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The mica group is composed of over 30 phyllosilicate (sheet silicate) minerals. All crystallise in the monoclinic
system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition.
Micas are translucent to opaque with a distinct vitreous or pearly luster, and different mica minerals
display colours ranging from white to green or red to black. Deposits of mica tend to have a flaky or platy
appearance
(Wiki).
The mica group includes muscovite and
biotite.
"Lepidomelane" is a general term for dark iron-rich micas that have not been sufficiently well analysed to
determine the species. Generally a biotite group mineral (Mindat).
Mica is a common constituent of most pegmatites,
granite,
diorite,
gabbro,
andesite and
gneiss.
It also may be found in
quartzolite,
kimberlite,
clay,
skarn,
amphibolite and
eclogite.
It is never found in the granulite facies rocks.
Localities
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Topaz is found in quartz-mica veins
in the granite
Tremolite is found abundantly in the skarn rocks, in intimate
association with actinolite; other associated minerals include
magnetite, garnet,
chlorite, mica and
fluorite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At Willow Springs, Pinal county, Arizona, USA, mica pseudomorphs after
schorl
occur in quartz (R&M 94.2.165-166).
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