Diopside
Diopside, CaMgSi2O6, is a monoclinic pyroxene, hardness 5.5 to 6.5, density 3.22 to 3.38. Chrome diopside is chromian diopside: over 0.01 Cr per formula unit.
Diopside is the calcium magnesium pyroxene, CaMgSi2O6, an IMA species (A, 1988 s.p.). It is monoclinic, hardness 5.5 to 6.5, density 3.22 to 3.38, with two {110} cleavages at about 87°. Green chrome diopside is not a separate species: under the 1988 IMA pyroxene rules it is chromian diopside, above 0.01 Cr per formula unit.
In short
- One species, many old names. The September 2026 IMA-CNMNC Master List gives diopside, CaMgSi2O6, status A, 1988 s.p., Italy. IMA pyroxene report (Morimoto 1988) accepted 20 pyroxene names, diopside among them, and discarded salite and ferrosalite as subdivision names of the diopside–hedenbergite series. Its list of obsolete names reads salite, baikalite, alalite and lavrovite as diopside, and malacolite as diopside with good (001) parting.
- The 0.24% chromium line. The report’s adjectival rule calls a pyroxene chromian above 0.01 Cr3+ per formula unit. Worked on end-member diopside (216.55 g/mol), 0.01 Cr is about 0.24% chromium by weight. The Handbook of Mineralogy sheet’s Dutoitspan mine, Kimberley analysis has Cr2O3 2.03%, which is 1.39% Cr, printed as Cr0.06: chromian diopside. The International Gem Society puts chrome diopside at about 0.5% Cr, which works out at 0.021 Cr, also over the line.
- Both sheet analyses pass the Wo check. The report draws diopside between 45 and 50% Wo, the Ca2Si2O6 component, on the magnesium side of the quadrilateral. Normalised to Ca + Mg + ΣFe, the Juva, Finland analysis is Wo 49.3, En 49.8, Fs 1.0; Dutoitspan is Wo 45.6, En 51.1, Fs 3.3, just 0.6 above the 45 line where augite begins.
- Weight in water overlaps its neighbours. At 3.22 to 3.38 a 20.00 g piece reads 13.79 to 14.08 g in water. Dioptase’s 13.90 to 14.03 g sits wholly inside that window, and jadeite’s 13.83 to 14.17 g largely overlaps it, so a balance confirms a pyroxene-weight stone and names nothing.
- Where. The Distribution field lists fine crystals from Schwarzenstein, Zillertal, Austria; Ala and St. Marcel, Italy; the Inagli massif and the Slyudyanka River near Lake Baikal, Russia; DeKalb, New York; large gemmy crystals from the Kunlun Mountains, China; and Ampandrandava, Madagascar.
| Name | Formula (IMA) | Crystal system | Hardness | Density (measured) | 20.00 g piece in water | Streak (sheet) | Cleavage (sheet) |
|---|---|---|---|---|---|---|---|
| Diopside | CaMgSi2O6 | Monoclinic | 5.5 to 6.5 | 3.22 to 3.38 | 13.79 to 14.08 g | White, gray, gray-green | Distinct {110}, about 87° |
| Hedenbergite | CaFe2+Si2O6 | Monoclinic | 5.5 to 6.5 | 3.56 | 14.38 g | Not printed | Good {110}, about 87° |
| Johannsenite | CaMnSi2O6 | Monoclinic | 6 | 3.27 to 3.54 | 13.88 to 14.35 g | Not printed | Good {110}, about 87° |
| Enstatite | Mg2Si2O6 | Orthorhombic | 5 to 6 | 3.2 to 3.9 | 13.75 to 14.87 g | White to grayish | Good {210}, about 88° |
| Jadeite | NaAlSi2O6 | Monoclinic | 6 to 7 | 3.24 to 3.43 | 13.83 to 14.17 g | White | Good {110}, about 87° |
| Dioptase | CuSiO3(H2O) | Hexagonal | 5 | 3.28 to 3.35 | 13.90 to 14.03 g | Green | Perfect {10-11} |
What is diopside? The calcium magnesium pyroxene
Diopside is the pyroxene with one calcium and one magnesium per formula, CaMgSi2O6. The Handbook of Mineralogy diopside sheet gives it monoclinic, point group 2/m, space group C2/c, as prismatic crystals with nearly square cross sections to 50 cm, and as granular, columnar or lamellar masses. Twins on {100} or {010} are common. The cleavage is distinct on {110}, the two planes meeting at about 87°, with partings on {100} and probably {010}. Hardness is 5.5 to 6.5 and measured density 3.22 to 3.38.
What the IMA list and the 1988 report record. The September 2026 list gives diopside, CaMgSi2O6, status A, 1988 s.p., with Italy as the country of first description; the 1988 date is the year of the special procedure, the pyroxene report, not the year of discovery. Morimoto et al. 1988, Nomenclature of pyroxenes (American Mineralogist 73, 1123) places diopside in the Ca pyroxenes, part of the Ca-Mg-Fe quadrilateral, and applies the 50% rule to the diopside–hedenbergite series. The Handbook adds a second series, with johannsenite, CaMnSi2O6. How the report sorts lithium and sodium pyroxenes is on is spodumene a pyroxene; its test is not repeated here.
Colour and streak. The sheet lists colourless, white, yellow, pale to dark green and black, colourless in thin section, with a white, gray or gray-green streak and a vitreous or dull lustre. Optically it is biaxial (+), α 1.664, β 1.672, γ 1.694. The name is from the Greek for double and appearance, apparently for two possible orientations of the prism zone.
Is chrome diopside a separate mineral? The 0.24% chromium line
No: by the 1988 IMA rules, chrome diopside is chromian diopside, and the line is 0.01 chromium per formula unit. The report’s Table 5 lists adjectival modifiers by cation content per formula unit M2M1T2O6, and gives chromian for Cr3+ above 0.01. Worked on end-member diopside, 216.55 g/mol, 0.01 × 51.996 / 216.55 is 0.24% chromium by weight. Neither scan of the report’s list of 105 obsolete names contains chrome diopside; the name is handled by the modifier rule, not discarded by name.
Worked on the Handbook’s own analysis. The sheet’s second analysis, from the Dutoitspan mine, Kimberley, Cape Province, South Africa, prints Cr2O3 2.03%. Multiplied by 2 × 51.996 / 151.99, that is 1.39% Cr, and the sheet’s formula gives Cr0.06; recalculating the printed oxides on six oxygens gives 0.058. Either way it is 0.06 against a line of 0.01, so the formal name is chromian diopside. The International Gem Society diopside article describes chrome diopside as diopside with deep green colouring from chromium, usually transparent, with a Cr content of about 0.5% by weight: 0.021 Cr on the same basis, also chromian. The Dutoitspan figure is 2.8 times the gem society’s.
Documented discrepancy on the sheet. The same Dutoitspan formula prints Na0.05, but the printed Na2O of 1.37% recalculates to about 0.10 Na on six oxygens. The chromium answer does not depend on it. The gem society also gives chrome diopside a density of 3.17 to 3.32, starting below the sheet’s 3.22, and names kimberlite, Slyudyanka, Outokumpu in Finland and Kenya for it.
Old diopside names: salite, malacolite, diallage, violane
The 1988 report discarded the names for slices of the diopside–hedenbergite series. It says subdivision names of the intermediate ranges, such as salite and ferrosalite of the diopside–hedenbergite series, have been discarded. Its Table 6 of obsolete names reads: salite = diopside; baikalite = diopside; alalite = diopside; lavrovite = diopside; malacolite = diopside with good (001) parting, also diopside from Sala, Sweden; diallage = diopside that has altered or that has good (100) parting; violan = magnesium-rich augite or diopside; schefferite = manganoan diopside. The Mineralogical Magazine printing of the 1988 pyroxene report (52, 535) prints the same list.
Where the references disagree. The International Gem Society article still lists salite and ferrosalite as intermediates between diopside and hedenbergite, alalite and malacolite as colour varieties, and violane as massive blue to violet diopside from St. Marcel, Piedmont. The IMA list has no line containing salite as a name of its own, nor violan, baikalite, malacolite or alalite. On an old label these read as diopside, or, for violane, diopside or augite; the analysis decides.
Star and cat’s-eye diopside. The gem society records asterism, usually four-rayed and occasionally six, and chatoyancy, naming Nammakal, India for star stones and cat’s-eyes. These are diopside showing an optical effect, not separate minerals.
Where diopside is found, and what it is mistaken for
The Distribution field gives selected localities for fine crystals: Schwarzenstein, Zillertal, and near Prägraten, Tirol, Austria; Ala, Piedmont, and St. Marcel, Val d’Aosta, Italy; Otokumpu, Finland (so printed; the gem society writes Outokumpu); the Akhmatovsk deposit near Zlatoust, the Inagli massif west of Aldan, and the Slyudyanka River near Lake Baikal, Russia; Bird’s Creek and Eganville, Ontario, and the Jeffrey mine, Asbestos, Quebec; DeKalb, Natural Bridge and Sing Sing, New York, and Ducktown, Tennessee; Ampandrandava and Andranodambo, Madagascar; large gemmy crystals from the Kunlun Mountains, China; Tange-Achin, Afghanistan; near Jaipur, India; and Khapalu and Chamachu, Pakistan.
Where it grows. The sheet calls it typical of metamorphosed siliceous Ca, Mg-rich rocks, common in skarns, Ca, Mg-rich gneisses and schists, and some kimberlites and peridotites, with calcite, forsterite, chondrodite, monticellite, wollastonite, grossular, vesuvianite, tremolite and quartz. Its neighbours enstatite and jadeite are pyroxenes too, and spodumene is the lithium one.
The copper look-alike. Green dioptase shares diopside’s first four letters and a density window, but not its chemistry or its matrix; the separating table is on is diopside the same as dioptase. Weighing in water is on how to measure specific gravity at home; it will not part these two. 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.