Is augite a pyroxene?
Yes. Augite is a monoclinic pyroxene species, hardness 5.5 to 6, with cleavages at 87°. Above 45% Wo it is diopside; hornblende, an amphibole, cleaves at 56°.
Yes. Augite is a monoclinic pyroxene, (Ca,Mg,Fe)2Si2O6 on the IMA list, one of the 20 names the 1988 IMA pyroxene report accepts: hardness 5.5 to 6, two cleavages at about 87°. It holds the band from 20 to 45% Wo; above 45 it is diopside, below 20 pigeonite. Hornblende, its look-alike, is an amphibole cleaving at 56° and 124°.
In short
- Yes, on the sheet and in the report. The Handbook of Mineralogy augite sheet says pyroxene group, monoclinic, C2/c. The 1988 IMA pyroxene report says pyroxenes contain single SiO3 chains of linked SiO4 tetrahedra, general formula M2M1T2O6, and that monoclinic pyroxenes are called clinopyroxenes. It lists augite, (Ca,Mg,Fe)2Si2O6, C2/c, among its Ca pyroxenes; the September 2026 IMA-CNMNC Master List gives it status A, 1988 s.p.
- The three-border rule. Augite is the pyroxene left inside three lines. Above 45% Wo it becomes diopside or hedenbergite. Below 20% Wo it becomes pigeonite, which the report says is a structural split, C2/c against P21/c, with many pyroxenes at 15 to 25% Wo mixtures of the two. Past J/(Q + J) = 0.2, where J = 2Na, it becomes a Ca-Na pyroxene, aegirine-augite or omphacite. The Handbook’s Stillwater analysis sits inside all three: Wo 39.8, J/(Q + J) 0.02.
- Not a pyroxene: hornblende. The magnesio-hornblende sheet gives amphibole (calcic) group, perfect {110} cleavage intersecting at 56° and 124°, biaxial (−), and says it forms as reaction rims on pyroxenes. Augite cleaves at about 87° and is biaxial (+). The IMA list has no row named hornblende; magnesio-hornblende (A, 2017-059) and ferro-hornblende (Rd, 2012 s.p.) are species.
- Documented failure: weight and magnet. A 20.00 g piece reads 13.73 to 14.38 g in water as augite, a 0.65 g window that holds all of diopside’s 13.79 to 14.08 g and shares 13.73 to 13.90 g with magnesio-hornblende. On the USGS separator augite’s best range, 0.40 to 0.90 amp, overlaps hornblende’s 0.30 to 0.60 and diopside’s 0.40 to 0.60. Neither tool names the mineral.
- Is augite magnetic? The augite sheet prints no magnetic property. USGS Open-File Report 99-529, Table 1 pulled 4 augite samples best at 0.40 to 0.90 amp, depending on chemistry, and 1 sample under ferroaugite, a name the 1988 report discarded as augite, at 0.20 to 0.30. USGS says iron-rich members of the pyroxene family are more magnetic than members with less iron.
| Criterion | Augite | Diopside | Pigeonite | Aegirine | Magnesio-hornblende | Separates augite? |
|---|---|---|---|---|---|---|
| IMA formula, status | (Ca,Mg,Fe)2Si2O6; A, 1988 s.p. | CaMgSi2O6; A, 1988 s.p. | (Mg,Fe,Ca)2Si2O6; A, 1988 s.p. | NaFe3+Si2O6; A, 1998 s.p. | ☐Ca2(Mg4Al)(Si7Al)O22(OH)2; A, 2017-059 | On paper |
| Group (sheet) | Pyroxene | Pyroxene | Pyroxene | Pyroxene | Amphibole (calcic) | Yes |
| Field (1988 report) | Wo 20 to 45 | Wo 45 to 50, Mg > Fe | Wo 5 to 20 | Na pyroxene, J/(Q + J) over 0.8 | Not a pyroxene | With an analysis |
| Space group (sheet) | C2/c | C2/c | P21/c | C2/c (synthetic) | C2/m | From pigeonite, by X-ray |
| Cleavage (sheet) | Good {110}, about 87° | Distinct {110}, about 87° | Good {110}, about 87° | Good {110}, about 87° | Perfect {110}, 56° and 124° | Yes: from hornblende |
| Optic sign, 2V (sheet) | Biaxial (+), 25°–61° | Biaxial (+), 59° | Biaxial (+), 0°–30° | Biaxial (−); Ca-, Mg-, Fe2+-rich (+); 60°–70° | Biaxial (−), 56° | Partly, with a microscope |
| Hardness | 5.5–6 | 5.5–6.5 | 6 | 6 | [5–6] | No |
| Density, measured | 3.19–3.56 | 3.22–3.38 | 3.17–3.46 | 3.50–3.60 | 3.10–3.28 | No |
| 20.00 g piece in water | 13.73–14.38 g | 13.79–14.08 g | 13.69–14.22 g | 14.29–14.44 g | 13.55–13.90 g | No: overlaps all four |
| Streak (sheet) | Gray-green | White, gray, gray-green | Not printed | Pale yellowish gray | Not printed | No |
| Colour (sheet) | Black, brown, greenish, violet-brown | Colourless, white, yellow, green, black | Brown, greenish brown-black | Dark green to greenish black, reddish brown, black | Green, brown | No |
| USGS best range, amp (samples) | 0.40–0.90, depends on chemistry (4) | 0.40–0.60 (4) | 0.50–0.60 (1) | 0.30–0.40, as acmite (5) | 0.30–0.60, as hornblende (12) | No: overlaps |
Is augite a pyroxene? Yes: the definition in one sentence
Augite is a pyroxene: a monoclinic single-chain silicate of calcium, magnesium and iron, accepted by name in the 1988 IMA nomenclature. Morimoto et al. 1988, Nomenclature of pyroxenes (Mineralogical Magazine 52, 535) says pyroxenes are silicates that, in their simplest form, contain single SiO3 chains of linked SiO4 tetrahedra, with a repeat of two tetrahedra along the chain and the general formula M2M1T2O6; any pyroxene is orthorhombic or monoclinic, and the monoclinic ones are called clinopyroxenes. The report formally accepts 20 names; its Table 2 lists augite, (Ca,Mg,Fe)2Si2O6, space group C2/c, among the Ca pyroxenes, and says the widely accepted term augite was retained.
The sheet agrees. The Handbook of Mineralogy augite sheet reads pyroxene group, monoclinic, C2/c, with a good {110} cleavage, the two planes at about 87°, hardness 5.5 to 6 and density 3.19 to 3.56. The September 2026 IMA list gives augite (Ca,Mg,Fe)2Si2O6, status A, 1988 s.p. The species facts, with both of the sheet’s analyses worked, are on augite.
A name search trap. Four IMA rows contain the letters augite: augite, aegirine-augite, janhaugite and kamphaugite-(Y). Janhaugite’s formula carries Si2O7 groups, not Si2O6, and kamphaugite-(Y), CaY(CO3)2(OH)(H2O), has no silicon at all. Only the first two are pyroxenes by the report.
When does augite stop being augite? The three-border rule
The three-border rule: augite is the clinopyroxene inside Wo 45, Wo 20 and J/(Q + J) = 0.2. The report plots calcium–magnesium–iron pyroxenes on the Wo–En–Fs quadrilateral, normalised to Ca + Mg + ΣFe with ΣFe = Fe2+ + Fe3+ + Mn, and its Fig. 4 draws augite from 20 to 45% Wo. Above 45 the name is diopside, or hedenbergite where iron beats magnesium. Below 20 it is pigeonite, and the report says that split is primarily structural, C2/c against P21/c, with many pyroxenes at 15 to 25% Wo proving to be mixtures of the two; augite below about 25% Wo is often called subcalcic augite.
The sodium border. The report counts Q = Ca + Mg + Fe2+ and J = 2Na per six oxygens and draws the line J/(Q + J) = 0.2: beyond it lie the Ca-Na pyroxenes, aegirine-augite and omphacite, and beyond 0.8 the Na pyroxenes, jadeite and aegirine. How the same diagram places spodumene at its origin is worked on is spodumene a pyroxene and is not repeated here. On the IMA list aegirine-augite is (Ca,Na)(Fe3+,Mg,Fe2+)Si2O6, status Rd, 1988 s.p.; the Handbook aegirine-augite sheet returned a 404 page and none of its numbers are used.
Worked once. The augite sheet’s Stillwater complex analysis gives Q 1.86 and J 0.04, so J/(Q + J) is 0.02, far inside the sodium border, and normalises to Wo 39.8, inside both Wo borders: augite on all three. The sheet’s Stöffel analysis is the case that fails, at Wo 49.7; it is on augite. Without an analysis only the hand tests below are available, and they cannot settle the borders.
Augite or hornblende? 87 degrees against 56 and 124
The cleavage angle separates augite from hornblende: about 87° in augite, 56° and 124° in hornblende. Both are monoclinic and prismatic, brown to green or black, and the magnesio-hornblende sheet even records it as reaction rims on pyroxenes. That sheet gives amphibole (calcic) group, perfect {110} cleavage intersecting at 56° and 124°, biaxial (−) with 2V 56°, and the ferrohornblende sheet gives the same angles. The augite sheet gives good {110} cleavage at about 87° and biaxial (+). On these sheets, two cleavage traces near a right angle mean pyroxene, and a diamond of 56° and 124° means amphibole. The method is on cleavage or fracture: how to tell.
What the IMA list says about hornblende. No row is named hornblende alone. Magnesio-hornblende, ☐Ca2(Mg4Al)(Si7Al)O22(OH)2, is status A, 2017-059, Namibia; ferro-hornblende, ☐Ca2(Fe2+4Al)(Si7Al)O22(OH)2, is Rd, 2012 s.p.; the Handbook hornblende sheet returned a 404 page. A label reading hornblende names an amphibole without naming the species. actinolite, another calcic amphibole, cleaves at the same 56° and 124° on its sheet.
Documented failure: hardness, weight and magnet. Hardness overlaps: augite 5.5 to 6, magnesio-hornblende [5 to 6], bracketed on its sheet. Worked as 20 − 20/D, a 20.00 g piece reads 13.73 to 14.38 g in water as augite and 13.55 to 13.90 g as magnesio-hornblende, sharing 13.73 to 13.90 g. On USGS Open-File Report 99-529 (Rosenblum and Brownfield) Table 1, augite’s best range is 0.40 to 0.90 amp and hornblende’s, across 12 samples, 0.30 to 0.60. Of the hand and bench tests in the table above, only the cleavage and the optics separate them. The methods are on how to measure specific gravity at home and how to test mineral hardness properly.
Is augite magnetic? What the sheet and USGS print
The augite sheet prints no magnetic property; USGS measured augite as paramagnetic on a laboratory separator. Table 1 of the report lists augite at 0.20 to 1.30 amp total, best 0.40 to 0.90, across 4 samples, marked depends on chemistry. USGS calls ferromagnetic only minerals strongly attracted to a magnet, like a piece of iron, and paramagnetic those separable at up to 1.70 amps. Neither source tests a hand magnet.
The old-name row. The same table has a row headed ferroaugite, 1 sample, best 0.20 to 0.30 amp. Ferroaugite is one of the names the 1988 report discarded, reading it as augite, so the best ranges for material that the 1988 names call augite run from 0.20 to 0.90 amp across the two rows. The report text says iron-rich end members of pyroxenes, amphiboles and other families are more magnetic than members with less iron. Diopside’s best range is 0.40 to 0.60 and hedenbergite’s 0.20 to 0.40, consistent with that.
Who each label is wrong for. Call it augite when a dark prismatic crystal shows two cleavages near 87° and positive optics and no analysis says otherwise; call it a hornblende when the cleavages meet at 56° and 124°. The epidote question is answered with a formula test on is epidote a pyroxene; the magnet question across the site is on which minerals are magnetic. The classification guides are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.