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Dominant species
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Illite
Chemical
Formula
(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]
Species
Silicates
Crystal
System
Monoclinic
Mohs
Scale
1-2
Specific
Gravity
2.6-2.9
Color
Grey-white to silvery-white, greenish-gray
Streak
white
Luster
Pearly
Refractive
Index
n = 1.535 - 1.570 n = 1.555 - 1.600 n = 1.565 - 1.605
Diaphaneity
Translucent
Cleavage
PerfectPerfect on .
Fracture
Micaceous
Crystal Habit:Micaceous aggregates
Geological Setting:Found in a wide variety of environments, the materials of this series form by either weathering or hydrothermal alteration of muscovite-phengite; but some is authigenic or could be derived from alteration of K-feldspars or recrystallization of smectites. Common in sediments, clays, marls, shales and some slates. Interstratified illite-smectite converts to illite at depths.
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