A quartzite that preserves some sedimentary features is best described as a metasandstone or metachert. This Las Vegas boulder is a metaquartzite. A quartzite in which all traces of the original grains and sedimentary structures are erased may also be called metaquartzite. In the first way, sandstone or chert recrystallizes resulting in a metamorphic rock under the pressures and temperatures of deep burial. This metamorphic rock forms in two different ways. It may be derived from sandstone or from chert by regional metamorphism. Quartzite is a tough stone composed mostly of quartz. It is quite durable in dry climates, as in the Mediterranean countries where ancient marble structures survive.Ĭommercial stone dealers use different rules than geologists to distinguish limestone from marble. Like limestone, marble tends to dissolve in acidic fluids. Like other metamorphic rocks, marble has no fossils and any layering that appears in it probably does not correspond to the original bedding of the precursor limestone. The color of marble can range from the purest white to black, ranging through the warmer colors in between depending on the other mineral impurities. For fine marble of the sort used in buildings and sculpture, the crystals are even smaller. In this hand specimen of Vermont marble, the crystals are small. 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.

Rocks that look like rose quartz