Actinolite
| Actinolite | |
|---|---|
Actinolite crystal from Portugal
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| General | |
| Category | Inosilicates |
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Formula (repeating unit) |
Ca2(Mg4.5-2.5Fe2+0.5-2.5) Si8O22(OH)2 |
| Strunz classification | 9.DE.10 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/m |
| Identification | |
| Color | pale to dark green, yellowish green and black. White or grey when in asbestos |
| Crystal habit | bladed, fibrous, radial |
| Twinning | simple or lamellar |
| Cleavage | perfect along {110}. |
| Fracture | uneven |
| Mohs scale hardness | 5–6 |
| Luster | vitreous to dull |
| Streak | white |
| Diaphaneity | translucent to transparent |
| Specific gravity | 3.00 (+0.10, -0.05) |
| Polish luster | vitreous |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.613–1.628 nβ = 1.627–1.644 nγ = 1.638–1.655 |
| Birefringence | 0.0250-0.0270 |
| Pleochroism | moderate, yellow to dark green (in stones that are transparent) |
| 2V angle | 78–82° (calc), 84–73° (meas) |
| Dispersion | r < v |
| Ultraviolet fluorescence | inert |
| Absorption spectra | faint line at 503 nm |
| References | |
Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2.
The name actinolite is derived from the Greek word aktis (ἀκτίς), meaning "beam" or "ray", because of the mineral's fibrous nature. (This word is also the origin of the name of the chemical element actinium.)
Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, and iron-rich ferro-actinolite, ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22(OH)2. Mg and Fe ions can be freely exchanged in the crystal structure. Like tremolite, asbestiform actinolite is regulated as asbestos.
Actinolite is commonly found in metamorphic rocks, such as contact aureoles surrounding cooled intrusive igneous rocks. It also occurs as a product of metamorphism of magnesium-rich limestones.
The old mineral name uralite is at times applied to an alteration product of primary pyroxene by a mixture composed largely of actinolite. The metamorphosed gabbro or diabase rock bodies, referred to as epidiorite, contain a considerable amount of this uralitic alteration.
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Wikipedia