GNEISS
Gneiss is a coarse to medium grained banded metamorphic rock formed from igneous or sedimentary rocks during regional metamorphism. Rich in feldspars and quartz, gneisses also contain mica minerals and aluminous or ferromagnesian silicates. In some gneisses thin bands of quartz feldspar minerals are separated by bands of micas; in others the mica is evenly distributed throughout. Gneisses formed from igneous rocks are called Orthogneisses. Gneisses formed from sedimentary rocks are called paragneisses. These are similar in composition to granite or granodiorite. Augen gneiss is a variety containing large eye shaped grains of feldspar. Injection gneisses are formed by injection of veinlets of granitic material into a schist or some other foliated rock. Banded gneisses called migmatites are composed of alternating light colored layers of granite or quartz feldspar and dark layers rich in biotite.
COLORS
Light-colored gneisses are composed primarily of rhyolite and quartz minerals. Intermediate gneisses form from granitic and arkosic minerals and include shades of red, white and pink. The darkest gneisses form from dark-colored minerals and parent rocks, such as diorite, gabbro, and pyroxene.
Plagioclase Feldspar: white to grey
Potassium Feldspar: pink
Quartz: clear and colorless.
Mica: black, brown, or clear, it is often found in large, flat sheets.
Hornblende: black or dark green, it is a common mineral in gneiss that forms long, slender crystals.
WHY HERE?
The GNEISS boulder you see here was taken from a local quarry in Antrim County MI, just a few miles away. This part of the Michigan Basin formed during early Mississippian Period between 360 and 325 million years ago.
LOGGING REQUIREMENTS
In order to log this EarthCache, send me your answers to the following questions either through email or messaging from my profile page.
1: What minerals are present in this Gneiss boulder?
2: Estimate the percentage of each mineral in this Gneiss boulder.
3: Post a pic of you or a signature item with the Gneiss Boulder in the background.
references
https://nature.berkeley.edu/
https://sites.pitt.edu/
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