Of the three principal rock types (igneous, sedimentary, and metamorphic),
igneous rocks can be thought of as "primary" rocks because
they crystallise from a liquid. Sedimentary and metamorphic rocks may be thought of as derivative
Igneous rocks are rocks formed from the crystallization of a liquid
(molten rock). Igneous rocks may be divided into three - categories.
Intrusive or plutonic rocks crystallise from magma beneath
the earth's surface.
Hypabysal rocks - crystalise from magma at shallow depth
below the surface
Extrusive or volcanic rocks crystallise from lava at the
The texture of an igneous rock (fine-grained vs coarse-grained) is
dependent on the rate of cooling of the melt:
slow cooling forms large crystals - magmas and their
resultant plutonic rock bodies cool and crystallise slowly
and are characterized by coarse-grained texture in which the mineral
crystals are visible to the unaided eye
fast cooling forms small crystals -lavas cool quickly at the
and are characterized by fine-grained texture, in which the crystals
are too small to be seen by the unaided eye.
very quickly cooled - lavas, typically those quenched
in water, will have a glassy texture. They cool too quickly to
Glasses do not have an orderly arrangement of atoms and there are
therefore no minerals, in the strict sense, in them. Volcanic glass is
In addition to texture, igneous rocks may are classified according to
their chemical composition.
The most general classification is based on the relative abundance in a
rock of felsic (feldspar and silica-quartz) minerals vs
mafic( magnesium and ferrum or iron) minerals.
felsic minerals (quartz, K feldspar, etc) are light colored
while - felsic minerals have the lowest melting points (600 to 750 °C)
mafic minerals (hornblende, pyroxenes) are normally dark
colored - mafic minerals have higher melting points (1000 to
Identification by crystal size, felsic / mafic composition and colour
A more detailed identification chart based on felsic / mafic mineral
can be thought of as an idealized order of crystallization of a
However, not all of these minerals will be crystallised together in the
mafic magmas will begin crystallizing olivine and continue
with pyroxenes and calcium rich plagioclase feldspar.
Some amphiboles may also crystallise before the melt is used up. Mafic
melts don’t have enough silica to crystallise potassium feldspar,
Minerals form in the cooling magama in a "discontinuous series"
, that is first one mineral --> then a different mineral -->
then a different mineral and so on...
felsic magmas don't have enough iron, magnesium, and
calcium to form olivine, pyroxene, or calcium plagioclase.
The first-formed minerals in a felsic melt are amphiboles (hornblende)
or biotite mica, along with some intermediate or sodium plagioclase.
Eventually, as the melt continues to cool and becomes richer in silica
(as the metal cations are used up preferentially in the double chain
and sheet silicates) potassium feldspar and quartz crystallise.
minerals form in a " continuous series" that is to say ... for
example: that the mineral feldspar remains as feldspar but its
composition (the percent of potassium vs calcium) increases as the
This system appears straight forward but as magmas cool their chemical
composition changes as higher temperature elements drop out of the
solution as minerals.
So for example a mafic magma can turn into a felsic magma as it cools...
is a feature of phaneritic rocks in which all of the minerals have
anhedral shapes; it is characteristic of some mafic and ultramafic rocks,
such as dunite and pyroxenite. Hypidiomorphic-granular
igneous texture in which most of the mineral grains are subhedral. Typical
of granite, granodiorite, quartz monzonite, etc. Porphyritic
rocks in which larger grains are contained in a finer grained matrix. May
occur in either intrusive or extrusive rocks, but it is most common in
extrusive rocks such as basalt, andesite, dacite, and rhyolite. Pyroclastic
produced by explosive volcanism, broken clasts of mineral, lithic clasts,
and glass shards are common. The clasts may be either sorted or unsorted.
Common in tuffs. Intergranular
plagioclase laths with interstitial pyroxene grains that are smaller than
the plagioclase; commonly found in basalts. Intersertal
Small feldspars with glass or altered glass interstitial to the feldspars;
common texture of basalts. Ophitic texture:
Pyroxene grains partially or completely surround plagioclase laths; common
texture of gabbros and basalts. Trachytic
Subparallel feldspars formed during flow in volcanic rocks. Spinifex texture:
Interlacing olivine or pyroxene, a texture formed by quenching in
Other features seen are:
Exsolution - occurs
within grains on certain minerals (pyroxenes and feldspars). This can
give an indication of pressure and hence depth.
Xenoliths - bits of the
rock into which the magma intruded
- when a mineral grows which is denser than the magma, it will sink to
the base of the chamber causing a cumulate layer. Minerals may form
from liquid trapped between the grains - interstitial minerals.