Chlorite

Hand Specimen identification: Deep green color, and micaceous habit and cleavage are usually adequate to identify chlorite. The many individual species and varieties are difficult to tell apart. Chlorite is sometimes confused with talc.

Chemical Composition : (Mg,Fe,Al)6(Si,Al)4O10(OH)8

Hardness : 2 to 21 2

Specific Gravity3

Cleavage/Fracture : perfect basal (001)/flexible

Luster/Transparency : vitreous/transparentto translucent

Color : green, variable

Streak : white, green

Optical Properties : Chlorite is generally green and sometimes green-brown in thin section. It usually exhibits yellow-green-brown pleochroism and has moderate to moderately igh re-lief. Birefringence is low. Interference colors normally are between anomalous blue, brown, or purple and first-order yellow. Biaxial 1- 2, a = 1.56 to 1.60,b = 1.57 to 1.61, g = 1.58 to 1.61, d = 0.006 to 0.020, 2V = 0° to 40°.

Crystallography : Monoclinic, a = 5.37, b = 9.30, c = 14.25, b = 97.4°,Z = 2; space group C2/m; point group 2/m.

Habit : Chlorite has habits similar to the other micas: Foliated books, scaly aggregates, and individual flakes in a quartz-feldspar matrix are common. Rare pseudohexagonal crystals are known.

Structure and composition : The structure of chlorite consists of stacked talc,Mg3Si4O101OH22 and brucite, Mg1OH22, layers. The stacking order is variable, leading to a great deal of va-iety. In both kinds of layers, Al replaces some of the Mg, and in most chlorites Fe is present as well. A gen-eral formula, 1Mg,Fe,Al261Si,Al24O101OH28, does not describe all of the compositional variability.

Occurrence and associations : Chlorite is a common mineral in low- to intermediate-grade metamorphic rocks, diagnostic of the greenschist facies. It is also a common secondary mineral after bi-otite, muscovite, and other mafic silicates in igneous and metamorphic rocks, and is sometimes found in sediments. Many greenish rocks owe their color to the presence of chlorite. Associated minerals include quartz and feldspars, epidote, muscovite, actinolite, al-bite, and a number of ferromagnesian silicates.

Varieties : Names given to some idealized chlorite compositionsare clinochlore, 1Mg5Al21AlSi3O1021OH28; chamosite,1Fe5Al21AlSi3O1021OH28; nimite,1Ni5Al21AlSi3O102 1OH28; and pennanite, 1Mn,Al261Si,Al24O101OH28.

Related Minerals : Chlorite has many varieties and polymorphs. Other sim- ilar minerals include cookeite, LiAl 1AlSi 2O 1OH2 ; 1 2 1 21 24 3 10 8 sudoite, Mg2 Al,Fe 3AlSi3O10 OH 8; and a numberof other hydrated aluminosilicates.

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