Microlites are minute crystals in an amorphous matrix. In

igneous petrology Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks uses a num ...

, the term microlitic is used to describe vitric (glassy, non-crystalline, amorphous) matrix containing microscopic crystals.''Dictionary of Geological Terms'', 1962, American Geological Institute Microlitic rocks are a type of hypocrystalline rocks.''Petrology The Study of Igneous...Rocks'', Loren A. Raymond, 1995, McGraw-Hill, p. 27 Unlike ordinary

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s, which can be seen with little or no magnification, microlites are generally formed in rapidly cooled (

quenched In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such a ...

)

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

, where cooling rates are too high to permit formation of larger crystals. Microlites are sometimes referred to as “small quench crystals”.Schmincke, Hans-Ulrich, ''Volcanism'', 2004, Springer-Verlag, Chapter 12, Fire and Water, Rapid Cooling. They form more easily in basaltic lava eruptions, which have relatively low

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

. Low viscosity permits rapid

nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

and ion migration, necessary for crystal formation. The high

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

content of

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

lavas gives them much higher viscosities. Such lavas tend to form glass (

obsidian Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...

) when they cool rapidly from a fully melted liquid state; though many obsidians also contain microlites. Low viscosity

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

s must be quenched very rapidly from a high temperature to form glass that is free of any crystalline content. Microlites have been found in

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

collected from Hawaiian lava fountains, where rapid cooling favors their formation. Sideromelane is a light brown basaltic glass, also formed in these eruptions, with and without microlites.

References

Igneous petrology Tephra {{igneous-petrology-stub