Actinolite

Formula:  Ca₂(Mg, Fe)₅Si₈O₂₂(OH)₂

Origin of name:  from Greek for ray, in allusion to it's common radiating fibrous habit

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  common parallel {100}, simple or lamellar;  also parallel {001}, lamellar

Unit cell:  a = 9.88 Å, b = 18.15 Å, c = 5.29 Å, b = 104.7°

Colour:  Light green to blackish green

Diaphaneity: Transparent to translucent

Luster:  Vitreous or dull

Habit:  Crystals long bladed; usually in fibrous to thin columnar aggregates, often radiated;  massive

Hardness:  5 to 6

Specific gravity:  3 to 3.44

Cleavage:  2; (110) good, (100) parting

Tenacity:  Brittle

Streak:  White

Synonyms/varieties:  jade (compact gem var.), nephrite (compact var.), byssolite (fibrous var.), smaragdite/uralite (replacing pyroxene), strahlstein, zillerite, zillerthite.  Intermediate member of the series tremolite - ferro-actinolite

Comments: Formed under low-grade regional or contact metamorphism of magnesium carbonate, mafic or ultramafic rocks; also in glaucophane-bearing blueschists