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Formula: Fe2+2Fe2+2Si8O22(OH)2
Inosilicate (chain silicate),
magnesium-iron-manganese amphibole subgroup,
amphibole supergroup,
polymorph of grunerite, forms
a series with anthophyllite
Hardness: 5½ to 6
Streak: White to greyish white
Colour: Clove-brown to dark brown, grey to white, pale green
Environments
Localities
At the Kitakami Mountains, Iwate Prefecture, Japan, fn the southern Kitakami mountainland, northeastern Japan, the
country rock is hornblende-biotite
granodiorite. Its contact aureole, 3 or 4 km wide, is divided into
the following four progressive metamorphic zones:
Chlorite-sericite zone:
chlorite, sericite,
chloritoid and chiastolite.
Andalusite-biotite zone:
andalusite, biotite and
muscovite.
Cordierite-almandine zone:
cordierite, almandine,
anthophyllite, andalusite,
biotite, muscovite,
spinel and corundum.
Sillimanite zone: sillimanite,
cordierite, almandine,
biotite, muscovite,
K-feldspar, spinel and
corundum.
Aluminian ferro-anthophyllite is found only in hornfels derived
from slate and tuffaceous
slate and belonging to the
cordierite-almandine zone.
The host rock is a deep green hornfels, compact and massive, with
porphyroblastic crystals of the aluminian ferro-anthophyllite and
chlorite. The aluminian ferro-anthophyllite also occurs as a
constituent of the matrix. The porphyroblasts are in radiating forms up to 1.3 cm in length, and usually free from dusty
inclusions of magnetite and
graphite which are abundant in the crystals of the matrix. The matrix of the
hornfels is composed of fine grained crystals of
andalusite, cordierite,
aluminian ferro-anthophyllite, garnet,
biotite, muscovite,
spinel, plagioclase,
quartz, magnetite and
graphite. Except garnet which
sometimes reaches 1.4 mm in size, these minerals are too fine-grained to be distinguished from each other with the
unaided eye
(AM 42.506-520).
The type locality is the Tamarack and Custer Mine, Placer Center Mining District, Shoshone county, Idaho, USA, which
is hosted in
quartzite
(Mindat).
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