Actinolite
Actinolite is a member of a series that contains varying amounts of iron and magnesium. Tremolite is the Mg end, and ferroactinolite the Fe end, with actinolite in the middle. Actinolites with more than 50% Fe are very rare. Catseye actinolite exists (S.G. 3.0, R.I. 1.63); when chatoyant material is cut, it exhibits a fine eye. Actinolite is easy to cleave and hard to cut and would make a poor jewelry stone. Actinolite is the chief constituent of nephrite (jade). Smaragdite is a chrome-rich tremolite from Tanzania.
Actinolite Value
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Actinolite Information
| Data | Value |
| Name | Actinolite |
| Varieties | Smaragdite |
| Crystallography | Monoclinic; bladed crystals, usually elongated; fibrous, columnar aggregates. Also massive, granular, Often twinned. |
| Colors | Pale to dark green, blackish green, black. |
| Luster | Vitreous, sometimes dull. |
| Hardness | 5.5 |
| Density | Usually 3.05; Tanzania: 3.03-3.07; Max.: 3.44. |
| Birefringence | 0.022-0.026. |
| Cleavage | 2 directions good, often fibrous nature. Brittle. Compact variety tough. |
| Stone Sizes | Actinolite is rarely facetable and usually in small fragments. Material from Chester, Vermont, could provide stones to about 10 carats. DG: 2.06 (greenish, step cut, Africa) |
| Luminescence | None (due to presence or Fe). |
| Spectral | Faint line at 5030. |
| Formula | Ca2(Mg, Fe)5Si8O22(OH)2 |
| Pleochroism | Yellow to dark green. Material transparent to nearly opaque. α: pale yellow/ yellowish-green. β: pale yellow-green/green. γ: pale green/ deep greenish blue. |
Comments: Actinolite is a member of a series that contains varying amounts of iron and magnesium. Tremolite is the Mg end, and ferroactinolite the Fe end, with actinolite in the middle. Actinolites with more than 50% Fe are very rare. Catseye actinolite exists (S.G. 3.0, R.I. 1.63); when chatoyant material is cut, it exhibits a fine eye. Actinolite is easy to cleave and hard to cut and would make a poor jewelry stone. Actinolite is the chief constituent of nephrite (jade). Smaragdite is a chrome-rich tremolite from Tanzania.
Name: Greek aktis, meaning ray, due to the fibrous nature.
Optics:
α = 1.619-1.622;
β = 1.632-1.634;
γ = 1.642-1.644 (Tanzanian material).
Biaxial (-); 2V = 78°
Occurrence: Contact metamorphic limestones and dolomites; magnesium-rich limestones and ultrabasic rocks; regionally metamorphosed rocks.
| Locality | Pleochroic Colors | α | β | γ | Birefringence | S.G. | Comments |
| Fowler, New York | red-violet | 1.602 | – | 1.630 | 0.028 | 3.03 | Hexagonite |
| Kenya | bright green | 1.602 | 1.613 | 1.628 | 0.026 | 2.99 | Tremolite |
| 1.607 | 1.618 | 1.632 | 0.025 | 3.01 | |||
| Tanzania (Lelatema) | yellow green/green | 1.608 | 1.616 | 1.631 | 0.023 | 3.01 | Tremolite |
| Tanzania | emerald green | 1.611 | 1.623 | 1.639 | 0.028 | 3.30 | Tremolite + Cr |
| Tanzania (Merelani) | yellow-green/emerald green | 1.608 | 1.618 | 1.63 | 0.022 | Smaragdite | |
| Taiwan | yellow-green/green tones, | 1.607- | – | 1.630- | 0.023- | 3.01 | Tremolite |
| also brownish | 1.609 | 1.633 | 0.024 | ||||
| 1.615- | – | 1.631- | 0.014- | 3.01 | Catseye tremolite | ||
| 1.619 | 1.633 | 0.016 | |||||
| Uganda | yellow-green/green | 1.619- | 1.632- | 1.642- | 0.021- | 3.04- | Tremolite-Actinolite |
| 1.622 | 1.634 | 1.644 | 0.024 | 3.07 | |||
| green/brown | 1.633 | – | 1.653 | 0.020 | 3.15 | Actinolite |
Chemical composition vs. optics and density of common hornblendes.
Adapted from W.A. Deere, R.A. Howie, and J. Zussman, 1962, The Rock Forming Minerals, vol.2 (New York: Wiley), p.296.
Chester, Vermont.
Madagascar: small, dark green crystals. Many of these are clean and suitable for faceting.
Tanzania: transparent crystals. (See also nephrite.)
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