Enstatite
Enstatite, Mg2Si2O6, is the orthorhombic magnesium pyroxene, hardness 5 to 6, density 3.2 to 3.9. Bronzite is now enstatite, and so is hypersthene if Mg leads.
Enstatite is the magnesium orthopyroxene, Mg2Si2O6, an IMA species: orthorhombic, hardness 5 to 6, density 3.2 to 3.9, with two {210} cleavages at about 88°. Under the 1988 IMA pyroxene rules it covers every orthopyroxene with more magnesium than iron, so material labelled bronzite, and some labelled hypersthene, is enstatite.
In short
- The 50% line, worked. IMA pyroxene report (Morimoto 1988) divides the enstatite–ferrosilite series at the midpoint, the 50% rule, and keeps orthopyroxene below about 5% Wo. The Handbook of Mineralogy sheet’s analysis from Maliba Matso, Lesotho, printed as Mg1.77Fe2+0.11Fe3+0.02Ca0.02, normalises to Wo 1.0, En 92.2, Fs 6.8: enstatite by 42 points. Recalculated from the printed oxides on six oxygens it gives the same to 0.1.
- Bronzite and hypersthene are retired names. The report says the subdivision names bronzite, hypersthene and eulite of the enstatite–ferrosilite series have been discarded. Its table of obsolete names reads bronzite = enstatite; hypersthene = enstatite or ferrosilite; eulite, orthoferrosilite and ferrohypersthene = ferrosilite. A script over the September 2026 IMA-CNMNC Master List finds 0 lines containing bronzite, hypersthene or eulite.
- The 1.12-gram window. At the sheet’s 3.2 to 3.9 a 20.00 g piece reads 13.75 to 14.87 g in water. The International Gem Society’s old-name densities, enstatite 3.20 to 3.30, bronzite 3.30 to 3.42 and hypersthene 3.43 to 3.90, all fall inside it. The balance cannot sort the old labels, and a stone in the hypersthene range, which the 1988 report reads as enstatite or ferrosilite, is not named by its weight.
- Three rows, one formula. A row parser over the list, checked by an independent line scan, finds exactly 3 rows written Mg2Si2O6: enstatite (A, 1988 s.p.), clinoenstatite (A, 1988 s.p.) and protoenstatite (A, 2016-117, USA). The 1988 report had said protoenstatite was not adopted as a mineral name.
- Where. The sheet names Bamble, Norway; The Lizard, Cornwall, England; Kraubath, Styria, Austria; fine crystals from Mbeya, Tanzania; Webster and Corundum Hill, North Carolina; and gem material from Embilipituya, Sri Lanka. It is also common in chondrite, achondrite and stony-iron meteorites.
| Name on the label | 1988 IMA report | IMA list, September 2026 | Specific gravity (source) | 20.00 g piece in water |
|---|---|---|---|---|
| Enstatite | Accepted name; Mg > Fe, Wo below about 5 | Mg2Si2O6, A, 1988 s.p., Czech Republic | 3.20 to 3.30 (IGS); 3.2 to 3.9 (Handbook) | 13.75 to 13.94 g (IGS); 13.75 to 14.87 g (Handbook) |
| Bronzite | Discarded: = enstatite | No line | 3.30 to 3.42 (IGS) | 13.94 to 14.15 g |
| Hypersthene | Discarded: = enstatite or ferrosilite | No line | 3.43 to 3.90 (IGS) | 14.17 to 14.87 g |
| Orthoferrosilite | Discarded: = ferrosilite | No line | 3.90 to 3.96 (IGS) | 14.87 to 14.95 g |
| Eulite | Discarded: = ferrosilite | No line | Not given | Not worked |
| Ferrosilite | Accepted name; Fe > Mg | Fe2+2Si2O6, Rn, 1988 s.p. | 3.96 (Handbook) | 14.95 g |
| Clinoenstatite | Accepted name; monoclinic | Mg2Si2O6, A, 1988 s.p. | 3.210 calculated; none measured (Handbook) | 13.77 g |
| Protoenstatite | Not adopted as a mineral name | Mg2Si2O6, A, 2016-117, USA | No Handbook sheet | Not worked |
What is enstatite? The magnesium orthopyroxene
Enstatite is the pyroxene of magnesium silicate, Mg2Si2O6, in its orthorhombic form. The Handbook of Mineralogy enstatite sheet gives point group 2/m 2/m 2/m and space group Pbca, crystals prismatic to 40 cm, commonly lamellar, fibrous or massive, with simple and lamellar twinning on {100}. Cleavage is good on {210}, the two planes at about 88°, with partings on {100} and {010}; brittle. Hardness 5 to 6, measured density 3.2 to 3.9. It is white, grayish, yellowish, greenish, olive-green or brown, with a white to grayish streak and a vitreous lustre, pearly on cleavages.
Where it forms. The sheet places it in pyroxenites, peridotites and dunites, in ultramafic inclusions in alkalic olivine basalts and kimberlite, in mafic volcanics, characteristic of charnockites and the granulite facies, and common in chondrite, achondrite and stony-iron meteorites. Its associates are olivine, phlogopite, clinopyroxene, diopside, spinel and pyrope. The sheet says it is dimorphous with clinoenstatite and forms a series with ferrosilite.
Gem enstatite. The International Gem Society enstatite article gives most gem enstatites indices of 1.663 to 1.673, describes colourless enstatite from Embilipitiya, Sri Lanka, with S.G. 3.25, and says India produces four-rayed star enstatites. The Handbook’s Distribution field ends with gem material from Embilipituya, Sri Lanka, spelled that way; the two references name the same place. The gem society reports asterism in bronzite and enstatite and no luminescence.
Is bronzite enstatite? The 50% line, worked
Yes: since the 1988 IMA pyroxene report, bronzite is an obsolete name for enstatite. Morimoto et al. 1988, Nomenclature of pyroxenes (American Mineralogist 73, 1123) says that whenever there is a complete solid-solution series between two end members, the division should be at A50B50, the 50% rule, and that subdivision names of the intermediate ranges, such as bronzite, hypersthene and eulite, have been discarded. Its Figure 5 draws two orthopyroxenes, enstatite on the magnesium side of the 50 line and ferrosilite on the iron side, and the text says the most calcium-rich orthopyroxene contains approximately 5% Wo.
Worked on the sheet. Normalise to Ca + Mg + ΣFe, with ΣFe = Fe2+ + Fe3+ + Mn as the report does. The Maliba Matso, Lesotho analysis prints Ca0.02, Mg1.77, Fe2+0.11 and Fe3+0.02: Wo = 100 × 0.02 / 1.92 = 1.0, En = 92.2, Fs = 6.8. Wo is under 5 and En is 42.2 points past 50, so the name is enstatite. As a control the printed oxides, SiO2 58.48, MgO 34.71, FeO 3.93, Fe2O3 0.72, CaO 0.50, were recalculated on six oxygens: Wo 1.0, En 92.2, Fs 6.9. Substitute any orthopyroxene analysis and the same three divisions name it.
Where the references disagree. The International Gem Society still says the series is arbitrarily broken into six regions of composition, lists bronzite, hypersthene, ferrohypersthene, eulite and orthoferrosilite in its orthopyroxene group, and gives bronzite as brown with fibrous inclusions giving it a bronze appearance. The 1988 report reads bronzite as enstatite and hypersthene as enstatite or ferrosilite, depending on which side of the 50 line the analysis falls. This page prints both. On a label, bronzite becomes enstatite; hypersthene needs an analysis.
Enstatite, clinoenstatite and protoenstatite: three rows, one formula
Three species on the September 2026 IMA list share the formula Mg2Si2O6. A row parser over the list, with an independent scan of every line for the token Mg2Si2O6, finds 3: enstatite, A, 1988 s.p., Czech Republic; clinoenstatite, A, 1988 s.p.; and protoenstatite, A, 2016-117, USA, with a second reference in American Mineralogist 102 (2017), 2146. Same chemistry, three structures: the formula alone does not name an Mg pyroxene, as it does not name an SiO2 mineral on is quartz a mineral.
Clinoenstatite. Its Handbook sheet gives monoclinic, space group P21/c on synthetic material, crystals to 2 mm, lamellar, fibrous or massive, polysynthetic twinning on {100}, hardness printed in brackets as [5–6] and no measured density, only a calculated 3.210. At that figure a 20.00 g piece reads 13.77 g in water, inside enstatite’s window.
Documented change since 1988. The 1988 report wrote that the high-temperature form of enstatite, space group Pbcn, is not quenchable and has not been found in nature, and that the name protoenstatite, used conventionally for it, was not adopted as a mineral name. The 2026 list carries protoenstatite as an approved species, IMA 2016-117. The list gives a formula, not a structure, so this page does not say which phase the 2016 approval describes; it records only that the name now has a row.
Where enstatite is found: Bamble, The Lizard and Sri Lanka
The Distribution field gives prominent localities for well-studied material: Zdjarberg, Czech Republic; Ødegården and Kjörrestad, Bamble, Norway; The Lizard, Cornwall, England; Dawros, Co. Connaught, Ireland; Kupferberg, Bavaria, and the Bellerberg volcano near Mayen, Eifel, Germany; Kraubath, Styria, Austria; fine crystals from Mbeya, Tanzania; Liahobong, Lesotho; Webster, Jackson Co., and Corundum Hill, Macon Co., North Carolina; Wood’s chrome mine and Texas, Lancaster Co., Pennsylvania; and gem material from Embilipituya, Sri Lanka. The X-ray pattern was taken on Bamble material.
The name. The sheet derives it from the Greek for opponent, in allusion to its refractory nature under the blowpipe; the gem society attributes the name to its high melting point.
Its relatives on this site. diopside is the calcium pyroxene it is found with; forsterite and olivine share its ultramafic rocks; jadeite and spodumene are the sodium and lithium pyroxenes, and the pyroxene question is set out on is spodumene a pyroxene. 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.