Seminar On

Chlorite group of minerals

Contents

Introduction

Chlorite structure

Physical properties

Members of chlorite group

Distinguishing from other minerals

Occurence

Economic importance

Conclusion

INTRODUCTION The chlorites are a group of phyllosilicate minerals.

Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.

Contd… In addition, zinc, lithium, and calcium species are known. The

great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and deeply buried sediments. The most common species in the chlorite group are clinochlore and chamosite.

CHL0RITE STRUCTURE The typical general formula is:

(Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)

6. This formula emphasises the structure of the group.

Chlorites have a 2:1 sandwich structure (2:1 sandwich layer = tetrahedral-octahedral-tetrahedral = t-o-t...), this is often referred to as a talc layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space (the space between each 2:1 sandwich filled by a cation) is composed of (Mg2+, Fe3+)(OH)6. This (Mg2+, Fe3+)(OH)6 unit is more commonly referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite (Mg(OH)2).

colour greenish black to white

Luster vitreous to pearly

Diaphaneity transparent to translucent

Cleavage Basal(001), perfect

Mohs Hardness 2 - 2.5

Specific Gravity 2.6 - 3.3

DistinguishingCharacteristics

Color, various shades of green, yellow, white, pink, rose-red

Streak greenish to colourless

Crystal System Monoclinic

Optical properties

Refractive indices increase with increasing Fe and Al contents.

Fe-rich chlorites are biaxial negative. Mg-rich chlorites are negative.

The sign of elongation of chlorites is opposite to the optic sign

and is much easier to obtain, especially in fine-grained examples.

Pleochroism strengthens with Fe content.

Mn chlorite – orange-brown; Nickel – yellow/green; Chromium –pinks/violet.

For the most part, mildly pleochroic with 1st order

grey birefringence.

Anomalous colours include browns (Mg-rich) and

violet-blues

MEMBERS OF CHLORITE GROUP Baileychlore

Borocookeite

Chamosite

Clinochlore

Cookeite

Corundophilite

Franklinfurnaceite Nimite

Orthochamosite

Pennantite

Sudoite

Distinguishing From Other Minerals

Chlorite is so soft that it can be scratched by a finger nail. The powder generated by scratching is green. It feels oily when rubbed between the fingers. The plates are flexible, but not elastic like mica.

Talc is much softer and feels soapy between fingers. The powder generated by scratching is white.

Mica plates are elastic whereas chlorite plates are flexible without bending back

OCCURRENCE Chlorite is commonly found in igneous rocks as an

alteration product of mafic minerals suchas pyroxene, amphibole, and biotite. In thisenvironment chlorite may be a retrogrademetamorphic alteration mineral of existingferromagnesian minerals, or it may be present asa metasomatism product via addition of Fe, Mg, orother compounds into the rock mass. Chlorite is acommon mineral associatedwith hydrothermal ore deposits and commonly occurswith epidote, sericite, adularia and sulfidemineralswith talc.

Contd…

. Chlorite is also a common metamorphic mineral,usually indicative of low-grade metamorphism. It isthe diagnostic species of lower greenschist facies. Itoccurs in the quartz, albite, sericite, chlorite, garnetassemblage of pelitic schist. Within ultramafic rocks,metamorphism can also produce predominantlyclinochlore chlorite in association

PROCHLORITE

(Ripidolite)

Composition-H4Mg3Si2O9

Crystal System - Monoclinic

Habit –massive foliated or granular

Hardness 1-2

Specific gravity-2.78-2.98

Translucent to opaque

Colour-green, olivine green blakish green

Pleochroism-distinct

Occurence-clorite schist other metamophic rock,serpentine

Common through out alps,switserland,rauris in salsberg.

CLINOCHLORE

Composition –H8 Mg5 Al2 Si3 O18

System- monoclinic

Cleavege-perfect

Hardness 2-2.5

Sp gravity 2.65-2.78

Colour -pale green to yellowish and white

Diaphinity –transparent to transulasent

Occurance with chloritic or schists and with serpentine. found in mussa alps in alla valley zermat in valis,foster iron mine newyork

CLINOCHLORE

PENNINITEApparantly rhombohedral in form butsrctly suedo rhombohedral

and monoclinic shows highly prefect cleavege

Hardness -2-2.5

Specific gravity -2.6-2.85

Lusture –pearly

Clour –emarald to olivine green also violet pink transparent to subtraslusent

Distinct pleochroism

Occures in valais switserland with serpentine, from the alla valley in pidmont with clinoclore also with clorite at various point in north carolina.

PENNINITE

CHAMOSITE

Composition-15(Fe,Mg)O.5Al2O3.11SiO2.16H2O

Habit-Compact or oolitic

Hardness-3

Specific gravity-3-3.4

Colour-greenish grey to black

Occurence-with various ore deposits,with sphallerite,galena,pyrite,siderite etc

Forms thick bed of limited extent in limestone at rhone valley,valais.With iron carbonate at scheimeilfield

ECONOMIC IMPORTANCE Used to extract chlorine

Used as gemstone

CONCLUSIONSMinerals of chlorite group are mainly products of low

temperatures and mainly of hydrothermal or low

temperature genesis. They also occur as products of

transformation of ferromagnesian minerals-

biotite,amphibole, pyroxene and others in amphibolite

rocks. Analyzing different varieties of amphibolite rocks, it

is represented that genesis of chlorite in them is often

followed with complex processes of mineral genesis,

where beside chlorite, created were other secondary

minerals like prenite, epidotite, clinocoisite, serpentine,

spinel,especially zeolitic. Also, occurrence of chlorite

together with appropriate minerals is important because

on th basis of this mineral association we can determine affiliation to metamorphic facies.

References

Rock forming minerals- ZUSSMAN

DANA’s text book of mineralogy-W.E. FORD

www.wikipedia.com/chloritegroup.htm

www.geology.com/chloriteminerals

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