Sturmanite
Sturmanite is a Ca-Fe sulphate-borate of the ettringite group from Kuruman, South Africa: bright yellow crystals to 40 cm, hardness about 2.5, density 1.847.
Sturmanite is a hydrated calcium ferric iron sulphate-borate of the ettringite group, Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12(H2O)25 on the IMA list, approved in 1981 (1981-011). It forms bright yellow hexagonal crystals to 40 cm at the Wessels and N'Chwaning II mines, Kuruman, South Africa: hardness about 2.5, density 1.847. On the IMA formula, 44.85% of its mass is water.
In short
- It is light. Its measured density of 1.847 would sit third-lowest among the 173 sheets in this site's reference set that print one. Only epsomite, 1.677, and borax, 1.715, are lighter. The sturmanite sheet is not in the set and is not counted.
- Nearly half of it is water. Worked from the IMA formula, it is 44.85% H2O by mass. The Kuruman analysis on the Handbook of Mineralogy sheet reads 46.7%.
- The two published formulas differ by half a sulphate, and the charge arithmetic shows why. The Handbook formula with all-ferric iron is +18 against −17. The IMA's (SO4)2.5 balances at −18. The sheet's own Kuruman analysis balances at ±17.8.
- The sturmanite-or-ettringite rule. Both occur at N'Chwaning, according to the two sheets. Sturmanite is bright yellow, 1.847, with ω near 1.50. Ettringite is colourless, pale yellow or milky white, 1.77, with ω 1.464. A 10 cm³ crystal weighs 18.5 g against 17.7 g, too close for the hand. Colour and refractive index do the work.
- Its sheet names one district: Kuruman, Cape Province, with large crystals from the Wessels and N'Chwaning II mines. Five of the 176 sheets in the set also name Kuruman, among them hematite and rhodochrosite.
| Species | Formula (Handbook) | Colour | Hardness | Density (measured) | Mass of 10 cm³ | ω | Localities on the sheet |
|---|---|---|---|---|---|---|---|
| Sturmanite | Ca6(Fe3+,Al,Mn2+)2(SO4)2[B(OH)4](OH)12·25H2O | Bright yellow | About 2.5 | 1.847 | 18.5 g | 1.499 to 1.500 | Kuruman district only: Wessels and N'Chwaning II mines |
| Ettringite | Ca6Al2(SO4)3(OH)12·26H2O | Colourless, pale yellow, milky white | 2 to 2.5 | 1.77 | 17.7 g | 1.464 | Germany, France, Ireland, Israel, N'Chwaning mine, USA |
| Charlesite | Ca6(Al,Si)2(SO4)2B(OH)4(OH,O)12·26H2O | Colourless, white, pale yellow, pale pink | About 2.5 | 1.77 | 17.7 g | 1.492 | Franklin, New Jersey only |
| Baryte | BaSO4 | Colourless, white, yellow, brown, grey, pale red, green, blue | 3 to 3.5 | 4.50 | 45.0 g | Biaxial (+); not comparable | Worldwide; an associate of sturmanite |
What is sturmanite?
Sturmanite is a calcium ferric iron sulphate with a borate group and a great deal of water, and a member of the ettringite group. Its 1983 description in The Canadian Mineralogist, the sheet's first reference, is titled Sturmanite, a ferric iron, boron analogue of ettringite. The September 2026 IMA-CNMNC Master List records it as approved, number 1981-011, type country South Africa. Type material is at the National Museum of Natural History, Washington, number 148261.
It is hexagonal. The sheet describes dipyramidal, tabular to elongated hexagonal crystals to 40 cm, showing {10-10}, {10-14} and {11-24}, typically in groups and sometimes in stacked parallel aggregates. The cleavage is perfect on {10-10} and it is brittle, hardness about 2.5. It is bright yellow with a pale yellow to greenish yellow streak, brownish orange if altered, and a vitreous lustre that is slightly greasy on fracture surfaces. It is weakly pleochroic, pale green to pale yellowish green.
It is unusually light. The measured density field, D(meas.), was parsed by script from all 176 sheets in the reference set, taking the upper end of any range; 173 print a number. Sturmanite's 1.847 would rank third-lowest among them: only epsomite, at 1.677 (synthetic material), and borax, at 1.715, are lighter. Hydrobasaluminite, at 1.86, is next above it. See how to measure specific gravity at home.
Why the Handbook and IMA formulas differ: a charge balance
The Handbook writes Ca6(Fe3+,Al,Mn2+)2(SO4)2[B(OH)4](OH)12·25H2O; the IMA list writes Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12(H2O)25. Count the charges. Six calcium and two ferric iron make +18. On the Handbook's two sulphates, the anions are −4 −1 −12 = −17, one short. The IMA's 2.5 sulphates make −5 −1 −12 = −18, balanced. The Handbook's brackets admit Mn2+ on the iron site, which lowers the cation charge, but at the analysis's 0.2 manganese that closes only 0.2 of the gap.
The Kuruman analysis on the sheet balances exactly. The sheet recalculates it to Ca6.0(Fe3+1.5Al0.3Mn2+0.2)(SO4)2.3[B(OH)4]1.2(OH)12.0·25.7H2O. That is +12 +4.5 +0.9 +0.4 = +17.8 against −4.6 −1.2 −12.0 = −17.8. The IMA list's second reference for sturmanite is a 2004 paper in The Canadian Mineralogist. The Handbook sheet does not cite it, so the reason for the revision is not given here.
A worked composition on the IMA formula, on standard atomic weights, formula mass 1325.60: CaO 25.38%, Fe2O3 12.05%, SO3 15.10%, B2O3 2.63% and H2O 44.85%. The analysis reads CaO 25.6%, Fe2O3 8.84% with Al2O3 1.13% and MnO 1.30% sharing the iron site, SO3 14.2%, B2O3 3.2% and H2O 46.7%. By element, the IMA formula is 8.43% iron and 0.82% boron.
Sturmanite or ettringite? The colour-and-index rule
Both occur at N'Chwaning. The sturmanite sheet gives large crystals from the Wessels and N'Chwaning II mines. The ettringite sheet, fetched this run and not in the reference set, gives large crystals from the N'Chwaning mine. Both are hexagonal, soft and light, with a perfect {10-10} cleavage.
- Colour. Sturmanite is bright yellow. Ettringite is colourless, pale yellow, milky white.
- Refractive index. Sturmanite has ω 1.499 to 1.500; ettringite 1.464. That is a clear gap for anyone with immersion liquids.
- Density: too close for the hand. 1.847 against 1.77, a 4.4% difference; a 10 cm³ crystal weighs 18.5 g against 17.7 g.
- Chemistry. Ettringite is Ca6Al2(SO4)3(OH)12·26H2O with no iron and no boron. Worked the same way it is Al2O3 8.12%, SO3 19.14% and H2O 45.93%, which the ettringite sheet's ideal column prints exactly. Sturmanite carries iron and boron.
The rule: a bright yellow crystal is sturmanite, a colourless to pale one ettringite, and a pale yellow one needs an index or an analysis. Charlesite, the aluminium and boron member, is recorded on its sheet only from Franklin, New Jersey. The ettringite sheet also says ettringite partially dehydrates on exposure, becoming opaque. The sturmanite sheet says nothing of the kind, and this page does not extend the statement to it.
Where sturmanite is found
The sheet gives one district: the Kuruman district, Cape Province, South Africa, with large crystals in the Wessels and N'Chwaning II mines. It calls sturmanite an uncommon secondary mineral in cavities in metamorphosed bedded manganese deposits. The associates are baryte, manganite, hausmannite and hematite. No other locality is recorded on the sheet.
Against the 176 sheets in the set (string search of each Distribution field), five name Kuruman. Hematite names the district. Hydroxyapophyllite gives large crystals from near Kuruman. Portlandite and tephroite give the Wessels mine. Rhodochrosite gives exceptional crystals from the Hotazel and N'Chwaning mines.
The name honours Bozidar Darko Sturman, born 1937, a Canadian mineralogist at the Royal Ontario Museum, Toronto. The rest of the sulphates 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.