a purple colour is characteristic of theaugite" in the Plateau rocks. The highly calcic diopsidic nature of theclinopyroxene in the X{ull Plateau (:olivine-basalt) magma, comparedwith the enstatite-augite varieties in the the Non-Porphyrit ic (: tho-Ieiit ic) magma type, was pointed out by Kennedy (1931, p. 63; 1933, pp.210-211). Similar mineralogical differences had been recognized earlier byWashington (1922, p. 800) in the "cone-" and "plateau-basalts" respec-tively. The ambiguity arising from usage of the terms "olivine-basalt"and "tholeiite" as applied to magma types, has been discussed by Til ley(1950, pp. rt0-41), who uses as an alternative for the former, the term"alkali olivine-basalt. "

(ii) There is no analytical evidence indicating that the lime content(usually greater than 20 weight per cent) of clinopyroxenes from alkaliolivine-basalt magma ever approaches that of pigeonite, as defined byPoldervaart and Hess (1951, p. 473), despite the fact that such pyroxenesmay have low 2V's in the range 30-38 degrees (cf. Yagi, 1953, p. 780).Yet many writers, following Kuno (1936a, p. 141) have referred to theseclinopyroxenes as "pigeonites" or "pigeonitic," with a consequent erro-neous composition. The clinopyroxenes under discussion are usually richerin TiOz (Table 2), Al2O3, FezOr and CaO than common tholeiit ic clino-pyroxenes (see Hess, 1949, p.633). There appears to be l itt le agreementin the l iterature as to the exact variation in 7,\c, 2V, refractive indices,etc. to be ascribed to a particular constituent, whose presence in manycases cannot be diagnosed opticaliy. The effects of TiOz on the optics aremost uncertain (Ditt ler, 1929; Kuno, 1936; Segnit, 1953). The variableoptics of the clinopyroxenes listed in Table 2 should be noted. Opticalproperties of titania-rich clinopyroxenes indicate a higher Fe and lowerCa content than revealed by analysis. Until other methods e.g. the cor-relation of cell dimensions with variation in Als+, p.r+, Ti++ and Na+(Kuno and Hess, 1953) become available, chemical analyses provide thesafest means of deriving accurate compositions of t itaniferous clino-pyroxenes.

726 J. F. G. WILIJNSON

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Hess (1941, pp. 574-578) summarized the work of Bar th (1931; 1936) 'Tsuboi (1932) and Wager and Deer (1939) and drew up a course of crys-

tail ization for thoieiit ic clinopyroxenes, a trend substantiated later by

Edwards (1942) and Walker and Poldervaart (1949) . The varying

crystaii ization trends suggested by these workers are shown in Fig. 1. In

certain cases it is clear that the suggested trends appiied to clinopyroxenesfrom both tholeiit ic and alkali olivine-basalt magmas.

Rasic data on the crystall ization trends of the clinopyroxenes from a

En FsFrc. 1. Schematic representation of pyroxene crystallization trends in basic rocks.

I Tsuboi (1932), followed by Kuno (1936a); II Barth (1936); III Wager and Deer (1939),

integrating data from Kennedy (1933) and Kuno (1936o); IV Hess (1941, 1951).

single differentiated intrusion of alkaii olivine-basalt type is now suppliedby the Garbh Eilean and Black Jack pyroxenes. The principal composi-tional relationships of the main mineral series in the latter intrusion havealready been summarized. In the former sil l , the range in clinopyroxenecomposition is from CaaT\,Ig+2Ferr, the pyroxene in the picrite, toCaaal\ '{93e.5Fezs.s, the iron-rich fraction of the clinopyroxene in thecrinanite 325 feet above sea ler, 'el (l{urray, 1954, Table 1). In both in-trusions, the modal olivine decreases with progressive magmatic evolu-tion, a feature shown by other differentiated intrusions of this magma(Table 4). The olivines in the Garbh Eilean (\Iurray, 1954, Fig. 2) andBlack Jack rocks are richer in ferrous iron and undergo considerablygreater iron enrichment than the associated clinopyroxene. The crystal-)ization trend of the latter minerals in both instances is parallel to thediopside-hedenbergite join (Fig. 2), principally in the fields of diopsideand salite.

Wot

CLINOPYROXENES OF ALKALI OLIVINE-BASALT MAGMA 727

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Frc. 2. Clinopyroxene crystailization trends in intrusions of alkali olivine-basalt type.

Open circles-clinopyroxenes from the Black Jack sill, near Gunnedah, New South Wales

trend AA'. Solid circles-clinopyroxenes from the Garbh Eilean sill, Shiant fsles, trend

BB' (Murray,1954),

One of the most striking features of the Garbh Eiiean and Black Jackclinopyroxenes is their relatively small range in composition, for in this

respect they lag strongly behind the associated minerals and parent

rocks (see l ' Iurray, 1954, Fig. 3). Such relationships provide marked con-

trast to tholeiit ic intrusions showing medium or strong fractionation,

where the principal pyrogenetic minerals all undergo comparable com-

positional changes (Tab1e 1).To stabil ize the trend indicated by the Garbh Eilean and Black Jack

Mg

Frc. 3. Compositions of titaniferous clinopl'roxenes. Solid circles-clinopyroxenes from

alkali olivine-basalt magma. Solid triangles titaniferous clinopyroxenes from tholeiitic

magma. Open circles-titaniferous clinopyroxenes from contaminated basic magma'

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CLINOPYROXENES OF ALKALI OLIVINE-BASALT MAGMA 729

minerals, additional clinopyroxenes have been plotted in Fig' 3. Since

alkali olivine-basalt clinopyroxenes generally possess relatively high

titania contents, attention was directed to clinopyroxenes containing

more than average amounts of this constituent. Relevant pyroxene data

concerning composition, optics and literature are l isted in Table 2. The

references in this table are not listed at the end of the paper.

The plotted analyses include nearly sixty clinopyroxenes from non-

thoieiit ic igneous rocks (Table 2, Analyses 1-48). To save repetit ion, the

Garbh Eilean clinopyroxenes (Murray, 1954, Table 1) have not been in-

cluded in the tabulation. The parent rocks are undersaturated (alkalic)

types, usually with olivine andf or feldspathoids, and include many rep-

resentatives of alkali olivine-basalt magma. The debatable question of

the possible genetic interrelationships of certain of the listed rocks is out-

side the scope of the present investigation.The additional analyses include titaniferous clinopyroxenes from

tholeiitic magma (Tabie 2, Analyses 49-53), and from basic feldspathoidal

rocks produced by reaction of basic magma with carbonate sediments(Analyses 54-56).

The plot of the clinopyroxenes from basic alkaline magmas (Fig. 3)

yields important results, especially when one considers the variation in

age, location and most important, the magmatic histories of the parent

rocks themselves. As a group, these minerals reveal l i tt le variation in

composition. The majority plots in the field of salite; a small number fall

in the augite field but close to the augite-salite boundary. There is an

absence of any marked trend extending through ferrosalite to heden-

bergite, only three analyses revealing significant iron enrichment (Tabie

2, Analyses 40 and 46; l ' Iurray, 1954, Table 1, Analysis 3l H).

The titaniferous clinopyroxenes from both tholeiit ic and contami-

nated basic rocks plot outside the field of clinopyroxenes from alkaline

rocks, the tholeiit ic examples behaving in a manner similar to their

titania-poor equivalents in difierentiated tholeiitic magma' with appreci-

able repiacement of both ca2+ and rrlg2+ by Fe2+' The carbonate-basicmagma reaction types in the three instances reveal l ime enrichment.

Comparing Figs. 2 and 3, it is seen that the compositional trends of the

Garbh Eilean and Black Jack clinopyroxenes embrace the normal com-

positional l imits of alkali olivine-basalt clinopyroxenes, i.e. the trend is

dominantly parallel to the diopside-hedenbergite join, within the field

embraced by Fee-Fe21 and Caso-Caa3" There is no tendency for a pyrox-

ene trend from diopside to ferrosil i te, a trend demonstrated in certain

tholeiit ic rocks (Kuno, 1955), but often considered to apply to basaltic

magmas in general (Fig. 1).X{urray (1954, Fig. 1) has extended the crystall ization course of the

I. F" G, WILKINSON

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CLINOPVROXENES OF ALI(ALI OLMNE-B,ISALT MAGM'I 73r

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J. F. G. WILKINSON

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CI.INOPYROXENES OII ALRALI OLIV INE-BASALT MAGMA

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734 J. F. G. WILKINSON

Garbh Eilean clinopyroxenes into the field of ferroaugite, mainly on ananalysis of clinopyroxene (Caa1l\,IguFe+r) from an orthophyre vein in thePortrush sill, County Antrim. Of the many analyses studied, only onefalls in the ferroaugite fi.eld, and then practically on the ferrosalite bound-ary (Fig.3). The Portrush clinopyroxene should find no place in deter-mining the trends of the present pyroxenes for the following reasons:

1. The origin of the orthophyre vein is related in part to reaction and

rheomorphic injection. Consequently it is doubtful if the associatedclinopyroxene is a "normal" type in that its precipitation occurredunder strictly magmatic conditions.

2. It appears that the Portrush sil l and its veins are the products oi

the consolidation of undersaturated tholeiitic magma (cf. Tilley'1950, pp. 40-41). In his original account of the Portrush rocks,Harris (1937, pp. 102-101) recognized hypersthene (Ofsr) in addi-tion to clinopyroxene. His descriptions include two clinopyroxeneanalyses, one of "enstatite-augite" (Ca42n{genFeu), the other poor

in lime and approaching subcalcic augite (Ca25Mgr1Fe3a). Theolivine-dolerites themselves carry normative hypersthene and arelow in alkalies. The clinopyroxene in question plots very close tothe trend of the Skaergaard clinopyroxenes from the ferrohortono-lite-ferrogabbros, a point commented on by Murray.

In tholeiitic magma rarely is fractionation so extreme that the composi-tion of the clinopyroxene passes WomEnroFsrl, i.e. the intersection of thecourse of clinopyroxene crystallization (after Hess, 1941) and the two-pyroxene boundary (Poldervaar t and Hess, 1951, p.483; F ig.5) . A l -though only one common pyroxene occurs in alkali olivine-basalt magma,it is important to note that only two clinopyroxenes in Table 2 pass thetwo-pyroxene boundary in the direction of hedenbergite. Examples ofclinopyroxenes of the diopside-hedenbergite series showing iron enrich-ment in excess of CaasMgzrFea6 must be rare in alkaline basic rocks. Theretention of a large percentage of the available iron in the ferrous statein a reducing (relatively anhydrous) magmatic environment (Kennedy,1948), and the continued crystallization of clinopyroxene after thedisappearance of olivine under conditions of continual Fe/NIg enrich-ment would be fundamental conditions for such iron enrichment.Extreme fractionation of ijolitic types may furnish examples. Iloweverat compositions more iron-rich then approximately CaaaMgaoFezo,continued fractionation of alkali olivine-basalt clinopyroxenes may resultin progressive enrichment in the acmite molecule (Yagi, 1953), a trendoften observed on a small scale in basic alkaline rocks as the zonalseries, salite---+aegirine-augite---+aegirine, especially where the normalclinopyroxene (diopside or salite) abuts against a micro-crystalline alka-line residuum.

CLINOPYROXENES OF ALKALI OLIVINLBASALT MAGMA

Tne CoxrnesrrNc BonevroR or,rup pynoxBNns

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The pyroxene assemblage in representatives of alkali orivine-basartmagma differs markedly from its equivalent in tholeiitic rocks in twormportant aspects, namely in the absence of orthopyroxene and pigeonite,and in the much more limited degree of clinopyroxene iron enrichment(Fig. 4), particularly in the replacement of Ca2+ by Fez+.

(i) In tholeiitic rocks, the frequently described mantling of olivine byorthopyroxene provides petrological testimony to the reaction relationbetween early-formed olivine and liquid (Bowen and Schairer, 1935).

Mg Fc

Frc. 4. Clinopyroxene trends in basaltic magmas.

A - At Alkali olivine-basalt clinopyroxene trend.Clinopyroxene course of crystallization in tholeiitic rocks (Poldervaart andHess . 1951 ) .

- - -++ Clinopyroxene trend of the tholeiitic Skaergaard intrusion (Muir, 195 1).

Although these authors consider the relation does not hold for certain"iron-rich basalts" which yield undersaturated differentiates, they regardthe phenomenon as independent of the parental magma.

In the magma types under discussion, the presence or absence oforthopyroxene depends on the chemistry of the residual liquids formedduring crystallization and subsequent fractionation. rn many basicalkaline rocks (essexites, theralites, teschenites, Iugarites), a microcrystal-line mesostasis, often analcimic, consisting of alkali feldspar microlites,clinopyroxerie and dusty iron ore, is frequently present, even in the earlystages of magmatic evolution (F-lett, 1910, p. 298; Browne , 1927, p. 375;Lehmann, 1930, Figs 3 and 4; Walker, 1930, p. 370;1936, p. 2g0; Benson,1942, p. 166). such a mesostasis is widespread in the teschenites from alllevels of the Black Jack sill (Table 3, Analysis 1). under suitabre condi-

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736 J. F. G. WILKINSON

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Microcrystalline mesostasis, occurring as vein in teschenite, Black Jacksill, Gunnedah, New South Wales. Anal. J.F.G'W.Analcine-syenite, forming felsic part of 80 foot sill, Utah. Anal. J. G.

Fairchild (Gilluly, 1927 , p. 2OS).

Analcime-syenite (1501), Morotu District, Sakhalin. Arral" K" Yagi(Yagi, 1953, Table 16).Segregation in analcime-syenite, Eilean Mhuire, Shiant Isles. Anal. E- G.

Radley (Walker, 1930, p. 387).

Syenitic segregation (potash alkali-syenite) in upper teschenite of Lugar

Sill boring, Mortonmuir, Cronberry, Ayrshire. Anal. G' A. Sergeant (Tyr-

rell, 1948, p. 165).

CLINOPYROXENES OF ALKALI OLIVINE-BASALT MAGMA l J l

tions, this type of residuum may crystallize as internal cross-cuttinganalcime-syenite (Table 3). The chemistry of this liquid prohibits anyreaction relation involving olivine. fn strong contrast the oversaturatednature of late-stage tholeiitic liquids is indicated by two analyses oftholeiit ic residua (Walker, 1935; Vincent, 1950), containing 31-36 percent normative quartz. Fractionation of alkali olivine-basalt magma doeshowever occasionally yield syenitic products with normative quartz(Yagi, 1953, Table 16) and in certain instances, modal quartz (Raggartand Whitworth, 1932, p. 223). At just what stage this divergence to yieldan acid alkali (pantelleritic) end product (Tilley, 1950, p. 43) occurs isnot precisely known, but it is not believed to materially afiect the pyrox-ene assemblage in relation to the non-appearance of orthopyroxene sinceolivine generally disappears before this stage is reached, and the ferro-magnesian assemblage is dominated by sodic clinopyroxenes and alkalineamphiboles.

In both magma types, the nature of the olivine (where present) reflectsthe presence or non-appearance of orthopyroxene. The olivines in tho-leiitic rocks are unzoned or rarely show slight zoning, indicating thereadiness with rvhich this mineral reacts with the magma, compared withthe associated clinopyroxene or plagioclase which may be strongly zoned.Widespread normal zoning is a feature of the olivines of alkali olivine-basalt derivatives (Tomkeieff, 1939; Benson, 7942, p. 164; Johnston,1953). The nature of the strongly fayalitic outer zones is analogous withthe compositionai relationships between the co-existing olivine and ortho-pyroxene in tholeiitic rocks, where the orthopyroxene may be the mostiron-rich ferromagnesian sil icate (Muir, 1951, p. 711; Ramberg and deVore, 1951, Fig. 2). However as progressive iron enrichment occurs, theolivine may be the most iron-rich mineral (Muir, 1954, p. 382; Rambergand de Yore, op cil.), a relationship in harmony with the experimentaldata of Bowen and Schairer (1935).

(ii) Edwards (1942), Walker and Poldervaart (1949) and Muir (1951)have correlated the observed crystallization trend in tholeiitic clino-pyroxenes with atomic structure. The higher titania content of alkaliolivine-basalt clinopyroxenes cannot be the reason for their relativelyinert behavior, since during fractionation, tholeiitic titaniferous clino-pyroxenes behave similarly to their titania-poor equivalents.

The following explanation of the two difiering clinopyroxene trends isbased mainly on petrological and mineralogical data, although physico-chemical and crystal chemical reasoning may well be applied.

In tholeiitic magma, even in its early undersaturated stages, the reac-tion relation between olivine and liquid results in the continuous forma-tion of "hypersthene molecules." While these are potential ortho-

738 J. F. G, WILKINSON

pyroxene or pigeonite, a variable percentage combines with highly diop-sidic clinopyroxene (the "primitive" clinopyroxene of both magma types)to give augite, so that the modal pyroxene of tholeiites is greater thanthat of alkali olivine-basalts for the major part of their differentiationhistories, even though in both cases a similar amount of lime is availablefor pyroxene formation.

Relevant data on medium grained differentiated intrusions (wheremore reliance can be placed on micrometric data) is set out in Table 4.These intrusions possess compositions representative of their parentalmagma types (see Walker, 1940, Table 3; Walker and Poldervaart,7949,Table 24; Nockolds and Allen, 1954, Table 17). Pegmatit ic facies or rockswith appreciable amphibole or mica are not included in the tabulation.The tholeiitic types possess a higher modal pyroxene content, averaging30-45 per cent (see Turner and Verhoogen, 1951, p. 183), compared withalkali olivine-basalt rocks where the modal pyroxene varies from 18-24per cent (see also Walker, 1934, Table 1). There is no notable divergencein the CaO content of both series, each carrying 50-60 per cent of modalplagioclase of similar basicity.

The principal pyroxene relationships visualized are as follows:

olivine* SiOz+orthopyroxeneolthopyroxene f diopside+augite

olivine* SiOz*diopside+augite (tholeiite)fn silica-deficient basic rocks-SiOg+olivine*diopside (alkali olivine-basalt)

Thus in the early magmatic stages, the tholeiitic clinopyroxene (augite)shows replacement of Ca2+ by Fez+, compared with the diopsidic type inalkali olivine-basalt at an equivalent stage. fn its subsequent differentia-tion, tholeiitic magma may involve nearly double the amount of clino-pyroxene in alkali magma.

While fractionation results in enrichment in iron relative to magnesium(and in certain instances, absolute iron enrichment), the response of theprincipal ferromagnesian mineral series to this enrichment varies in eachmagma type. fn tholeiitic rocks, most of the Fe/Mg enrichment occurs inthe olivine (if it still persists) , orthopyroxene or pigeonite; the clinopyrox-ene lags behind these minerais. While falling temperature favors clino-pyroxene iron enrichment, the additional and more difficult replacementof Ca2+ by Fer+ is induced by the limited amount of available magmaticlime. Exceptions to the relatively high pyroxene content of tholeiitesmay be noted. The Skaergaard marginal gabbros, considered to representthe parental magma, contain 23 per cent pyroxene, a figure which in-creases to 41 per cent in the middle gabbros (Wager and Deer, 1939,p. 95 and p. 144). With this increase in pyroxene, which now reveals ap-preciable replacement of Ca2+ by Fe2+ (Fig. 4), the modal olivine de-

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creases. Tholeiites may retain their initial moderately low pyroxeneeontent (when no great degree of iron enrichment would be anticipated),if reaction is prevented by the residual liquids difiusing through thecrystal interstices. However olivine frequently disappears at a rela-tively early stage in tholeiitic evolution, and indeed this mineral may beantipathetic with nrthopyroxene (Walker and poldervaart, 1,g4la, p.143). ln the later stages of pyroxene fractionation when re-entry of Caz+occurs (Edwards, 1942), and the replacement is dominantly Fe2+ forMg2+, the additional withdrawal of lime is reflected in a decrease in modalpyroxene, despite albite enrichment in the plagioclase e. g. d.ecrease inmodal pyroxene occurs at this stage in the Palisades, Hangnest and Dills-burg sills. Possibly the two-pyroxene boundary is reached about thisstage and the main Fe/Mg enrichment now occurs in the clinopyroxene.

In alkali olivine-basalt magma, the ferromagnesian assemblage con-sists of diopsidic clinopyroxene and olivine. rn the initial stages, the earlyseparation of forsteritic olivine greatly increases the normative wo con-tent of the magma (Kennedy, 1933, p. 252).In the absence of possiblelocalized crystal accumulation, the failure of the reaction relation resultsin a moderately low near-constant amount of clinopyroxene. With de-creasing temperature, the bulk of Fe/Mg enrichment is borne by an ever-diminishing quantity of olivine, which becomes more fayaritic (and inmany instances, more strongly zoned). Exsolution lamellae are uncom-mon in alkali olivine-basalt clinopyroxenes. The absence of the reactionrelation limits the amount of En-Fs in solid solution (cf. poldervaart andHess, 1951, p. a88). In these clinopyroxenes, the lack of alteration in thepresence of residual magmatic solutions (except in dykes of ,,white trap',),(Flett, 1931, p. 47 ; 1932, p. 145; Browne, 1924, p. 244; Raggatt and Whit-worth, 1932, p. 217; Campbell, Day and Stenhouse, 1932, p. 352) maywell be consequent on the greater structural stability resulting from morelimited replacement of ca2+ by Fe2+, compared with their tholeiitic equiv-alents (see also Harry, 1954, p. 85).

AcxlrowrnoGMENTS

The bulk of this study was carried out in the Department of Minerar-ogy and Petrology, Cambridge, during the writer's tenure of a RoyalDutch/Shell commonwealth Post-Graduate Scholarship. This financialassistance is gratefully acknowledged.

The writer expresses his thanks to Professor C. E. Tilley and Dr. S. R.Nockolds for helpful discussions on pyroxene problems, and to professorH. H. Hess and Dr. G. M. Brown for critically reading the manuscript.Dr. S. O. Agrell gave permission to quote the analysis of the Camphousetitaniferous clinopyroxene, carried out by Mr. J. H. Scoon.

CLINOPI.ROXENES OF ALKALI OLIVINE-BASAL'I' MAGMA 747

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Monu,script receiveil Dec. 23, 1955.