Microlites are minute crystals in an amorphous matrix. In
igneous petrology, 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
phenocrysts, which can be seen with little or no magnification, microlites are generally formed in rapidly cooled (
quenched)
basaltic
lava, 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. Low viscosity permits rapid
nucleation
In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
and ion migration, necessary for crystal formation. The high
silica content of
rhyolitic 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.
Obsidian is produced from felsic lava, rich in the lighter elements s ...
) when they cool rapidly from a fully melted liquid state; though many obsidians also contain microlites.[ Low viscosity mafic magmas 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 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
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