ferroactinolite

Ferroactinolite is the iron-rich member of the actinolite subgroup of amphibole minerals. It is part of the calcic amphibole family and represents the end-member of the tremolite–actinolite–ferroactinolite series in which iron substitutes for magnesium on the M sites. The idealized formula is Ca2Fe5Si8O22(OH)2, though natural samples often contain small amounts of Mg and other substituents.

Crystallography and appearance: Ferroactinolite crystallizes in the monoclinic system and typically forms prismatic crystals. It has two cleavages at about 56 and 124 degrees, a Mohs hardness of about 5–6, and a specific gravity around 3.3–3.5. Color ranges from pale to dark green; higher iron content can give deeper greens or brownish hues. It commonly exhibits pleochroism like other amphiboles.

Geologic occurrence: Ferroactinolite forms in metamorphic rocks under conditions that favor high Fe/Mg ratios, particularly in amphibolite- to granulite-facies environments. It is commonly associated with other calcic amphiboles such as hornblende, tremolite, and garnet, and may occur as a replacement product in iron-rich rocks or in metasomatic settings.

Significance: The mineral helps indicate the iron richness and pressure–temperature history of metamorphic rocks. Because it is part of a continuous solid-solution series with tremolite and actinolite, ferroactinolite composition is typically determined by precise analytical methods, such as electron microprobe analysis.

Notes: In some literature, ferroactinolite is described as the Fe-rich end-member of actinolite; in others, it is treated as a distinct end-member of the tremolite–actinolite series. Ferroactinolite is relatively rare compared with other amphiboles.