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

Metamorphic environments


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).

Back to Minerals