Augite
Augite, (Ca,Mg,Fe)2Si2O6, is the Ca-Mg-Fe clinopyroxene, black to brown: hardness 5.5 to 6, density 3.19 to 3.56, cleavages at 87°; it spans 20 to 45% Wo.
Augite is the calcium–magnesium–iron clinopyroxene, written (Ca,Mg,Fe)2Si2O6 on the IMA list (A, 1988 s.p.): monoclinic, black to brown, hardness 5.5 to 6, density 3.19 to 3.56, with two cleavages at about 87°. The 1988 IMA pyroxene report gives it the band from 20 to 45% Wo; above 45 the name is diopside or hedenbergite.
In short
- Two formulas, one species. The September 2026 IMA-CNMNC Master List writes augite (Ca,Mg,Fe)2Si2O6, status A, 1988 s.p.; the Handbook of Mineralogy sheet writes (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6, carrying the sodium, aluminium and titanium its analyses contain. The 1988 IMA pyroxene report says the 50% rule was not applied rigorously to the Ca-Mg-Fe pyroxenes, and that widely accepted terms such as augite were retained.
- The Wo 20 to 45 window. The report plots Ca-Mg-Fe pyroxenes on the Wo–En–Fs quadrilateral, normalised to Ca + Mg + ΣFe = 100, with ΣFe = Fe2+ + Fe3+ + Mn. A clinopyroxene with 20 to 45% Wo is augite; 45 to 50 is diopside or hedenbergite; below 20 is pigeonite, a distinction the report calls primarily structural, C2/c against P21/c.
- Worked on the sheet: Stillwater. The Stillwater complex analysis, printed as (Ca0.76Fe2+0.22Na0.02)(Mg0.88Al0.05Fe3+0.04Ti0.01Mn0.01)(Si1.92Al0.08)O6, normalises to Wo 39.8, En 46.1, Fs 14.1: augite. With 0.13 aluminium per formula unit, over the report’s 0.10 limit, it is aluminian augite. Recalculated from the printed oxides on six oxygens: Wo 39.9.
- Documented failure: the Stöffel case. The sheet’s second analysis, from Stöffel, Germany, normalises to Wo 49.7, En 36.1, Fs 14.2, inside diopside’s field, with Si 1.69 under the 1.75 subsilicic limit, 0.35 Al and 0.13 Fe3+. Counted as calcium over all M-site cations it is 45.0%, on the line. The report’s own footnote says normalising puts some augites above even 50% Wo. One analysis does not settle a name.
- Old labels. Of the 105 pyroxene names the 1988 report discarded, 16 resolve to augite alone, among them ferroaugite, titanaugite (now titanian augite) and coccolite (iron-rich augite); 6 more, fassaite and violan among them, read augite or another name. A whole-word script finds none of the 22 on the September 2026 IMA list.
| Name or analysis | IMA formula, status | Wo field (1988 report) | Space group (sheet) | Hardness | Density (measured) | 20.00 g piece in water | Streak (sheet) | Reading |
|---|---|---|---|---|---|---|---|---|
| Augite | (Ca,Mg,Fe)2Si2O6; A, 1988 s.p. | 20 to 45% | C2/c | 5.5–6 | 3.19–3.56 | 13.73–14.38 g | Gray-green | The species |
| Diopside | CaMgSi2O6; A, 1988 s.p. | 45 to 50%, Mg > Fe | C2/c | 5.5–6.5 | 3.22–3.38 | 13.79–14.08 g | White, gray, gray-green | Inside augite’s weight window |
| Hedenbergite | CaFe2+Si2O6; A, 1988 s.p. | 45 to 50%, Fe > Mg | C2/c | 5.5–6.5 | 3.56 | 14.38 g | Not printed | At augite’s heavy end |
| Pigeonite | (Mg,Fe,Ca)2Si2O6; A, 1988 s.p. | 5 to 20% | P21/c | 6 | 3.17–3.46 | 13.69–14.22 g | Not printed | Split from augite by structure |
| Enstatite (orthopyroxene) | Mg2Si2O6; A, 1988 s.p. | Below about 5% | Pbca | 5–6 | 3.2–3.9 | 13.75–14.87 g | White to grayish | Orthorhombic, not a clinopyroxene |
| Stillwater analysis (augite sheet) | - | Wo 39.8, En 46.1, Fs 14.1 | - | - | - | - | - | Aluminian augite (Al 0.13) |
| Stöffel analysis (augite sheet) | - | Wo 49.7, En 36.1, Fs 14.2 | - | - | - | - | - | Diopside field on paper; Ca 45.0% of M-site cations |
What is augite? The calcium–magnesium–iron clinopyroxene
Augite is a monoclinic pyroxene of calcium, magnesium and iron, with aluminium, titanium and sodium in both of the Handbook’s analyses. The Handbook of Mineralogy augite sheet gives monoclinic, point group 2/m, space group C2/c, as stubby prismatic crystals, square or octagonal in section, to 10 cm, also elongated, acicular, skeletal or dendritic, with exsolution lamellae and overgrowths common; twins are simple or multiple on {100}, also on {001}. It cleaves well on {110}, the two planes meeting at about 87°, with partings on {100} and {010}; hardness 5.5 to 6, measured density 3.19 to 3.56 against a calculated 3.31.
Colour and optics. Black, brown, greenish or violet-brown, colourless to gray in thin section, often with clear cores zoned to dark rims; streak gray-green; lustre vitreous, resinous to dull. Biaxial (+), α 1.671 to 1.735, β 1.672 to 1.741, γ 1.703 to 1.774, 2V 25° to 61°. The sheet prints no magnetic property; the USGS separator figures are on is augite a pyroxene.
What the IMA list records. The September 2026 list gives augite (Ca,Mg,Fe)2Si2O6, status A, 1988 s.p., with the country of first description given as unknown and a first reference to the Bergmannisches Journal of 1792. The sheet’s occurrence line: essential in mafic igneous rocks, basalt and gabbro; common in ultramafic rocks; in some high-grade metamorphic rocks and metamorphosed iron formations. Its associates are orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes.
Augite or diopside? The Wo 20 to 45 window, worked
On the 1988 rules, a calcium–magnesium–iron clinopyroxene with 20 to 45% Wo is augite. Morimoto et al. 1988, Nomenclature of pyroxenes (Mineralogical Magazine 52, 535) plots these pyroxenes on the quadrilateral of Ca2Si2O6 (Wo), Mg2Si2O6 (En) and Fe2Si2O6 (Fs), normalised to Ca + Mg + ΣFe = 100, where ΣFe = Fe2+ + Fe3+ + Mn2+. Its Fig. 4 draws diopside and hedenbergite from 45 to 50% Wo, augite from 20 to 45, and pigeonite from 5 to 20. It calls the augite–pigeonite distinction primarily structural, C2/c against P21/c, and says many pyroxenes with 15 to 25% Wo have proved to be mixtures of the two; augite below about 25% Wo is often called subcalcic augite. The diopside side of the 45 line, and the Wo check on diopside’s own analyses, are on diopside.
Worked on the Stillwater analysis. The augite sheet prints it as (Ca0.76Fe2+0.22Na0.02)(Mg0.88Al0.05Fe3+0.04Ti0.01Mn0.01)(Si1.92Al0.08)O6. Ca 0.76, Mg 0.88 and ΣFe 0.27 (0.22 + 0.04 + 0.01) sum to 1.91, so Wo 39.8, En 46.1, Fs 14.1: augite, well inside the window. Aluminium totals 0.13 per formula unit, over the report’s 0.10 limit, so the full name is aluminian augite. Recalculated from the printed oxides on six oxygens it gives Wo 39.9, En 46.2, Fs 14.0. Substitute any analysis in the same division.
Documented failure: the Stöffel case. The sheet’s second analysis, from Stöffel, Germany, prints (Ca0.91Na0.07Fe2+0.04)(Mg0.66Fe3+0.13Fe2+0.09Ti0.08Al0.04)(Si1.69Al0.31)O6. Normalised the report’s way it is Wo 49.7, En 36.1, Fs 14.2, which falls in diopside’s field, not augite’s; on the report’s Table 5 it would carry subsilicic (Si 1.69, under 1.75), aluminian (0.35 Al) and ferrian (0.13 Fe3+). Counted instead as calcium over all 2.02 M-site cations it is 45.0%, exactly on the line, and the report’s own footnote warns that normalising puts some augites above even the 50% line. The sheet prints the analysis under augite; the rule read literally puts it in diopside. This page does not rename it; it shows why an aluminium-rich black pyroxene is named by a full calculation, not by one number.
What do ferroaugite, titanaugite and fassaite mean? Old augite labels
Sixteen names discarded in 1988 now read simply as augite, with or without a modifier. Table 6 of the report, read from the rendered page of the American Mineralogist printing of the same report (73, 1123) and counted by script, lists 105 obsolete pyroxene names. Sixteen resolve to augite alone: aegerine-hedenbergite, anthochroite, ferroaugite, ferrohedenbergite, korea-augite, protheite, trachyaugite and violaite as augite; asteroite and coccolite as iron-rich augite; endiopside and enstatite-diopside as magnesium-rich augite; calc-pigeonite as subcalcic augite and pigeonite-augite as probably subcalcic augite; titanaugite as titanian augite; vanadinaugite as vanadium-bearing augite.
Six more read augite or another name. Fassaite is ferrian aluminian diopside or augite; jeffersonite zincian manganoan diopside or augite; mansjoite augite, diopside or hedenbergite; urbanite iron-rich augite or aegirine-augite; violan magnesium-rich augite or diopside; lime-bronzite probably pigeonite or enstatite plus augite. A whole-word scan of every line of the September 2026 IMA list finds none of the 22. On an old label each describes real material under a retired name, and for the six, only an analysis says which.
Modifiers the report allows. For the augite series it says the adjectives iron-rich, magnesium-rich and subcalcic are convenient, and that terms such as ferro-augite should not be used as mineral names. Element modifiers follow its Table 5: aluminian, ferrian, titanian or sodian above 0.10 atoms per formula unit, subsilicic below 1.75 Si. The old orthopyroxene names, bronzite and hypersthene, are decoded on enstatite and are not repeated here.
Where augite is found, and what it is mistaken for
The Distribution field opens widespread, listing only a few classic localities: Arendal, Norway; in Italy, Vesuvius, Frascati in the Alban Hills, Mt. Monzoni in Val di Fassa, Traversella and Mt. Etna; around the Laacher See, Eifel district, Germany; Ústí nad Lábem (Aussig), Bílina and Vlčí Hora near Černošín, Czech Republic; the Azores and Cape Verde Islands; Renfrew and Haliburton Cos., Ontario, and Otter Lake, Quebec; Franklin and Sterling Hill, New Jersey, and Diana and Fine, New York; Tomik, Gilgit district, Pakistan; and Kangan, Andhra Pradesh, India.
What a balance cannot do. At the sheet’s 3.19 to 3.56, a 20.00 g piece reads 13.73 to 14.38 g in water, a window 0.65 g wide that holds all of diopside’s 13.79 to 14.08 g and most of pigeonite’s 13.69 to 14.22 g. The same window contains epidote’s 14.08 to 14.27 g, which is worked on is epidote a pyroxene, and the formula test that separates epidote from a pyroxene is there too. The method is on how to measure specific gravity at home.
The amphibole confusion. Black augite and black hornblende share rocks and colour; the decision between them is the cleavage angle, 87° against 56° and 124°, set out with the sheets on is augite a pyroxene. The pyroxene neighbours with their own pages are diopside, enstatite and jadeite. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.