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 utilizes a ...

, 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 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

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, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ph ...

)

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

, 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

. Low viscosity permits rapid nucleation 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 , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

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 mineral ...

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

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

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, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

collected from Hawaiian lava fountains, where rapid cooling favors their formation.

Sideromelane Sideromelane is a vitreous basaltic volcanic glass, usually occurring in palagonite tuff, for which it is characteristic. It is a less common form of tachylite, with which it usually occurs together; however it lacks the iron oxide crystals di ...

is a light brown basaltic glass, also formed in these eruptions, with and without microlites.

References

Igneous petrology Tephra {{Geology-stub