Powellite
Powellite, Ca(MoO4): tetragonal calcium molybdate, hardness 3.5–4, density 4.26, 47.97% molybdenum, fluoresces creamy white to golden yellow under shortwave UV.
Powellite is calcium molybdate, Ca(MoO4) on the September 2026 IMA list: tetragonal, hardness 3.5 to 4, density 4.26, straw-yellow to brown or blue, fluorescing creamy white or yellow to golden yellow under shortwave UV. Worked from the formula it is 47.97% molybdenum. It forms a series with scheelite, Ca(WO4), which has tungsten in place of molybdenum and is far heavier.
In short
- The species. The September 2026 IMA-CNMNC Master List lists powellite as Ca(MoO4), status G, 1891, USA. The Handbook of Mineralogy sheet writes CaMoO4: tetragonal, dipyramidal crystals to 8 cm, also paper-thin plates, crusts and ocherous masses; cleavage indistinct; hardness 3.5 to 4; density 4.26. A 20.00 g piece reads 15.31 g in water, the figure on the scheelite page.
- Under UV, on the sheet. “Fluoresces creamy white or yellow to golden yellow under SW UV.” The scheelite sheet prints bright bluish white under SW UV and X-rays. How the glow moves from blue to yellow along the series is set out on is scheelite fluorescent and is not repeated here. UV lamp exposure is covered by the ICNIRP 2004 guidelines on ultraviolet exposure; this page is not safety advice.
- The same-box check. Worked from the two sheets’ cells, powellite (a 5.222, c 11.425 Å) holds 311.55 cubic ångströms and scheelite (a 5.2429, c 11.3737 Å) 312.64: 0.35% apart. The formula masses are 200.02 and 287.91, 43.94% apart. The cells give densities of 4.264 and 6.117, a ratio of 1.434; the measured 4.26 and 6.10 give 1.432.
- Documented failure: two printed calculated densities. Powellite’s cell gives 4.264 against the printed 4.255, and scheelite’s 6.117 against the printed 6.09 and measured 6.10. Both are small, both are reported, and neither sheet is corrected. The sheet’s Western Altai analysis recasts to Ca 1.007 per Mo with 0.34% CuO, which the sheet puts down to malachite.
- Powellite in amethyst or quartz. Collectors search for “powellite amethyst” and “powellite quartz”. The powellite sheet names molybdenite, ferrimolybdite, stilbite, laumontite and apophyllite as associates and does not mention quartz or amethyst, and no source we would cite was found that covers it; this page makes no claim either way.
| Quantity | Powellite | Scheelite | Difference |
|---|---|---|---|
| IMA formula; status | Ca(MoO4); G, 1891, USA | Ca(WO4); G, 1821, Sweden | Mo against W |
| Formula mass | 200.02 | 287.91 | +43.94% |
| Cell a, c (Å) | 5.222, 11.425 | 5.2429, 11.3737 | a +0.40%, c −0.45% |
| Cell volume (ų) | 311.55 | 312.64 | +0.35% |
| Density from cell, worked | 4.264 | 6.117 | Ratio 1.434 |
| Density printed | 4.26 meas., 4.255 calc. | 6.10 meas., 6.09 calc. | Ratio 1.432 (measured) |
| 20.00 g piece in water | 15.31 g | 16.72 g | 1.42 g |
| Metal by mass, worked | Mo 47.97% | W 63.85% | — |
| Analysis on the sheet | Western Altai: MoO3 71.67, CaO 28.11, CuO 0.34; Ca 1.007 per Mo | Schwarzenberg: WO3 80.17, MoO3 0.07, CaO 19.49; Mo 0.14 mol% of Mo + W | — |
| Hardness | 3.5–4 | 4.5–5 | — |
| Optics | Uniaxial (+); ω 1.974, ε 1.984 | Uniaxial (+); ω 1.918–1.921, ε 1.935–1.938 | — |
| UV on the sheet | Creamy white or yellow to golden yellow, SW | Bright bluish white, SW and X-rays | — |
What is powellite? A calcium molybdate
Powellite is Ca(MoO4) on the September 2026 IMA list, status G, 1891, with the USA as the country of first description; the first reference is American Journal of Science 41 (1891), 138, and the second Acta Crystallographica E76 (2020), 121. The Handbook of Mineralogy powellite sheet writes CaMoO4. Worked on standard atomic weights the formula mass is 200.02: molybdenum 47.97%, calcium 20.04%, oxygen 31.99%. As oxides that is MoO3 71.96 and CaO 28.04, the sheet’s ideal column exactly. How the molybdates are classed against the oxides is on is wulfenite an oxide.
What the sheet records. Tetragonal, point group 4/m, space group I41/a (synthetic), a = 5.222 and c = 11.425 Å, Z = 4. Crystals are dipyramidal {111}, with {011} and {112}, less commonly flat tabular to paper-thin on {001}, with many minor forms, to 8 cm; it is also aggregated into crusts, pulverulent, ocherous or massive. Cleavage on {112}, {011} and {001}, all indistinct; fracture uneven. Hardness 3.5 to 4; density 4.26 measured, 4.255 calculated. Transparent to translucent; straw-yellow, greenish yellow, yellow-brown, brown or colourless, may be zoned, and also blue to nearly black; lustre subadamantine, resinous, pearly. Uniaxial (+), ω 1.974, ε 1.984, pleochroic in deeply coloured specimens, O blue and E green.
Where it forms. “An uncommon secondary mineral, typically formed in the oxidation zone of molybdenum-bearing hydrothermal mineral deposits, rarely in basalts, tactites, and granite pegmatites.” Its associates are molybdenite, ferrimolybdite, stilbite, laumontite and apophyllite.
Powellite and scheelite: the same-box check
One series, two metals. The powellite sheet says it “Forms a series with scheelite”, and the scheelite sheet says the same of powellite. A scan of the IMA formula column finds both as single-cation rows with a bracketed (MoO4) or (WO4) group, beside wulfenite, Pb(MoO4), and stolzite, Pb(WO4). The two calcium end members share the space group I41/a and Z = 4.
The check, worked. Powellite’s cell is 311.55 cubic ångströms and scheelite’s 312.64, so swapping molybdenum for tungsten enlarges the box by 0.35% (a grows 0.40%, c shrinks 0.45%). The formula mass rises 43.94%, from 200.02 to 287.91. The cell densities are therefore 4.264 and 6.117, a ratio of 1.434, and the measured 4.26 and 6.10 give 1.432. A collector feels the difference as weight: a 20.00 g piece reads 15.31 g in water as powellite and 16.72 g as scheelite, on the method at how to measure specific gravity at home.
Documented failure: the printed calculated figures. Powellite’s cell reproduces its measured 4.26 but not quite its printed calculated 4.255 (worked 4.264); scheelite’s gives 6.117 against the printed 6.09. The differences are 0.2% and 0.4%. The sheet’s one powellite analysis, from the Western Altai, Russia, recasts to 1.007 Ca per Mo and 47.77% molybdenum, with 0.34% CuO that the sheet says is “probably from malachite”. The scheelite sheet’s Schwarzenberg analysis carries MoO3 0.07: molybdenum is 0.14 mole% of its Mo + W.
Is powellite fluorescent? The sheet’s words
Yes, under shortwave, on the sheet. The physical-properties line reads: “Fluoresces creamy white or yellow to golden yellow under SW UV.” There is no “may” before it, and no longwave response is named. Scheelite’s sheet reads “Bright bluish white cathodoluminescence and fluorescence under SW UV and X-rays.”
What is not repeated here. The molybdenum content at which scheelite’s glow turns from blue to white to yellow, from Modreski (1987), and the gem-society panels, are on is scheelite fluorescent. The lamp background is on fluorescent minerals explained. This page names no activator: the sheet prints none.
Lamps. Shortwave lamps fall under the ICNIRP 2004 guidelines on ultraviolet exposure. This is a routing line, not safety advice; nothing on this page is.
Where is powellite found? Michigan, Nasik and the rest
Distribution, as printed. In the USA: the Peacock Lode, Seven Devils district, Idaho; large crystals at the Isle Royale, Calumet & Hecla, Tamarack and other mines, Houghton Co., Michigan; the Pine Creek tungsten mine, near Bishop, California; the Tonopah-Divide mine, Esmeralda Co., and many other places in Nevada. Huepac, Sonora, Mexico. In Chile, Chuquicamata, Antofagasta, and Inca de Oro and the Las Escobas mine, near Copiapó, Atacama. Ultevis, Lappland, Sweden. Traprain Law, near Haddington, East Lothian, Scotland. “Extraordinary crystals from Pandulena Hill, eight km south of Nasik, Maharashtra, India.” Tsumeb, Namibia. “There are a number of other older localities known, most now considered quite minor.”
Name and type material. The name honours Major John Wesley Powell (1834–1902), American geologist and explorer and Director of the U.S. Geological Survey. Type material is at the National Museum of Natural History, Washington, D.C., 80674. The sheet marks no type locality; the IMA list gives the USA. See what is a type locality.
Elsewhere on this site. More molybdates and tungstates are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.