Schist - info to start with
Posted 16 September 2007 - 03:02 PM
Schist is formed by dynamic high-temperature, high-pressure metamorphism that involves a lot of strain. Schists are commonly described in terms of their predominant minerals. Schists have intermixed quartz, feldspar, and mica (biotite and/or muscovite), and often amphibole. Schist is a medium-grade metamorphic rock, so it experienced more heat and pressure than both slate and phyllite. Schist may contain garnet. Schist is composed predominantly of mica minerals, which impart a platy or layered texture to the rock. Schists are distinguished from the other foliated rocks, slates and gneisses, by the size of their mineral crystals; these are larger than those of slates, being visible to the naked eye, but smaller than those of gneisses, which are coarsely foliated rocks as opposed to finely foliated, or schistose, rocks.
Schists are abundant in the Precambrian (Archeozoic and Proterozoic) rocks. Schist breaks easily into thin layers. Schists are commonly rich in quartz and contain some feldspars and carbonates. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced.
The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others.
The characteristic flaky texture of schist gives rise to the adjective "schistose". Most schists have in all probability been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as basalts and tuffs. Most schists are mica schists, but graphite and chlorite schists are also common. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. The graphitic schists may readily be believed to represent sediments once containing coaly matter or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon.
The main change from phyllite to schist is that foliation is much more distinct due to the recrystallization of mica andchlorite mineral crystals. Under the microscope the appearance presented by mica-schists differs according to whether the rock is cut parallel to or across the planes of foliation. The association of quartzites and quartz-schists, graphiteschists and crystalline limestones with mica-schists in the field is explained by the fact that all these rocks are altered sediments.
Don't be afraid to learn more about Schist, it will help you find gold!
Posted 16 September 2007 - 03:19 PM
Posted 16 September 2007 - 03:26 PM
I always feel like i`m going to score a nuggie when schist is around or as bedrock
Good info ..
Posted 16 September 2007 - 03:32 PM
Posted 16 September 2007 - 03:59 PM
Posted 16 September 2007 - 09:57 PM
Yep, and with respect to gold, thats important. That level of heat and pressure cause some minerals to change and release water contained in their crystal latice. Because its under high heat and pressure, that water picks up into solution nasty heated stuff like sulfur and quartz. It also collects this rare element called gold. Water under pressure will flow to lower pressure areas if it has a chance, and often faults and fractures provide the essential conduit that allows the water to flow upward toward the surface where the pressure is a lot less. As it flows upward the rocks become cooler and eventually the water can no longer hold its cargo of disolved quartz, iron, copper, silver, sulfur and of course, gold. As the water cools and the pressure decreases, that stuff is deposited in the walls of the fracture in which the nasty hot water is flowing.
Thats how gold bearing quartz veins in schist are formed in Arizona and many other locations.
Millions of years later, the surface above is erroded away, and the gold bearing veins become exposed at the surface. Some veins still in place get mined by hard rock methods, some parts get erroded away and contibute their gold to placers that we all detect.
Yep, schist and other metamorphic rocks are important.
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