Picotite
Picotite
The chrome-rich, aluminium-bearing dark spinel of ultramafic rocks
Picotite is a varietal name for dark brown to nearly black spinel coloured by elevated chromium and aluminium content, occurring as accessory grains and small masses in chromite-rich ultramafic rocks. It is a member of the spinel group with a composition that is best written as (Mg,Fe)(Al,Cr)2O4, sitting between magnesiochromite and pleonaste in the chemical continuum that defines the chrome-spinel series. In strict mineralogical use the term is reserved for the chrome-aluminium spinels of serpentinites and related ultramafic terrains; in looser gemmological use the name is sometimes extended to any opaque or near-opaque chrome-rich spinel, regardless of host rock.
Origin of the name
The variety takes its name from the French mineralogist François Picot, and the type locality is in the Binn Valley, Switzerland, where the species was first described in association with serpentinite. The name predates much of the modern spinel-group nomenclature and persists in older literature and in the field language of geologists working ultramafic terrains. In contemporary gemmology the term has narrowed to a varietal label rather than a formally accepted mineral species in its own right; the species is spinel, the variety is picotite.
Composition and structure
Spinel is a cubic oxide with the general formula AB2O4, where the A-site is occupied by divalent cations — typically magnesium and ferrous iron — and the B-site by trivalent cations such as aluminium and chromium. Picotite sits at the chrome-rich end of this aluminium-chromium spectrum, with chromium occupying enough of the B-site positions to push the colour into deep brown, brown-black, or sooty olive-black. The hardness is the spinel-group standard of around 7.5 to 8 on the Mohs scale, and the specific gravity runs slightly higher than for end-member magnesium spinel because of the chromium and iron content, typically 4.0 to 4.7 depending on composition.
Geological occurrence
Picotite is a rock-forming accessory in chromite-rich ultramafic bodies — peridotites, dunites, and the serpentinites that derive from them. It commonly occurs as small octahedral crystals or rounded grains scattered through the silicate matrix, sometimes as the dark cores of chromite grains, and sometimes as discrete bands in podiform chromite deposits. Worldwide, the variety is found wherever ultramafic rocks are exposed: the Bushveld and Great Dyke complexes of southern Africa, the ophiolite belts of Oman and the eastern Mediterranean, the Ural Mountains, the Stillwater Complex in Montana, the Zhob Valley of Pakistan, and the New Caledonian and Philippine ultramafics. The Quebec type-locality material remains a reference for older descriptions, but is dwarfed in volume by these larger ultramafic terrains.
Appearance and gem use
Picotite is most often opaque to barely translucent, with a vitreous to slightly resinous lustre and a colour that runs from dark brown through olive-black to true black. Transparent crystals of gem quality are scarce; when they occur, they tend to be small — under a carat — and are cut chiefly for systematic mineral collectors and species-completionist faceters rather than for the commercial jewellery trade. A faceted dark-green to brown picotite carries some of the visual character of dark green tourmaline or chrome-rich pyrope, but without the depth and colour shift that make those species commercially significant. The variety has no real foothold in jewellery, and most gem-grade material trades in mineral-specimen channels rather than at gem shows.
Trade usage and confusion
The principal source of confusion is the looseness with which the name is applied. Some authors restrict picotite to the dark brown chrome-aluminium spinels of serpentinites; others extend it to any chrome-rich spinel, including transparent dark-green and olive material. Gemmological reports from major laboratories generally identify the species simply as spinel, with chemistry noted only when the question is material to the report. A buyer being offered "picotite" as a faceted stone should expect a chrome-bearing dark spinel of broadly the composition described here, but should not assume any particular host rock or origin without further documentation.
Distinguishing from chromite
Picotite is closely related to chromite — the two sit on the same chemical continuum — and the boundary is drawn by the relative occupancy of the B-site. When chromium dominates over aluminium, the mineral is chromite; when aluminium dominates, the mineral is picotite or, with less chromium, pleonaste. The transition is gradual and the field distinction is rarely confident; in petrographic work the determination is made by microprobe analysis. For gem purposes the question is largely academic — both chromite and picotite are dark, opaque to nearly opaque, and rarely cut.
In the trade
Picotite is a name a working coloured-stone dealer rarely encounters and a name a coloured-stone buyer almost never asks for. It is mineralogically interesting and geologically significant — the chrome spinels of ultramafic rocks are a window into mantle chemistry — but commercially marginal. Cut stones reach the market through specialist mineral and faceting circles, and any larger or transparent example is a curiosity rather than a category. Buyers asking about picotite are generally collectors or students of the spinel group rather than jewellery clients; the conversation belongs to that more technical end of the trade.