Where can i find gneiss

During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes.

Gneisses from western Greenland comprise the oldest crustal rocks known more than 3. Gneiss is an old German word meaning bright or sparkling. A cabochon cut from this type of material is rarely seen, but it would be an interesting gem for a geologist. The stone is approximately 38 x 27 millimeters in size. This specimen is about four inches across and has a round blue sapphire crystal on the left side.

Don't be surprised if you see gneiss labeled as " granite " at a cabinet shop or monument company. In the dimension stone trade, any rock with visible, interlocking grains of feldspar is considered to be "granite" in that industry. Seeing gneiss, gabbro , labradorite , diorite , and other types of rock marketed as "granite" disturbs many geologists.

However, this long-time practice of the dimension stone trade simplifies discussions with customers since not everyone knows the technical names of unusual igneous and metamorphic rocks. The best way to learn about rocks is to have specimens available for testing and examination. Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch.

If samples of these rock units are broken into one-inch pieces, many of them will be too small to exhibit the banding features of gneiss. This will confuse many students and cause others to incorrectly identify the rock. Teachers can avoid these problems by collecting specimens that clearly display a banded structure. Teachers who purchase specimens must examine them carefully before they are presented to students.

After students have learned to identify gneiss and many other rock types, presenting specimens of gneiss that do not exhibit banding can be a challenging way to have students: A consider possibilities that are not obvious, and, B realize that a single rock specimen may not adequately represent a rock unit. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. What Is Gneiss? How Does Gneiss Form? Rock, Mineral and Fossil Collections.

Hardness Picks. Flint, Chert, and Jasper. Tumbled Stones. It was originally a mining term, meaning a country rock in the Ore Mountains Erzgebirge which contained metalliferous veins.

The term gneist was first recorded in print by Agricola Georg Bauer in his famous posthumously published book De Re Metallica which remained the most important mineralogy and mining textbook for the next two centuries. The book was published in 1. Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in this rock is a result of mineral segregation into separate, typically light- and dark-colored layers. Light-colored layer is usually composed of feldspars and quartz.

Most important dark minerals are hornblende and biotite. Individual bands are usually mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatites. It is often difficult to distinguish it from migmatite because there is a gradational transition from one to another.

It is not well understood how the segregation takes place, but it must be the result of extreme pressure and shear stress deep in the crust. The protolith of gneiss may be an igneous rock , in this case it is called an orthogneiss. It forms probably because of shear in vicous granitic magma. Paragneiss is a variety with a sedimentary protolith. Even in the latter case, gneissic banding has nothing to do with original layering of sedimentary rocks.

These original features are completely obliterated by the metamorphic processes involved in the formation of this rock type. Paragneiss in most cases is thought to be the end product of metamorphism of a pelitic clay-rich sedimentary rock shale , argillite, claystone, etc. Still deeper burial or more intense heating may result in migmatization and finally complete melting of gneiss. Despite being clearly oriented, this rock is not considered to be foliated because it is not fissile along the layering.

So, when hammered, gneiss behaves like a uniform homogenous rock. In this sense it is similar to igneous rocks like granite and gabbro and not similar to related metamorphic rocks like schist and phyllite which are foliated.