spinifex
Composed of dendritic plates, which present as the scored texture of bundled, spine-like structures typical of komatiites.
spinifex: the skeletal crystal texture of fast-cooled ultramafic rock
Spinifex texture appears in komatiites and other ultramafic lavas that cool rapidly enough that olivine crystallizes as thin, needle-like dendrites rather than chunky grains. The result looks like a bundle of spines or skeletal plates radiating outward, often with a distinctive cross-hatched or scored pattern when you look at a cut surface. This texture is diagnostic: if you see spinifex, you are looking at a rock that solidified from a melt hotter than normal basalt, typically above 1200 degrees Celsius.
The name comes from the spinifex grass of Australian deserts, which grows in a similar radiating, spiky pattern. Early petrographers who first described these rocks in komatiite formations in Western Australia saw the resemblance and the name stuck. In the mining industry, spinifex texture is important as a field identifier and a clue to the thermal history of the rock.
Where spinifex matters in mining
Komatiites with spinifex texture often host nickel sulfide deposits, particularly in Archean greenstone belts. The rapid cooling that produces spinifex typically occurs in thin lava flows or the upper margins of thicker flows, and these zones can concentrate ore minerals. Geologists use spinifex as a textural marker to predict where ore bodies are likely to occur and to understand the original orientation and thickness of mineralized flow units.
Spinifex should be distinguished from other needle-like crystal structures. Acicular (needle) pyroxene can create a somewhat similar appearance, but pyroxene crystals are usually more uniform and less branching than true dendritic olivine spinifex. Under the microscope, the dendritic character, the composition of the minerals involved, and the way they interlock make spinifex unmistakable.
In drill core and hand specimen, spinifex can appear quite variable depending on the plane of cut relative to the crystal growth direction. A cut perpendicular to the spine axes shows the classic radiating pattern; a parallel cut shows mainly linear striations. This anisotropy in appearance has caught out geologists new to komatiite terrains, so orientation of the core or sample matters when you are trying to confirm the texture.