This texture allows the rock to be broken into thin slabs along the alignment direction of the platy mineral grains. This type of breakage is known as schistosity. In rare cases the platy metamorphic minerals are not derived from the clay minerals of a shale. The platy minerals can be graphite , talc , or hornblende from carbonaceous, basaltic, or other sources. Chlorite schist: A schist with chlorite as the dominant visible mineral is known as a "chlorite schist.
Schist is a rock that has been exposed to a moderate level of heat and a moderate level of pressure. These are usually shales or mudstones. The best way to learn about rocks is to have specimens available for testing and examination.
In the convergent plate boundary environment, heat and chemical activity transform the clay minerals of shales and mudstones into platy mica minerals such as muscovite, biotite, and chlorite. The directed pressure pushes the transforming clay minerals from their random orientations into a common parallel alignment where the long axes of the platy minerals are oriented perpendicular to the direction of the compressive force.
Slate is has a dull luster, it can be split into thin sheets along the parallel mineral alignments, and the thin sheets will ring when they are dropped onto a hard surface. If the slate is exposed to additional metamorphism, the mica grains in the rock will begin to grow. The grains will elongate in a direction that is perpendicular to the direction of compressive force. This alignment and increase in mica grain size gives the rock a silky luster.
At that point the rock can be called a "phyllite. Garnetiferous schist: This rock is composed of fine-grained muscovite mica with numerous visible grains of red garnet. The large crystal is about 21 millimeters in length. Gneiss vs Eclogite. Gneiss vs Amphibolite. Gneiss vs Migmatite. More More Compare Metamorphic Rocks. Metamorphic Rocks »More. Eclogite Definition Uses Formation Properties.
Granulite Definition Uses Formation Properties. Gossan Definition Uses Formation Properties. Compare Metamorphic Rocks »More. It is water-resistant and when it splits, it breaks in straight lines with flat surface top and bottom. Piece of slate, metamorphic rock. Slate is a good roofing material, but people also use it for decorative gardening stones, as a base for snooker tables and it was used as a writing board or writing slate in the Victorian classroom.
Marble is limestone that has undergone some changes. For this reason, a simple eased edge is recommended, because any of the more detailed edge treatments are likely to end up with mica flakes poking out. Also, areas that have a lot of mica will be softer than a typical granite. That said, the non-mica parts of the stone will behave similar to granite. For this reason, schist can have planes of weakness where the rock is prone to split.
If the stone will be supported underneath, like on a floor, this is less of a concern. But when the stone is overhanging, like the edge of a countertop, be wary. Try to avoid an overhang that runs parallel to the grain of the stone. If possible, try to avoid using mica-rich parts of a slab on areas that will be overhanging or unsupported.
Galaxy schist. Photo courtesy of Ashfield Stone. In general, schists with small mica grains like Galaxy schist or Quicksilver schist are safe bets. These stones have an overall shimmer rather than a full-on glittery look. Because the mica pieces are smaller, they are less likely to flake off and cause problems.
If you are considering schist for a countertop or other interior use, spend some extra time inspecting potential slabs. Ideally, look at both a fabricated piece as well as a raw slab. Pay close attention to the edges, where you can see into the stone a bit.
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