Metamorphic Rocks Examples

Gneiss   Description      Banded foliation composed of layers/zones of light colored quartz, feldspar interlayered with layers/zones of dark colored biotite and amphibole. The minerals bands in this specimen are just beginning to develop, but are distinctly separate from each other putting it in the gneissic category .      Tectonic Association      Gneisses are typically associated with major mountain building events when shales or clay rich sandstones (wackes), or felsic igneous rocks (e.g. granite, granodiorite, etc.) are metamorphosed through depth of burial, and proximity to batholiths

Hornfels Composition Hornfels can have any of a wide diversity of microscopic minerals, all too small to see or identify without thinsection and a specialized microscope. Description      Hornfels is a nondescript rock which varies widely in appearance. This specimen is typical, a dark gray to black,dense, hard, compact, massive rock breaking with conchoidal fracture into sharp angular pieces. It is harder than glass and has a dull luster.      Be cautions; variations on this, especially color, are common – as well as confusion with similar looking rocks, such as basalt.

Hornfels Continued Tectonic Association      Hornfels results from the contact metamorphism of shales or other clay rich rocks in the viscinity of an igneous intrusion. As a result they have no specific tectonic association or meaning. They can form just about anywhere.

Marble-Limestone Description           Recrystallized limestone producing interlocking grains of calcite. This is a saw cut specimen, wet with water to bring out the grains.      Marble is an extremely variable rock in appearance since it depends on what limestone was the parent, and the degree of metamorphism it underwent. It can be virtually any color from white to black to red to green, etc., and sometimes it will contain metamorphosed fossils or other components. The most definitive test for the rock would be acid (vigorous bubbling), and a hardness less than glass.

Marble-Limestone Type of Metamorphism Composition      None specific. Limestone will convert into marble with virtually any type of metamorphism. The main exception is hydrothermal metamorphism which introduces silica rich fluids, in which case the rock turns into calcium rich silicate skarn minerals. Composition Calcite: CaCO3

Quartzite Description This is a typical quartzite. They are dense, hard rocks, generally uniform in texture, composed of fused quartz sand grains. Often they rock has a granular appearance on a weathered surface, but along a broken surface the sand grains are usually split. Also, unlike most of the nondescript rocks, quartzite has a glassy luster Composition      Fused quartz sand grains. Typically white or gray, although other pale colors are common; rarely a darker color.

Quartzite Continued Tectonic Association      Quartzite results from the almost any metamorphism of a quartz sandstone, and as such have no specific tectonic association or meaning. They can form just about anywhere.

Schist Composition Tectonic Association NORMAL Quartz Feldspar Biotite ACCESSORY In red if present Garnet Staurolite      Schists have intermixed quartz, feldspar, and mica (biotite and/or muscovite), and often amphibole. This specimen is dominated by biotite, quartz, and feldspar. Often other minerals are present too, such as garnet and staurolite, but none are present in this particular specimen. Tectonic Association      Schists are typically associated with major mountain building events when shales or clay rich sandstones (wackes) are metamorphosed through depth of burial, and proximity to batholiths

Schist Continued Description      Schistose foliation composed of intermixed quartz, feldspar, and biotite (with some possible amphibole). The minerals in this specimen are well intermixed, characteristic of the schistose pattern, although when looking at the enlarged specimen it begins to look more like gneissic banding. It is a matter of scale though; if you have to get this close to see the minerals separating into bands you are still in the schist realm.      Schists are hard to photography and capture their details; all the minerals tend to be mashed together. The details are even hard to see when the specimen is in front of you. However, go to a very large picture, and you can see the intermixing of the minerals better.

Slate Composition NORMAL Chlorite ACCESSORY In red if present Muscovite      Individual crystals are too small to be seen by eye. Color typically green (color of chlorite) but ranges gray to black (organic matter), to green, red, purple, brown (iron oxides). At higher grades of metamorphism organic matter and iron oxides are recrystallized and rock becomes color of minerals.

Slate Continued Description Tectonic Association      Slatey cleavage foliation composed of the lining up of the basal cleavages in the new chlorite and/or muscovite minerals. The result is slatey cleavage, the breaking of the rock into sheets along smooth, flat faces - as seen in this specimen.      A distinct, weak sheen (light reflectance) is generally visible on cleavage faces, as seen here. The sheen shows up best when the rock is rotated in the light.       The rock will often " ring " like a bell when tapped on a hard surface, another feature not found in the shale parent, or the next rock in the metamorphic sequence, phyllite Tectonic Association      Slates are typically associated with major mountain building events when shales or clay rich sandstones (wackes) undergo low grade (Barrovian greenschist facies) metamorphism