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In cross section they have the shape of an eye. [1] Feldspar, quartz, and garnet are common minerals which form augen. [2] Augen form in rocks which have undergone metamorphism and shearing. The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet.
Augen gneiss, from the German: Augen, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this ...
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground
Gneiss, a foliated metamorphic rock. Quartzite, a non-foliated metamorphic rock. Foliation in geology refers to repetitive layering in metamorphic rocks. [1] Each layer can be as thin as a sheet of paper, or over a meter in thickness. [1] The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. [1]
Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding.; Ultramylonites usually have undergone extreme grainsize reduction. In structural geology, ultramylonite is a kind of mylonite defined by modal percentage of matrix grains [clarify] more than 90%. [4]
Banded gneiss with dike of granite orthogneiss; competent banded gneiss is boudinaged by ductile shear. Boudin on the Island of Uto, Stockholm Archipelago, Sweden Chocolate-tablet boudinage structures in a low grade metasedimentary rock outcropping in Deception Pass, Washington
A thin section showing two feet penetrating into the lens capsule. The capsule appears just under 10 microns thick in this micrograph though the apparent thickness will vary with the angle of section cut so the actual thickness may be less. Large numbers of small vesicles can also be seen One of the feet and higher magnification.
Thin section of garnet mica schist from Salangen, Norway. A garnet porphyroblast (black) contains curved trails of small inclusions (white and grey). Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings.