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The 'matrix' or 'groundmass' of rock is the fine-grained mass of material in which larger grains or crystals are embedded.
The matrix of an igneous rock consists of fine-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 a fine-grained clay or silt in which larger grains are embedded.

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Cementations

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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 that 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.

References