Phosphate Minerals as Gem Material
Phosphate Minerals as Gem Material
Apatite, turquoise, lazulite, variscite, and the wider phosphate class
The phosphate mineral class encompasses a substantial group of gem and ornamental materials whose chemistry is built around the phosphate anion, PO43-, paired with a wide range of cations to produce species as varied as turquoise, apatite, lazulite, variscite, amblygonite, herderite, and brazilianite. Phosphates are second only to silicates among rock-forming and gem-forming mineral classes by chemical diversity. They form principally in low- to moderate-temperature hydrothermal, sedimentary, and pegmatitic environments, and the species span the full range of jewellery use, from the genuinely commercial — turquoise above all — to the strictly collector-only.
Major phosphate gem species
Turquoise — hydrated copper aluminium phosphate, CuAl6(PO4)4(OH)8·4H2O — is by a wide margin the most commercially important phosphate gemstone, prized for its blue to greenish-blue colour and produced principally in Iran, the southwestern United States, China, and historically in the Sinai. Hardness is typically 5 to 6, making turquoise suitable for jewellery use with reasonable care.
Apatite — the species name for a group of calcium phosphates with the general formula Ca5(PO4)3(F,Cl,OH) — produces gem-quality crystals in a remarkable range of colours: blue from Madagascar and Brazil, the saturated neon-blue Paraíba-style apatite from Madagascar, yellow from Mexico, and green from a number of localities. Apatite's hardness of 5 limits jewellery wear to protected settings.
Lazulite, a magnesium aluminium phosphate, produces blue gem material from Brazil, Madagascar, and the Yukon. Variscite — hydrated aluminium phosphate, AlPO4·2H2O — produces the yellow-green to apple-green ornamental material famous from Utah and Australia. Brazilianite is a yellow-green sodium aluminium phosphate from Minas Gerais. Amblygonite and montebrasite produce yellow to colourless gem material from Brazil and elsewhere. Herderite produces yellow to colourless gem material from Brazilian pegmatites.
Phosphophyllite — hydrated zinc iron phosphate — produces some of the most beautiful and most fragile gem crystals known, the blue-green Bolivian material being the standard reference.
Phosphate formation environments
The variety of phosphate gem species reflects the variety of environments in which phosphate minerals form. Sedimentary phosphate deposits — including the giant marine phosphorite formations exploited for fertiliser — are the principal source of apatite at non-gem scales. Hydrothermal phosphate veins associated with copper, aluminium, and manganese mineralisation produce turquoise, lazulite, and variscite. Granite pegmatites yield brazilianite, amblygonite, herderite, and the rarer phosphate species in association with quartz, feldspar, lepidolite, and tourmaline.
Identification
Phosphate gemstones are identified by their crystal structure, chemistry, and standard gemmological properties — refractive index, specific gravity, dichroism — as documented in Dana's System of Mineralogy and the GIA reference materials. Spectroscopic methods including ultraviolet-visible-near-infrared and infrared spectroscopy support identification of the more challenging phosphate species and of treatments such as the wax and resin impregnation routinely used for porous turquoise.
In the trade
Among the phosphates, turquoise dominates the commercial trade and is the only phosphate gemstone routinely encountered outside specialist dealers and lapidary venues. Apatite, particularly the Paraíba-style neon blue Madagascar material, has a small but enthusiastic collector following, and the rarer phosphates — phosphophyllite, brazilianite, herderite — trade primarily in the collector-stone and mineral-specimen markets. For working with phosphate gemstones, careful attention to hardness and to the presence of cleavage is essential; many phosphates are softer and more fragile than the silicate gem species more familiar in commercial jewellery.