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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