Actinolite

What is actinolite? The IMA's magnesium rule, worked

Actinolite is a calcic amphibole: two calcium, five magnesium or ferrous iron, eight silicon, twenty-two oxygen and two hydroxyl. The Handbook writes Ca2(Mg,Fe2+)5Si8O22(OH)2 and says it forms a series with tremolite and ferro-actinolite. The September 2026 IMA list writes ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2, where the box is the vacant A site. It records actinolite as Rd, redefined, 2012 s.p. Its first reference is Elements of Mineralogy, 2nd ed., vol. 1. Elmsly, London (1794).

The boundaries are in the three IMA formulas. Tremolite is Mg 5.0 to 4.5, actinolite 4.5 to 2.5 and ferro-actinolite 2.5 to 0.0, out of five. Divided by five, that is Mg/(Mg+Fe2+) of 0.90 or more for tremolite, 0.90 to 0.50 for actinolite and below 0.50 for ferro-actinolite. The tremolite sheet’s own group line gives the same 0.90.

Worked on the Handbook’s analysis, electron microprobe, southeast Anglesey, Wales: MgO 16.21% and FeO 11.07%. Divided by the oxide masses (40.30 and 71.84 on standard atomic weights), that is 0.4022 Mg to 0.1541 Fe, and Mg/(Mg+Fe) = 0.723 if all the iron is ferrous. The sheet’s recalculated formula, Mg3.41 Fe2+1.26 with Fe3+ 0.05, gives 0.730. Both are inside 0.90 to 0.50, so this is actinolite. Colour alone would not have settled it; the number does.

Actinolite or tremolite, and amphibole or pyroxene

Against tremolite, look at colour first, then density. Tremolite’s sheet gives white, grey, lavender to pink, density 2.99 to 3.03. Actinolite’s gives bright green to greyish green, 3.03 to 3.24. Hardness is 5 to 6 on both and cannot separate them. A 10 cm³ piece weighs 30.3 to 32.4 g as actinolite and at most 30.3 g as tremolite. The honest boundary, though, is the magnesium number above, and pale green material near 0.90 needs an analysis.

The 56/124 corner: amphibole or pyroxene. Method: the Mineral Group field of all 176 sheets was searched for amphibole. Two match, actinolite and tremolite, and both give prismatic cleavage intersecting at 56° and 124°. The diopside sheet, a pyroxene with an overlapping green, gives about 87°. On a clean broken end of a prism, a lozenge with sharp and blunt corners points to an amphibole; a near-square points to a pyroxene. See cleavage or fracture: how to tell.

Against epidote, a green prism on actinolite’s association line: epidote is harder (6 to 7) and denser (3.38 to 3.49), with one perfect and one imperfect cleavage rather than two equal ones at 56° and 124°, and a pistachio-green colour. See epidote.

Is nephrite jade actinolite?

Yes, by the Handbook. Its tenacity line reads brittle, tough in fibrous aggregates (“nephrite jade”). The habit line runs from bladed crystals to 15 cm, through columnar, kinked or bent, to radiating fibrous to asbestiform, granular and massive. The name records the fibres: it is from the Greek for ray, in allusion to the mineral’s common radiating fibrous habit.

Where the sheet records nephrite: south and east of Lander, Fremont Co., Wyoming; north from Cape San Martin, Monterey Co., California; around Jade Mountain, near the Kobuk River, Alaska; along the Fraser River, British Columbia; around Mt. Cook, South Island, New Zealand; and fine material from the Kunlun Mountains, Sinkiang Uighur Autonomous Region, China. The sheet names no other jade mineral, and this page does not either.

How unusual these words are. A case-insensitive search of all 176 sheets finds nephrite on one, actinolite’s, and asbestiform on one, actinolite’s. A ligature-aware search for fibrous finds 58 sheets, where a literal search finds 43. It matches fi or the stray glyph that replaces the ligature on older sheets, and the controls, actinolite and tremolite, both print ¯brous and both match. So fibrous habits are common; asbestiform is not.

Actinolite asbestos: what the regulation names

The legal definition, quoted. The Control of Asbestos Regulations 2012, regulation 2(1), say that “asbestos” means the following fibrous silicates: (a) asbestos actinolite, CAS No 77536-66-4; (b) asbestos grunerite (amosite); (c) asbestos anthophyllite; (d) chrysotile; (e) crocidolite; and (f) asbestos tremolite, CAS No 77536-68-6. Actinolite and tremolite, the two amphibole sheets in this site’s reference set, are both named, each with the word asbestos in front.

This page gives no health, safety or legal advice. It does not say whether any specimen falls under the regulation. For that, go to the Health and Safety Executive, the HSE EH40 workplace exposure limits and COSHH 2002, or a clinician. The collection-level summary is toxic minerals in a collection, and the same regulation is read against another fibrous mineral on is scolecite asbestos.

Where actinolite is found

The Handbook puts actinolite in low-grade regional or contact metamorphism of magnesium carbonate, mafic or ultramafic rocks, and in glaucophane-bearing blueschists. Its associates are talc, epidote, chlorite, glaucophane, pumpellyite, lawsonite and albite. Going the other way, actinolite is on the association lines of three other sheets in the set: talc, epidote and tremolite. It is also on the ferroaxinite sheet, covered on axinite.

The distribution, in the sheet’s order, begins many localities. It names Mt. Greiner, Zillertal, and Untersulzbachtal, Austria; Zermatt, Valais, Switzerland; Snarum and Arendal, Norway; and the Ural Mountains, Russia. In the USA it names Gouverneur, New York; Franklin and Newton, New Jersey; Chester, Vermont; the Fairfax quarry, Centreville, Virginia; Crestmore, California; and Salida, Colorado. The nephrite localities follow. The distribution names no British locality. The sheet’s analysis is from southeast Anglesey, Wales, and one of its references is a study of greenschist and blueschist in Anglesey.

The rest of the silicates are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

Questions