The Chlorite Group

a)   

·       The chlorite group of minerals are in many respects closely related to the micas. They crystallize in the monoclinic system. The mica like basal cleavage is prominent in crystallized forms. Chromium enters in some forms. Manganese replaces ferrous iron in a few cases Calcium and alkalies. Characteristic of micas are absent.

·       Chlorites are a mixture of magnesium and iron- aluminum silicates, with water. They often form as alteration of rocks rich in pyroxenes, amphiboles and biotite.

       i.          Classification of chlorites

Penninite, clinochlore and prochlorite belong to orthochlorites. The leptoechlorites are complicated in composition, have a higher iron content and less water, they have higher refractive indices than the orthochlorites. The rock forming chlorites are recognized in thin sections by their characteristic appearance in thin leaves, scales or fibers, sometimes aggregated into spherulites by their greenish colour, pleochroism. Extinction parallel to the cleavage, low interference colours. The composition of the orthochlorites is given below;

·       Penrinite = H4Mg3Si2O9 > H4Mg2A12SiO9

·       Clinochlore =  lH4Mg3Si2O9. lH4Mg2A12SiO9

·       Prochlorite = H4Mg3Si2O9 < H4Mg2A12SiO9

     ii.          Physical Properties:

·       The chlorites are characterized by a green colour.

·       Hardness = 2 to 2.5.

·       Specific Gravity = 2.8

·       Lustre = pearly

·       Streak = greenish or white.

·       The crystals have a perfect basal cleavage and split into thin sheets as in micas.

   iii.          Optical Properties:

·       Green in colour and pleochroic

·       RI higher than Canada balsam

·       Birefringence is low (0.001 to 0.011)

·       1st order interference colour

·       Parallel extinction