This type of metamorphic rock consists of recrystallized calcite (in limestone) or dolomite (in dolomite rock). Marble is made by regional metamorphism of limestone or dolomite rock, causing their microscopic grains to combine into larger crystals. In German, the word means bright or sparkling. Pretty much everywhere on the continents, you will drill straight down and eventually strike gneiss. Gneiss makes up the largest part of the Earth's lower crust. The oldest Earth rocks known are gneisses from Acasta, in northern Canada, that are more than 4 billion years old. With still more metamorphism, gneisses can turn to migmatite and then totally recrystallize into granite.ĭespite its highly altered nature, gneiss can preserve chemical evidence of its history, especially in minerals like zircon which resist metamorphism. Thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. You can see that unlike schist, which is more strongly aligned, gneiss doesn't fracture along the planes of the mineral streaks. In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. The streaks contain minerals, like hornblende, that don't occur in sedimentary rocks. Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and recrystallize. This type of metamorphic was created by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. It means a type of rock texture, not a composition. Gneiss ("nice") is a rock of great variety with large mineral grains arranged in wide bands. For instance, a blueschist-facies ultramafic rock consists mainly of serpentine (antigorite), olivine and magnetite.Īs a landscaping stone, blueschist is responsible for some striking, even garish effects. It depends on the original rock that is metamorphosed. In other samples, lawsonite, jadeite, epidote, phengite, garnet, and quartz are also common. In this hand specimen from Ward Creek, California, glaucophane is the major blue mineral species. Petrologists often prefer to talk about the glaucophane-schist metamorphic facies rather than blueschist, because not all blueschist is all that blue. The bluest, most schistose blueschist-like this example-is made from sodium-rich mafic rocks like basalt and gabbro. Blueschist is a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and a strongly layered fabric has been imposed. High-pressure, low-temperature conditions are most typical of subduction, where marine crust and sediments are carried beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks. Blueschist signifies regional metamorphism at relatively high pressures and low temperatures, but it isn't always blue, or even a schist.
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