Quartz Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons. Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid. When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces. It is also found as microcrystalline masses, such as in the rock chert, and as fibrous masses, such as in chalcedony. Vitreous lusture. As visible crystals, Quartz is one of the more common rock forming minerals. It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks. It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand. Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary r
TYPES OF METAMORPHIC ROCKS
Figure 1.1
There are two major subdivisions of metamorphic rocks :-
1. Foliated metamorphic rocks
2. Non foliated metamorphic rocks
Figure 1.2 ( foliated rock)
Figure 1.7 ( Amphibolite)
Figure 1.8 ( Hornfels)
1. Foliated – These have a planar foliation caused by the preferred orientation (alignment)
of minerals and formed under differential stress.
They have a significant amount of sheet silicate (platy minerals and are classified by
composition, grain size, and foliation type.2. Non-foliated – These have no evident planar fabric or foliation, crystallized under
conditions where there was no differential stress, and are comprised of equant minerals
only. These are classified mainly by the minerals present or the chemical composition of
the protolith.
Foliated Metamorphic Rocks
Figure 1.3 ( Slate)
Slate - Slates form at low metamorphic grade by the growth of fine grained chlorite and
clay minerals. The preferred orientation of these sheet silicates causes the rock to easily
break along the planes parallel to the sheet silicates, causing a slatey cleavage. Note that
in the case shown here, the maximum stress is applied at an angle to the original bedding
planes, so that the slatey cleavage has developed at an angle to the original bedding.
Figure 1.4 ( Phyllite)
Phyllite - Fine mica-rich rock, formed by low – medium grade metamorphism. In a
phyllite, the clay minerals have recrystallized into tiny micas (biotite and muscovite
which reflect a satiny luster. Phyllite is between slate and schist.
Schist - The size of the mineral grains tends to enlarge with increasing grade of
metamorphism. Eventually the rock develops a near planar foliation caused by the
preferred orientation of sheet silicates (mainly biotite and muscovite). Quartz and
Feldspar grains, however show no preferred orientation. The irregular planar foliation at
this stage is called schistosity
Quartz, Feldspars, Kyanite, Garnet, Staurolite, and Sillimanite.
When these non-mica minerals occur with a grain size greater than the rest of the
rock, they are called pophyroblasts.
Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark
colored minerals like hornblende and pyroxene start to grow. These dark colored
minerals tend to become segregated in distinct bands through the rock, giving the rock a
gneissic banding. Because the dark colored minerals tend to form elongated crystals,
rather than sheet- like crystals, they still have a preferred orientation with their long
directions perpendicular to the maximum differential stress.
Granulite - At the highest grades of metamorphism all of the hydrous minerals and sheet
silicates become unstable and thus there are few minerals present that would show a
preferred orientation. The resulting rock will have a granulitic texture that is similar to a
phaneritic texture in igneous rocks.
Migmatites – If the temperature reaches the solidus temperature (first melting
temperature), the rock may begin to melt and start to co-mingle with the solids. Usually
these melts are felsic with the mafic material remaining metamorphic.
NON-FOLIATED METAMORPIC ROCKS
Figure 1.6 ( Quartzite)
Non-foliated rocks lack a planar fabric . Absence of foliation possible for several reasons:
Rock not subjected to differential stress.
Dominance of equant minerals (like quartz, feldspar, and garnet).
Absence of platy minerals
Amphibolite - These rocks are dark colored rocks with amphibole (usually hornblende) as their
major mineral. They are usually poorly foliated and form at intermediate to high grades of
metamorphism of basaltic or gabbroic protoliths.
Hornfels - These are very fine grained rocks that usually form as a result of magma intruding
into fined grained igneous rocks or shales. The magma causes a type of metamorphism called
contact metamorphism (to be discussed later).
Quartzite - A rock made up almost entirely of quartz. They are formed by metamorphism of
quartz arenites (sandstones). Since quartz is stable over a large range of temperatures and
pressures, no new minerals are formed during metamorphism, and the only metamorphic effect
that occurs is recrystallization of the quartz resulting in interlocking crystals that make up a
very hard rock. (figure 1.6)
Figure 1.9 ( Marble)
Marble - A limestone or dolostone made up only of calcite or dolomite will metamorphose to a
marble which is made mostly recrystallized calcite or dolomite. The recrystallization usually
obliterates all fossils. Marbles have a variety of colors and are often complexly banded. They
are commonly used as a decorative stone.
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