Peridotite
The rock the Earth's mantle is made of — olivine and pyroxene, rarely seen at the surface, and the source of chromium and much of the world's magnesium.
Peridotite is the most abundant rock in the Earth by volume and one of the rarest at its surface. The upper mantle is made of it — a dense, dark, coarse rock of olivine with subordinate pyroxene — and the crust we live on is, in effect, the light scum that separated from it.
Where it does outcrop, it is generally because a slice of oceanic mantle was pushed onto a continent during a collision rather than subducted. Those slices, called ophiolites, are how the sea floor and the mantle beneath it can be walked over on dry land.
How it forms
Peridotite is not so much formed as left behind. Partial melting of the mantle extracts the components that make basalt, and the residue — enriched in magnesium and depleted in the elements that went into the melt — is peridotite. The more melt has been extracted, the more magnesian and the more refractory the remaining rock.
At the surface it is thermodynamically out of place. Olivine reacts readily with water to form serpentine, so exposed peridotite converts to serpentinite from the outside in, and fresh peridotite is generally found only in the interiors of larger bodies or in xenoliths carried up rapidly by basalt or kimberlite.
That reactivity has become interesting for a reason unrelated to geology: the same reaction consumes carbon dioxide, and peridotite is among the most studied rocks for permanent mineral carbon storage.
Economic significance
Chromium is the metal peridotite supplies most directly. Chromite crystallises early from mafic magma and accumulates in ultramafic bodies, and essentially all the world's chromium — and so all its stainless steel — comes from these rocks or from the layered intrusions that are their close relatives.
Nickel arrives less directly. Weathering a peridotite in a tropical climate strips the magnesium and silica and leaves a nickel-enriched laterite behind, and roughly half the world's nickel now comes from those weathered profiles rather than from sulfide ore.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Olivine — mineral · 40–90% · the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition
- Forsterite — mineral · the magnesian end of the olivine series, and what mantle olivine actually is — around ninety per cent forsterite in most peridotite
- Chromite — mineral · an early-crystallising accessory that can accumulate into seams of near-pure ore — the source of essentially all the world's chromium
is a source for
- Serpentinite — rock · serpentinite is hydrated peridotite; the protolith is the mantle rock itself
- Basalt — rock · partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt
- Gabbro — rock · the same mantle melt as basalt, trapped at depth and cooled slowly enough to grow visible crystals
- Kimberlite — rock · generated far deeper in the mantle than ordinary basalt, and carrying fragments of the peridotite it passed through as xenoliths
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing