Matrix (geology)

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Potassium feldspar phenocrysts within a fine
grained matrix in a granite porphyry.

The matrix or groundmass of rock is the finer grained
mass of material in which larger grains, crystals or clasts are
embedded.

The matrix of an igneous rock consists of
finer grained, often microscopic, crystals in which larger crystals (phenocrysts)
are embedded. This porphyritic texture
is indicative of multi-stage cooling of magma. For example, porphyritic andesite will
have large phenocrysts of plagioclase in a
fine-grained matrix. Also in South Africa, diamonds are often
mined from a matrix of weathered clay-like rock (kimberlite) called "yellow
ground".

The matrix of sedimentary rocks is
finer grained sedimentary material, such as clay or silt, in which larger grains or
clasts are embedded. It also used to describe the rock material in which a
fossil is embedded.

[edit]
Cementations

All sediments are at first in an incoherent condition (e.g. sands, clays and
gravels, beds of shells, etc.), and in this state they may remain for an
indefinite period. Millions of years have elapsed since some of the early Tertiary strata
gathered on the ocean floor, yet they are quite friable (e.g. the London
Clay) and differ little from many recent accumulations. There are few
exceptions, however, to the rule that with increasing age sedimentary rocks
become more and more indurated, and the older they are the more likely it is
that they will have the firm consistency generally implied in the term
"rock".

The pressure of newer sediments on underlying masses is apparently one cause
of this change, though not in itself a very powerful one. More efficiency is
generally ascribed to the action of percolating water, which takes up certain
soluble materials and redeposits them in pores and cavities. This operation is
probably accelerated by the increased pressure produced by superincumbent
masses, and to some extent also by the rise of temperature which inevitably
takes place in rocks buried to some depth beneath the surface. The rise of
temperature, however, is never very great; we know more than one instance of
sedimentary deposits which have been buried beneath four or five miles of
similar strata (e.g. parts of the Old Red Sandstone),
yet no perceptible difference in condition can be made out between beds of
similar composition at the top of the series and near its base.

The redeposited cementing material is most commonly calcareous or siliceous.
Limestones,
which were originally a loose accumulation of shells, corals, etc., become
compacted into firm rock in this manner; and the process often takes place with
surprising ease, as for example in the deeper parts of coral reefs, or even in
wind-blown masses of shelly sand exposed merely to the action of rain. The
cementing substance may be regularly deposited in crystalline continuity on the
original grains, where these were crystalline; and even in sandstones (such as Kentish
Rag) a crystalline matrix of calcite often envelops the sand
grains. The change of aragonite to calcite and of
calcite to dolomite, by forming new
crystalline masses in the interior of the rock, usually also accelerates
consolidations. Silica is less easily soluble in
ordinary waters, but even this ingredient of rocks is dissolved and redeposited
with great frequency. Many sandstones are held together by an infinitesimal
amount of colloid or cryptocrystalline silica; when freshly dug from the quarry
they are soft and easily trimmed, but after exposure to the air for some time
they become much harder, as their siliceous cement sets and passes into a rigid
condition. Others contain fine scales of kaolin or of mica. Argillaceous materials may
be compacted by mere pressure, like graphite and other scaly
minerals.[1]

[edit]
References

^
This article incorporates text from a
publication now in the public
domain: Chisholm,
Hugh, ed (1911). "Petrology". Encyclopædia
Britannica (11th ed.). Cambridge University Press.

Retrieved from "http://en.wikipedia.org/w/index.php?title=Matrix_(geology)&oldid=455512454"

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This page was last modified on 14 October 2011 at
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