Miarolitic cavities (or miarolitic texture) are typically crystal-lined irregular cavities or
vugs most commonly found in
granitic
A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...
pegmatites, and also in a variety of
igneous rocks
Igneous rock ( ), or magmatic rock, is one of the three main Rock (geology)#Classification, rock types, the others being sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rocks are formed through the cooling and solidifi ...
. The central portions of
pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...
s are often miarolitic as the pegmatite
dike crystallizes from the outside walls toward the center. The
volatile portion of the
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 ...
is gradually excluded from the forming crystal phases until it becomes trapped within the body and forms the cavities which often contain
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
s of elements incompatible with the typical
silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
granitic mineralogy.
The miarolitic cavities and miarolitic pegmatites are sources of rare and unusual minerals containing elements not found in abundance in normal igneous rocks. Minerals containing lithium, rubidium, beryllium, boron, niobium, tantalum, tin, bismuth, fluorine and other elements can be found.
The term ''miarolitic'' comes from the
Italian
Italian(s) may refer to:
* Anything of, from, or related to the people of Italy over the centuries
** Italians, a Romance ethnic group related to or simply a citizen of the Italian Republic or Italian Kingdom
** Italian language, a Romance languag ...
''miarolo'' in reference to a variety of granite, rich in cavities, from Baveno in northern Italy.
References
PDFLondon, David, ''Formation of tourmaline-rich gem pockets in miarolitic pegmatites'' American Mineralogist, Volume 71, pages 396–405, 1986
Mineralogy
Petrology
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