Legrandite
Legrandite, Zn2(AsO4)(OH)(H2O): monoclinic zinc arsenate, hardness 4.5, density 3.98 to 4.01, yellow prisms to 28 cm from the Ojuela mine, Mapimí, Durango.
Legrandite is a hydrated zinc arsenate, Zn2(AsO4)(OH)(H2O) on the IMA list (G, 1932, Mexico): monoclinic yellow prisms to 28 cm, hardness 4.5, density 3.98 to 4.01. It is adamite’s formula plus one water molecule, 8.87% water by mass. A 20.00 g piece reads 14.97 to 15.01 g in water; adamite reads 15.37 g or more.
In short
- The one-water step. A script over the September 2026 IMA-CNMNC Master List finds four rows whose formula begins Zn2(AsO4)(OH): adamite and paradamite with no water molecule, legrandite with one, and ianbruceite (approved 2011, Namibia) with three. Worked from those formulas the water by mass is 3.14%, 8.87% and 18.51%. Legrandite is the one-water member.
- Water costs density. On the Handbook of Mineralogy sheets: paradamite 4.55, adamite 4.32 to 4.48, legrandite 3.98 to 4.01, ianbruceite 3.197 (calculated). Worked as 20 − 20/D, a 20.00 g piece reads 15.60, 15.37 to 15.54, 14.97 to 15.01 and 13.74 g in water. Legrandite is at least 0.36 g lighter than adamite and 0.59 g lighter than paradamite.
- Shape and hardness separate it too. Legrandite forms prisms elongated along [001] to 28 cm, in sprays or sheaflike aggregates, at hardness 4.5; the adamite sheet gives 3.5. Both are yellow on their sheets, so colour does not separate them.
- Documented failure. Austinite, CaZn(AsO4)(OH), which its sheet also records at the Ojuela mine, reads 15.16 g at density 4.13: only 0.15 g above legrandite’s top figure. smithsonite reads 15.49 g, inside adamite’s band. Weight sorts the Zn2(AsO4)(OH) family; it does not sort every yellow mineral from the same mine.
- Where. The Distribution field gives large crystals from the Ojuela mine, Mapimí, Durango (Ojuela), and also the Flor de Peña mine, Nuevo León, and the Potosí mine, Santa Eulalia, Chihuahua; Tsumeb, Namibia; Sterling Hill, New Jersey; the Ogibira and Toroku mines, Japan; and Boa Vista, Minas Gerais, Brazil.
| Species | Formula (IMA) | IMA status | Water molecules on the Zn2(AsO4)(OH) core | Water by mass, worked | System (sheet) | Hardness | Density | 20.00 g piece in water | Colour (sheet) | Optical class (sheet) |
|---|---|---|---|---|---|---|---|---|---|---|
| Paradamite | Zn2(AsO4)(OH) | G, 1956, Mexico | 0 | 3.14% | Triclinic | n.d. | 4.55 | 15.60 g | Pale yellow | Biaxial (−) |
| Adamite | Zn2(AsO4)(OH) | G, 1866, Chile | 0 | 3.14% | Orthorhombic | 3.5 | 4.32 to 4.48 | 15.37 to 15.54 g | Pale yellow, honey-yellow, brownish yellow and others | Biaxial (+), may be biaxial (−) |
| Legrandite | Zn2(AsO4)(OH)(H2O) | G, 1932, Mexico | 1 | 8.87% | Monoclinic | 4.5 | 3.98 to 4.01 | 14.97 to 15.01 g | Bright yellow, wax-yellow, colourless | Biaxial (+) |
| Ianbruceite | Zn2(AsO4)(OH)(H2O)·2H2O | A, 2011-049, Namibia | 3 | 18.51% | Monoclinic | 1 | n.d.; calc. 3.197 | 13.74 g (calc.) | Sky blue to pale blue | Biaxial (−) |
| Köttigite | Zn3(AsO4)2(H2O)8 | G, 1850, Germany | Different core | 23.32% | Monoclinic | 2 to 3 | 3.33 | 13.99 g | White, colourless; light carmine, peach-blossom-red; blue, greyish blue | Biaxial (+) |
| Scorodite | Fe3+(AsO4)(H2O)2 | G, 1818, Germany | No zinc | 15.61% | Orthorhombic | 3.5 to 4 | 3.27 | 13.88 g | Pale leek-green, greyish green, liver-brown and others | Biaxial (+) |
| Smithsonite | Zn(CO3) | G, 1832, United Kingdom | Carbonate | 0% | Hexagonal | 4 to 4.5 | 4.43 | 15.49 g | White, pale to dark grey, brown; more rarely red, pink, yellow, green, blue and others | Uniaxial (−) |
| Austinite (failure) | CaZn(AsO4)(OH) | G, 1935, USA | Has calcium | 3.45% | Orthorhombic | 4 to 4.5 | 4.13 | 15.16 g | Colourless, white to pale yellowish | Biaxial (+) |
What is legrandite? A zinc arsenate with one water molecule
Legrandite is Zn2(AsO4)(OH)(H2O) on the September 2026 IMA list, status G (grandfathered), 1932, with Mexico as the country of first description. The Handbook of Mineralogy legrandite sheet writes it Zn2(AsO4)(OH)·H2O. It is monoclinic, point group 2/m, in prismatic crystals elongated along [001] to 28 cm, with dominant {110}, striated parallel to {001} and {−111}, with {100} and {001}, typically in sprays or sheaflike aggregates. The {−111} overbar was read from the rendered sheet, because the text layer drops it.
On the sheet. Cleavage fair to poor on {100}, fracture uneven, brittle; hardness 4.5; measured density 3.98 to 4.01, calculated 4.015. Transparent to translucent, bright yellow, wax-yellow or colourless, with a vitreous lustre. Biaxial (+), α 1.702, β 1.709, γ 1.740, 2V 50°, pleochroic from colourless to yellow. The sheet prints no streak and no fluorescence; this page claims neither. The name honours a Mr. Legrand, a Belgian mine manager who collected the first specimen; type material is at the Natural History Museum, London, Harvard University and the National Museum of Natural History, Washington.
Its chemistry, worked. On standard atomic weights the IMA formula has a formula mass of 304.70 and gives ZnO 53.42%, As2O5 37.72% and H2O 8.87%, the ideal column the sheet prints. The sheet’s Ojuela analysis reads ZnO 50.1 with FeO 1.4 and MnO 1.7, As2O5 37.7 and H2O+ 8.8. Arsenic is 24.59% by mass; arsenic minerals are covered on toxic minerals in a collection and is orpiment toxic, and nothing on this page is health or safety advice.
How is legrandite related to adamite? The one-water step
Legrandite is adamite’s formula plus one water molecule. A script over the September 2026 IMA list (row parser over pdftotext -layout text, cross-checked by a raw line scan) finds four rows whose formula begins Zn2(AsO4)(OH): adamite, G, 1866, Chile; paradamite, G, 1956, Mexico; legrandite; and ianbruceite, Zn2(AsO4)(OH)(H2O)·2H2O, A, 2011-049, Namibia. Adamite and paradamite are dimorphs on their sheets. The four are 4 of the 13 parsed rows built only of zinc, arsenic, oxygen and hydrogen.
Worked. Counting OH as half a water molecule, the water by mass is 3.14% for adamite and paradamite, 8.87% for legrandite and 18.51% for ianbruceite. The added molecule is 5.91% of legrandite’s mass. On their sheets, density falls in the same order: paradamite 4.55, adamite 4.32 to 4.48, legrandite 3.98 to 4.01, ianbruceite 3.197 calculated. The table shows the pattern across four rows; it does not prove a rule for other minerals, and this page claims none.
In water. Worked as 20 − 20/D, a 20.00 g piece reads 15.60 g as paradamite, 15.37 to 15.54 g as adamite and 14.97 to 15.01 g as legrandite. The gap to adamite is 0.36 to 0.56 g, wide enough for a 0.01 g scale on solid crystals; matrix and sprays with gaps pull the reading towards the rock. The method is on how to measure specific gravity at home. Adamite’s own page is adamite, and its ultraviolet response is on is adamite UV reactive.
How do you tell legrandite from adamite and paradamite?
Habit and hardness first, weight second. Legrandite on its sheet forms prisms to 28 cm elongated along [001] in sprays and sheaves, hardness 4.5. The adamite sheet gives hardness 3.5; the paradamite sheet gives no hardness, rarely equant striated crystals, and a perfect {010} cleavage, against legrandite’s fair to poor {100}. Colour does not decide it: all three sheets print yellow.
Optics overlap more than weight. Legrandite’s α is 1.702; adamite’s runs 1.708 to 1.722, only 0.006 higher at the nearest point, and paradamite’s is 1.726. Legrandite’s birefringence, γ − α, is 0.038; paradamite’s is 0.054 and adamite’s spans about 0.036 to 0.065 across its printed ranges. A refractometer reading needs a clean face and still leaves adamite in play; the in-water weight does not.
Where the weight check fails
Austinite and smithsonite break it. The austinite sheet records austinite, CaZn(AsO4)(OH), at the Ojuela mine, at density 4.13: a 20.00 g piece reads 15.16 g in water, only 0.15 g above legrandite. Its sheet gives colourless, white to pale yellowish bladed crystals, hardness 4 to 4.5, biaxial (+) with α 1.759, which is 0.057 above legrandite’s α. smithsonite, on the same association line, reads 15.49 g, inside adamite’s band, and is uniaxial. scorodite and köttigite read 13.88 g and 13.99 g, both well below legrandite.
So the check has a scope. It separates legrandite from the other Zn2(AsO4)(OH) rows. A yellow Ojuela mineral outside that family needs its habit and optics, and a small or massive one needs an analysis. Other Ojuela species, such as hemimorphite, ojuelaite and mapimite (both named for the mine and district on their sheets), are covered on the Ojuela page and their own sheets.
Where is legrandite found?
The Distribution field names, in Mexico, the Flor de Peña mine, Lampazos, Nuevo León; large crystals in the Ojuela mine, Mapimí, Durango; and the Potosí mine, Santa Eulalia, Chihuahua. Also Tsumeb, Namibia; Sterling Hill, Ogdensburg, Sussex Co., New Jersey; the Ogibira mine near Osa, Okayama Prefecture, and the Toroku mine, Miyazaki Prefecture, Japan; and Boa Vista, near Galiléia, Minas Gerais, Brazil. The sheet calls it an uncommon secondary mineral in the oxidized zone of Zn–As-bearing deposits, rare in granite pegmatite.
Its companions differ by locality. The association field gives adamite, paradamite, köttigite, scorodite and smithsonite at the Ojuela mine; leiteite, smithsonite and reniérite at Tsumeb; and köttigite, adamite, pharmacosiderite and scorodite at Sterling Hill. 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.