Part II: Non-foliated Metamorpic Rocks

Classification of Non-foliated Metamorphic Rocks
Composition
Quartz (SiO2) Calcite (CaCO3) Amphibole
Rock NameQuartziteMarbleAmphibolite
Parent RockQuartz SandstoneLimestoneBasalt/Gabbro

QUARTZITE

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

    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

    Amphibolite is a granoblastic rock formed by the metamorphism of gabbro or basalt. This dark-colored rock is predominantly composed of macroscopic amphibole.
  • Grade: medium
  • 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

    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

    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

    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

    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.

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