Actinolite
Image: Didier Descouens (CC BY-SA 4.0)
Mineral NameActinolite
Chemical FormulaCa2(Mg,Fe)5Si8O22(OH)2
Mohs Hardness5.5
Crystal Systemmonoclinic
Lustervitreous
Streak Colorwhite
Cleavageperfect on {110} (two directions at ~56° and ~124°)
Fractureuneven to subconchoidal
Specific Gravity3
Colorsdark green, black, grayish-green
Transparencytranslucent
Type LocalityMontijos Quarry, Monte Redondo, Leiria, Leiria District, Portugal
Mineral GroupSilicates
Usescollector, ornamental stone (nephrite jade)
Associated Mineralscalcite
Rarityuncommon
Description
This specimen features a dense, radiating cluster of dark greenish-black acicular (needle-like) actinolite crystals, prominently displayed on a matrix that is largely covered by a granular coating of tiny, yellowish-brown calcite crystals.
Geological Context
Actinolite is a common metamorphic mineral, typically forming in greenschist facies metamorphic rocks from the alteration of mafic igneous rocks or magnesium-rich sedimentary rocks. It is indicative of low-to-medium grade regional metamorphism.
