Tremolite
Tremolite is the magnesium end of the calcic amphiboles: white to lavender blades to 20 cm, hardness 5 to 6, density 2.99 to 3.03. One sheet analysis is fluoro.
Tremolite is the magnesium-rich calcic amphibole, ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2 on the September 2026 IMA list. The Handbook records white, grey or lavender to pink monoclinic blades to 20 cm, hardness 5 to 6, density 2.99 to 3.03, and two cleavages at 56° and 124°. It is named for the Tremola Valley, Switzerland.
In short
- The magnesium end of a series. The IMA amphibole report (Hawthorne et al. 2012) lists ☐Ca2Mg5Si8O22(OH)2 as the tremolite end member of the calcium amphiboles, and gives tremolite the range from Mg5 to Mg4.5Fe2+0.5. The IMA list writes the same range into its formula. Beyond it lies actinolite, whose page works the magnesium boundary on an analysis; it is not repeated here.
- The W-site check. The amphibole report gives the prefix fluoro- where F exceeds OH and Cl. The Handbook sheet’s Gouverneur, New York analysis recalculates to OH 1.34 and F 0.66: tremolite. Its Balmat No. 3 mine analysis recalculates to F 1.64 and OH 0.35: fluorine-dominant, which on the W site is the side of fluoro-tremolite, ☐Ca2Mg5Si8O22F2, a separate IMA row approved as 2016-018.
- The wollastonite window. A 20.00 g piece of tremolite reads 13.31 to 13.40 g in water. Wollastonite, another white bladed calcium silicate, reads 13.01 to 13.53 g on its sheet: its range contains tremolite’s entirely. A balance cannot separate them; cleavage and hardness can (see the table).
- Nephrite. The International Gem Society nephrite article places nephrite in the tremolite-actinolite series and says stones closer to tremolite carry more magnesium and lighter colours, such as the creamy white called mutton fat. The jade question is on is jade a mineral.
- Where. The Distribution field opens with Campolungo Alp, Ticino, and Bristenstock, Uri, Switzerland, then St. Marcel, Piedmont; Bilin, Czech Republic; Pierrepont, Gouverneur, Edwards and Macomb, St. Lawrence Co., New York; Franklin, New Jersey; Wilberforce, Ontario; Kozano, Afghanistan; Lelatema, Tanzania; and the Brumado mine, Bahia, Brazil.
| Species | IMA formula, status | System | Hardness | Density (measured) | 20.00 g piece in water | Cleavage (sheet) | Colour (sheet) |
|---|---|---|---|---|---|---|---|
| Tremolite | ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, Rd 2012 s.p. | Monoclinic | 5 to 6 | 2.99 to 3.03 | 13.31 to 13.40 g | Perfect {110} at 56° and 124° | White, grey, lavender to pink |
| Actinolite | ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2, Rd 2012 s.p. | Monoclinic | 5 to 6 | 3.03 to 3.24 | 13.40 to 13.83 g | Good {110} at 56° and 124° | Bright green to greyish green |
| Wollastonite | CaSiO3, A 1962 s.p. | Monoclinic or triclinic | 4.5 to 5 | 2.86 to 3.09 | 13.01 to 13.53 g: contains tremolite’s range | Perfect {100}; {100} to {001} at 84.5° | White, colourless, brown, red, yellow, pale green |
| Diopside (pyroxene) | CaMgSi2O6, A 1988 s.p. | Monoclinic | 5.5 to 6.5 | 3.22 to 3.38 | 13.79 to 14.08 g | Distinct {110} at about 87° | Colourless, white, yellow, pale to dark green, black |
| Scolecite (zeolite) | Ca(Si3Al2)O10·3H2O, A 1997 s.p. | Monoclinic, pseudotetragonal | 5 to 5.5 | 2.25 to 2.29 | 11.11 to 11.27 g | Two perfect prism cleavages | Colourless to white |
| Talc | Mg3Si4O10(OH)2, G | Triclinic or monoclinic | 1 | 2.58 to 2.83 | 12.25 to 12.93 g | Perfect {001} | Light to dark green, brown, white |
What is tremolite? The calcium amphibole with magnesium in all five C sites
Tremolite is the amphibole ☐Ca2Mg5Si8O22(OH)2 and the compositions close to it: two calcium, five magnesium, eight silicon, a vacant A site and two hydroxyls. The Hawthorne et al. 2012, Nomenclature of the amphibole supergroup (American Mineralogist 97, 2031) builds every amphibole on the general formula AB2C5T8O22W2; its Table 3 lists ☐Ca2Mg5Si8O22(OH)2 as tremolite and ☐Ca2Fe2+5Si8O22(OH)2 as ferro-actinolite, and its text gives tremolite the range from Mg5 to Mg4.5Fe2+0.5. The September 2026 IMA-CNMNC Master List writes that range into the formula, ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, with status Rd, redefined, 2012 s.p., and Switzerland as the country of first description.
What the sheet records. The Handbook of Mineralogy tremolite sheet gives Ca2(Mg,Fe2+)5Si8O22(OH)2, amphibole (calcic) group, forming a series with actinolite and ferro-actinolite. Monoclinic; elongated, stout prismatic or flattened bladed crystals to 20 cm, also fibrous, granular or columnar aggregates; twinning simple or multiple, common parallel to {100}. Cleavage perfect on {110} at 56° and 124°; hardness 5 to 6; density 2.99 to 3.03 measured. White, grey, lavender to pink; vitreous. Biaxial (−), indices 1.605 to 1.636, 2V 86° to 88°.
Worked: the ideal composition. On standard atomic weights ☐Ca2Mg5Si8O22(OH)2 has a formula mass of 812.35 and is 59.17% SiO2, 24.81% MgO, 13.81% CaO and 2.22% H2O. The sheet’s Gouverneur analysis reads 56.57%, 24.41% and 12.25% for the first three; Balmat reads 59.30%, 24.58% and 12.03%. Where tremolite ends and actinolite begins is a magnesium count, worked on actinolite and not repeated here.
Is my tremolite fluoro-tremolite? The W-site check
An amphibole with more fluorine than hydroxyl in its W site takes the prefix fluoro-, and one of the two analyses on the tremolite sheet does. The amphibole report’s Table 1 defines the prefix: fluoro, F greater than OH and Cl. The IMA list has a row for it: fluoro-tremolite, ☐Ca2Mg5Si8O22F2, approved as 2016-018, country USA.
Worked on the sheet. Analysis (1), Gouverneur, New York: F 1.52% by weight, recalculated by the sheet to (OH)1.34 F0.66 Cl0.01. Hydroxyl dominates; F is 0.33 of the site. Analysis (2), Balmat No. 3 mine, St. Lawrence Co., New York, by electron microprobe: F 3.84%, recalculated to F1.64 (OH)0.35 Cl0.01. Fluorine dominates; F is 0.82 of the site. The ideal fluoro-tremolite is 4.65% F by mass, so Balmat carries 82.5% of the end member’s fluorine and Gouverneur 32.7%.
What the check does and does not say. It says the printed Balmat composition falls on the fluoro- side of the report’s rule. It does not say every Balmat specimen is fluoro-tremolite: the sheet’s analyses are of particular samples, and a label change needs an analysis of the piece. The sheet was published in 2001, before fluoro-tremolite was approved, which is why it prints both analyses under one name. How to record a superseded name beside a current one is on how to label and catalogue a mineral collection.
Tremolite or wollastonite? The wollastonite window
Wollastonite’s in-water range, 13.01 to 13.53 g for a 20.00 g piece, contains tremolite’s 13.31 to 13.40 g entirely, so weighing cannot separate them. Both are white, bladed calcium silicates and the wollastonite sheet records parallel fibrous material too. The separators are on the sheets: tremolite has two equal cleavages at 56° and 124° and hardness 5 to 6; wollastonite has one perfect cleavage on {100}, others good, with {100} to {001} at 84.5°, and hardness 4.5 to 5. The weighing method is on how to measure specific gravity at home; the angle is read on a clean broken end, as set out on cleavage or fracture: how to tell.
The other pale look-alikes. Diopside is heavier, 13.79 to 14.08 g, and its prism cleavages meet at about 87°. Scolecite, a zeolite that grows in white radiating sprays, is far lighter at 11.11 to 11.27 g. Talc, which the tremolite sheet lists as an associate, is hardness 1 and reads 12.25 to 12.93 g. Actinolite touches tremolite at 13.40 g and is green.
In the hand, the order is colour, angle, then weight. A lavender to pink blade fits tremolite’s colour line and no other row in the table. A white blade needs the cleavage angle. The hardness test marks the piece, so it comes last; see how to test mineral hardness properly.
Is tremolite asbestos? One line, and where the answer is
Regulation 2 of the Control of Asbestos Regulations 2012 names asbestos tremolite, CAS No 77536-68-6, among six fibrous silicates. The tremolite sheet records fibrous aggregates among its habits, beside crystals to 20 cm. What the regulation and HSE’s guidance say about that is set out on is tremolite asbestos, and the general page is toxic minerals in a collection. This page gives no health, safety or legal advice.
Nephrite, from the gem side. The International Gem Society places nephrite in the tremolite-actinolite series: closer to tremolite, more magnesium and lighter colours; closer to actinolite, more iron. The IMA list has rows for tremolite and actinolite and none for nephrite, and the actinolite sheet, not the tremolite sheet, records the word nephrite. The jadeite side is on jadeite.
Where tremolite is found
The sheet puts tremolite in contact metamorphism of Ca-Mg siliceous sediments, and in greenschist-facies metamorphic rocks derived from ultramafic or magnesium carbonate rocks. Its associates are calcite, dolomite, calcian garnet, wollastonite, talc, diopside, forsterite, cummingtonite, magnesio-cummingtonite, riebeckite and winchite.
Notable localities, in the sheet’s order: Campolungo Alp, Ticino, and Bristenstock, Uri, Switzerland; St. Marcel, Piedmont, Italy; Bilin, Czech Republic; Pierrepont, Gouverneur, Edwards and Macomb, St. Lawrence Co., New York; Franklin, Sussex Co., New Jersey (see Franklin and Sterling Hill); Lee, Berkshire Co., Massachusetts; Wilberforce, Ontario, Canada; Kozano, Badakhshan Province, Afghanistan; Lelatema, Tanzania; and the Brumado mine, Bahia, Brazil. The X-ray pattern is from St. Gotthard, Switzerland, and the name from the Tremola Valley there.
The other amphiboles and chain 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.