How spinifex texture is formed?

The distinctive spinifex texture is formed by skeletal, platy, or acicular crystals of olivine, orthopyroxene, or clinopyroxene, or their pseudomorphs in ultramafic and mafic lavas or silicate-rich furnace slag. The complex skeletal or platy crystals may show random orientation, or may display a preferred arrangement.

How is spinifex texture different from the skeletal texture?

Large skeletal crystals may have crystallized at depths, many metres below the surfaces of the flows where cooling rates must have been low. In contrast, the morphologies of the olivine or pyroxene crystals in spinifex-textured lavas resemble those produced experimentally at cooling rates never less than about 30°C/hr.

What are Komatiites Why are Komatiites rare?

Komatiite is a very rare ultramafic extrusive igneous rock, only formed from extremely hot magma. These hot lava flows may rapidly cool, resulting in a characteristic “spinifex” texture of komatiite. The source lava is so hot that the lava is thin, almost watery in consistency.

Is komatiite an igneous?

They are one variety of extrusive ultramafic igneous rock (although a komatiite in Canada has been interpreted to be an intrusive sill). They are named after the Komati River in South Africa, the type locality. Komatiite is an exceedingly rare type of lava. No volcano on Earth erupts this material today.

What is Trachytic texture?

Trachytic is a texture of extrusive rocks in which the groundmass contains little volcanic glass and consists predominantly of minute tabular crystals, namely, sanidine microlites. The microlites are parallel, forming flow lines along the directions of lava flow and around inclusions.

What is Glomeroporphyritic texture?

Glomeroporphyritic or glomerophyric is a term used to describe a porpyritic texture in which phenocrysts are clustered into aggregates called glomerocrysts or crystal clots. Glomeroporphyritic textures are common and often included plagioclase and pyroxenes in basic rocks.

What are Komatiites rare?

Komatiites are very rare igneous rocks. Komatiite is an exceedingly rare type of lava. No volcano on Earth erupts this material today. Komatiites are essentially restricted to the Archean (4.55 to 2.5 billion years ago), when Earth’s heat flux was much higher.

Are ultramafic rocks rare?

Ultramafic rocks are dominated by olivine or olivine and pyroxene. Such rocks are rare on the Earth’s surface, but they dominate the mantle.

Why are there no longer ultramafic lava flows forming on Earth?

Structure of the Earth. Image taken from here. Although ultramafic rock makes up most of the Earth, geologists rarely find ultramafic rocks on Earth’s thin crust. This is because when the Earth’s mantle melts to produce magmas, it does not melt 100%.

What does Granophyric texture mean?

A granophyric texture is an intergrowth of quartz and alkali feldspar in an igneous rock that is less well defined than a graphic texture and often is somewhat radiating. Granophyric texture can be similar to micrographic texture and to the coarser graphic intergrowths of quartz and alkali feldspar common in pegmatite.

What is Intergrowth texture?

From Wikipedia, the free encyclopedia. In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica.

What causes the texture of spinifex in komatiite?

Solidification under the conditions of supercooling (low nucleation rates and rapid crystal growth rates) produces a few large skeletal or dendritic crystals. However, this model cannot explain the occurrence of spinifex texture within thick komatiite flows well below the upper chilled crust.

What is the platy olivine texture in komatiites?

The formation of platy olivine spinifex, the texture that characterizes komatiite lavas, has long been enigmatic. A major problem is that the dendritic morphology of the olivine resembles that of crystals grown in laboratory experiments at high cooling rates (>50°C/h),…

Where does the name Spinifex texture come from?

The term derives from an Australian spiky grass ( Triodia spinifex ). Petrologists who initially studied spinifex texture noticed that the skeletal olivine phenocrysts in komatiites resemble quench crystals (formed at very rapid cooling rates) in experimental melt.

How does the cooling rate affect the spinifex texture?

When the cooling rate is high, the thermal gradient has a negligible effect on the texture and the crystals have a random orientation, like that in the upper parts of komatiite flows. Spinifex is one of the most spectacular magmatic textures.