Part II: Non-foliated Metamorpic Rocks
Classification of Non-foliated Metamorphic Rocks
| Quartz (SiO2)
|| Calcite (CaCO3)
|| Amphibole |
|Parent Rock||Quartz Sandstone||Limestone||Basalt/Gabbro|
Quartzite is a non-foliated, granoblastic (intergrown, interlocking, equidimensional, enlarged grains), metamorphic rock composed predominantly of quartz. It forms by the regional, burial or contact metamorphism of quartz-rich sandstones or chert.
Grade: low to high
Parent Rock: quarrz-rich sandstone or chert
Nature of Metamorphism: burial, contact or regional (cannot be distinguished in hand sample)
Impt. Characteristics: composed almost entirely of granoblastic quartz; more lithified and coarser grained than sandstone
Marble is a non-foliated metamorphic rock composed of mostly calcite. It forms by the metamorpism of limestone and other calcite-rich rocks.
Grade: low to high
Parent Rock: limestone or chalk
Nature of Metamorphism: burial, contact or regional (can not be distinguished in hand sample)
Impt. Characteristics: composed almost entirely granoblastic calcite; more lithified and coarser grained than limestone or chalk
Amphibolite is a granoblastic rock formed by the metamorphism of gabbro or basalt. This dark-colored rock is predominantly composed of macroscopic amphibole.
Parent Rock: basalt or gabbro
Nature of Metamorphism: burial or regional
Impt. Characteristics: dark-colored, composed mostly of visible amphibole grains, sometimes foliated
Part III: Common Metamorphic Minerals
There are several index minerals found in metamorphic rocks in addition to the common rock-forming minerals such as quartz, mica and calcite. These index minerals can be used to delineate the pressure and temperature conditions (grade) of metamorphism. The following diagram shows the general grade of four metamorpic index minerals.
(LOW) Chlorite-> Garnet-> Staurolite-> Kyanite (HIGH)
Chlorite is a green, sheet-silicate that commonly occurs in low-grade metamorphic rocks. A single well-defined direction of cleavage is commonly aligned perpendicular to the directed stress during regional metamorpism. The above sample is a chlorite schist that has been deformed by multiple episodes of regional metamorphism.
Garnet is a medium-grade silicate mineral commonly found in schist and gneiss. It is characterized by a distinctive equidimensional 12-sided crystal form, glassy luster, high hardness and absence of cleavage. The color is highly variable, but red-brown is most common. It is used as both a gemstone and abrasive.
Staurolite is a metamorphic mineral that is commonly found in aluminum-rich medium-grade metamorphic rocks such as mica schists. It is brown to black in color and prismatic in shape. Twinned (intergrown) crystals commonly form cross shapes resulting in the 'fairy cross' name also being used for this mineral.
Kyanite is a blue to light-green mineral which forms blade-like crystals. It forms at medium temperatures and high pressures in aluminum-rich regional metamorpic rocks. It is commonly used in the manufacture of high-temperature ceramics.