Gabbro
Basalt's coarse-grained equivalent, cooled slowly at depth — and, where it settled into layers, the source of the world's platinum, most of its chromium and much of its nickel.
Gabbro is what basalt would have been if it had cooled slowly. Same magma, same chemistry, different history: trapped at depth instead of erupted, it had time to grow crystals large enough to see, and the result is a coarse dark rock of plagioclase and pyroxene, often with olivine.
Most of the lower oceanic crust is gabbro, which makes it one of the commonest rocks on the planet even though little of it is exposed. Where it is quarried it is usually sold as black granite, a trade name that is geologically wrong in every particular and has been in use for over a century.
How it forms
Gabbro crystallises from mantle-derived magma in chambers within the crust. The interesting cases are the large intrusions where that chamber stayed molten long enough for crystals to settle through it, sorting themselves by density and crystallisation order into layers that can be followed for tens of kilometres.
Those layered intrusions are where the ore is. Chromite crystallises early and settles into seams of near-pure ore; the platinum-group metals and nickel-copper sulfides concentrate at specific horizons where the magma became saturated in sulfur, often as layers only a metre or two thick. The Bushveld Complex in South Africa is the type example and holds most of the world's platinum in exactly this arrangement.
What makes those horizons form where they do — magma recharge, contamination by wall rock, or a change in pressure — is still argued about, which is a problem for exploration because the layers are the entire deposit.
Economic significance
The platinum-group metals are a near-perfect example of geological concentration deciding an industry. They are extraordinarily rare in the crust as a whole, and almost all of what is accessible sits in a handful of layered gabbroic intrusions — Bushveld, Great Dyke, Stillwater, Noril'sk. Catalytic converters, hydrogen electrolysers and a good deal of chemical industry rest on that.
Gabbro is also a serious dimension stone. It takes a high polish, holds it outdoors far better than most light-coloured stone, and is the standard material for memorials and for building cladding where a dark finish is wanted.
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
- Plagioclase — mineral · 35–65% · one of the two essential minerals, generally calcium-rich labradorite rather than the sodic plagioclase of granite
- Anorthite — mineral · the calcium end of the plagioclase series, towards which gabbroic plagioclase sits — the same crystals as the group edge above, named more precisely
- Olivine — mineral · in the olivine gabbros, and abundant in the lower layers of a layered intrusion
- Chromite — mineral · in layered intrusions specifically, as seams that settled out of the magma early — not a feature of ordinary gabbro
- Pentlandite — mineral · the nickel sulfide of layered intrusions, concentrated where the magma became saturated in sulfur
- Sperrylite — mineral · the platinum arsenide of the reef horizons, and one of the few platinum minerals abundant enough to matter commercially
- Ilmenite — mineral · with magnetite in the iron-titanium oxide layers, which are worked for titanium and vanadium as well as iron
- Magnetite — mineral · in the upper layers of a layered intrusion, where vanadium-bearing magnetitite seams form
is an input to
- Quarrying — process · for the dark polished cladding and memorials sold as black granite
is an alternative to
- Granite — rock · as dimension stone, both sold as granite by the trade. Gabbro gives the dark polished finish and granite the pale speckled one, and neither is chosen for anything but appearance and price
is sourced from
- Peridotite — rock · the same mantle melt as basalt, trapped at depth and cooled slowly enough to grow visible crystals
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing