Vauxite
Vauxite, Fe2+Al2(PO4)2(OH)2(H2O)4·2H2O: a sky-blue to Venetian blue Bolivian phosphate, triclinic, hardness 3.5, density 2.39 to 2.40, crystals to 6 mm.
Vauxite is a hydrated ferrous aluminium phosphate, Fe2+Al2(PO4)2(OH)2(H2O)4·2H2O on the IMA list (G, 1922, Bolivia). Its Handbook sheet gives sky-blue to Venetian blue triclinic crystals and aggregates to 6 mm, hardness 3.5, density 2.39 to 2.40, and two localities, both in Bolivia: the Siglo XX mine, Llallagua, Potosí, and Huanuni, Oruro.
In short
- Three minerals, two water counts. The September 2026 IMA-CNMNC Master List writes vauxite Fe2+Al2(PO4)2(OH)2(H2O)4·2H2O (G, 1922, Bolivia); paravauxite Fe2+Al2(PO4)2(OH)2(H2O)6·2H2O (G, 1922, Bolivia); metavauxite Fe2+Al2(PO4)2(OH)2·8H2O (G, 1927, Bolivia). Vauxite has six waters, the other two eight, and the Handbook of Mineralogy sheets call paravauxite and metavauxite dimorphous. By mass that is 28.54% water against 33.93%.
- The water check. Counting the two OH as one H2O, the formula holds 7 H2O for every two phosphorus. The sheet’s analysis from Llallagua prints H2O+ 22.92 and H2O− 8.49, 31.41% in all, 2.87 points over the formula. Scaled to two phosphorus it gives 8.11 H2O in total and 5.92 counting H2O+ alone. The same reckoning gives 9.10 and 8.86 on the paravauxite and metavauxite sheets, against 9.
- The cell check. The sheet’s triclinic cell (a 9.142, b 11.599, c 6.158 Å; α 98.29°, β 91.93°, γ 108.27°) has a volume of 611.42 ų. Two formula units of 441.85 in it give 2.400, the printed 2.40. That is 305.7 ų per formula unit; paravauxite’s and metavauxite’s cells give 334.4 and 335.8, about 29 to 30 ų more for two more waters.
- Documented failure: the balance does not sort the Llallagua phosphates. A 20.00 g piece reads 11.63 to 11.67 g in water as vauxite, 11.53 g as paravauxite or wavellite, 11.47 g as metavauxite: 0.10 to 0.20 g apart. Colour separates better on the sheets: of the three sheets, only vauxite’s prints blue, but its own sheet adds greenish on exposure, and wavellite’s lists turquoise-blue.
- Where. The Distribution field reads, in full: In Bolivia, from the Siglo XX mine, Llallagua, Potosí, and at Huanuni, Oruro. The IMA list gives Bolivia as the country of first description; the sheet marks no type locality. The wavellite sheet also names Llallagua, and the vauxite sheet lists wavellite as an associate at both places.
| Mineral (IMA formula) | System | H2O per formula: ideal / from the analysis | Water by mass: ideal / analysis | Hardness | Density (measured) | 20.00 g in water | Volume per formula unit | Colour (sheet) |
|---|---|---|---|---|---|---|---|---|
| Vauxite, Fe2+Al2(PO4)2(OH)2(H2O)4·2H2O | Triclinic | 7 / 8.11 (H2O+ alone 5.92) | 28.54% / 31.41% | 3.5 | 2.39–2.40 | 11.63–11.67 g | 305.7 ų (cell gives 2.400) | Sky-blue to Venetian blue, greenish on exposure |
| Paravauxite, Fe2+Al2(PO4)2(OH)2(H2O)6·2H2O | Triclinic | 9 / 9.10 | 33.93% / 34.15% | 3 | 2.36 | 11.53 g | 334.4 ų (2.373) | Pale greenish white to colourless |
| Metavauxite, Fe2+Al2(PO4)2(OH)2·8H2O | Monoclinic | 9 / 8.86 | 33.93% / 33.22% | 3 | 2.345 | 11.47 g | 335.8 ų (2.363) | Colourless, white, pale green |
| Wavellite, Al3(PO4)2(OH)3(H2O)5 | Orthorhombic | — | 28.42% ideal | 3.5–4 | 2.36 | 11.53 g | — | White, green, yellow, turquoise-blue, brown and others |
| Variscite, Al(PO4)(H2O)2 | Orthorhombic | — | 22.81% ideal | 4.5 | 2.57–2.61 | 12.22–12.34 g | — | Pale green, emerald-green, bluish green, colourless |
What is vauxite? A blue ferrous aluminium phosphate
The formula. The September 2026 IMA list gives Fe2+Al2(PO4)2(OH)2(H2O)4·2H2O, status G (grandfathered), 1922, Bolivia, with a first reference in Science 56 (1922) and a second in the Canadian Mineralogist 54 (2016). The Handbook of Mineralogy vauxite sheet writes the six waters together: Fe2+Al2(PO4)2(OH)2·6H2O. On standard atomic weights the formula mass is 441.85, 12.64% iron, 12.21% aluminium and 14.02% phosphorus by mass; the ideal oxides, FeO 16.26, Al2O3 23.08, P2O5 32.12 and H2O 28.54, are the sheet’s column (2) to the second decimal.
What the sheet describes. Triclinic, space group P1̄, as crystals tabular on {010} and elongated along [001] or [101], or as radial to subparallel aggregates to 6 mm, and nodular; twinned on {010}. Hardness 3.5, brittle; no cleavage is printed. Sky-blue to Venetian blue, greenish on exposure, pale blue in transmitted light, with a white streak and vitreous lustre. Biaxial (+), α 1.551, β 1.555, γ 1.562, with strong pleochroism: X and Z colourless, Y blue. The sheet does not say what the exposure is or why the colour changes, and this page does not supply a reason.
The water check: six waters on paper, eight in the analysis
Method. Each oxide of the sheet’s analysis (1), from Llallagua, is divided by its molar mass and scaled so that P2O5 is one, which is two phosphorus. Iron comes out at 1.006 Fe2+ and aluminium at 1.954, close to the formula’s 1 and 2. Water is the exception. The formula’s two OH and six H2O are 7 H2O in oxide terms; the analysis prints H2O+ 22.92 and H2O− 8.49, which give 5.92 and 2.19, 8.11 together. By mass the analysis carries 31.41% water against 28.54% for the formula.
Documented failure, and the control. Neither reading of the vauxite analysis gives 7: with H2O− it is about one water too many, without it about one too few. The same arithmetic on the eight-water minerals’ sheets, also Llallagua analyses, comes out at 9.10 for paravauxite and 8.86 for metavauxite, against 9. The sheet cites Baur and Rama Rao (1968) on the crystal structure and the chemical composition of vauxite, but prints no explanation, and this page offers none.
Why vauxite is denser than paravauxite: the cell check
Worked. For a triclinic cell, V = abc·√(1 − cos²α − cos²β − cos²γ + 2·cosα·cosβ·cosγ). The vauxite cell gives 611.42 ų; with Z = 2 and a formula mass of 441.85, the density is Z·M·1.66054/V = 2.400, the sheet’s calculated 2.40 and the top of its measured 2.39 to 2.40. Paravauxite’s cell (Z = 1) gives 2.373 against its printed [2.37], and metavauxite’s monoclinic cell (Z = 2) 2.363 against [2.36].
What it shows. Per formula unit, vauxite occupies 305.7 ų, paravauxite 334.4 and metavauxite 335.8. The two extra waters add 36.03 to the formula mass and about 29 to 30 ų to the volume, so the eight-water minerals are lighter: 2.36 and 2.345 measured against vauxite’s 2.39 to 2.40. On a balance that is 11.53 and 11.47 g in water for 20.00 g, against 11.63 to 11.67 g; method on how to measure specific gravity at home. A 0.10 g step is close to what home weighing can resolve on a small aggregate.
Where is vauxite found? Bolivia, as the sheet prints it
Two places. The sheet’s Distribution field names only the Siglo XX mine, Llallagua, Potosí, and Huanuni, Oruro, both in Bolivia. Its X-ray powder pattern is from Llallagua. The IMA list records Bolivia as the country of first description, but the sheet marks no type locality, and this page does not supply one; see what is a type locality. Paravauxite’s sheet names Llallagua and places in the USA, Mexico, Germany, Portugal and Sweden; metavauxite’s names Llallagua and Tazna, Potosí.
What it grows with. The sheet calls vauxite a secondary mineral derived from alteration of apatite. Its associates are wavellite and paravauxite at Llallagua, and marcasite and wavellite at Huanuni. The wavellite sheet lists fine examples from Llallagua, Potosí, and Oruro; the paravauxite sheet adds sigloite, crandallite, childrenite and quartz at Llallagua. Other phosphates are in the species index.
Vauxite, variscite and wavellite: telling the blue and green phosphates apart
Iron is the chemical difference. Vauxite carries 12.64% iron; variscite, Al(PO4)(H2O)2, and wavellite, Al3(PO4)2(OH)3(H2O)5, carry none in their IMA formulas. In the hand the sheets give vauxite hardness 3.5 against variscite’s 4.5 and wavellite’s 3.5 to 4, and density 2.39 to 2.40 against 2.57 to 2.61 and 2.36. Variscite reads 12.22 to 12.34 g in water for 20.00 g, well clear of vauxite’s 11.63 to 11.67 g; wavellite’s 11.53 g is not.
Documented failure: the refractive index. Vauxite’s α 1.551 sits inside variscite’s 1.550 to 1.563, so a single refractometer reading does not separate them; the optic sign does, biaxial (+) against variscite’s (−). Variscite against turquoise is a separate question, worked on variscite vs turquoise. The sheet records no luminescence by a ligature-aware search; the Fluomin vauxite record lists a pinkish white response under longwave (365 nm) and shortwave (254 nm) and names Llallagua. UV exposure limits are in the ICNIRP 2004 guidelines on limits of exposure to ultraviolet radiation; this is not safety advice. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.