Orthosilicate mineral, hydrated calcium iron aluminum silicate
It is found in metamorphic and igneous
rocks as monoclinic system prismatic crystals and fibrous or granular masses;
it is typically pistachio green, glassy, and brittle. Certain large crystals,
7.5–25cm (3–10in), are collector's items.
Hardness 6–7; relative density 3.4.
AND PLANETARY SCIENCE