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Slaty Cleavage
Foliation in slate is caused by the parallel orientation of platy minerals in the rock, such as microscopic grains of clay minerals and mica. These parallel mineral grain alignments give the rock an ability to break smoothly along planes of foliation. People exploit this property of slate to produce thin sheets of slate that are…
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Uses of the Word “Slate”
The word “slate” has not been used consistently over time and in some industries. Today most geologists are careful not to use the word “slate” when talking about “shale.” However, in the past the word slate was often used freely in reference for shale. This confusion of terms partially arises from the fact that shale…
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How Does Slate Form?
The tectonic environment for producing slate is usually a former sedimentary basin that becomes involved in a convergent plate boundary. Shales and mudstones in that basin are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone. Foliation develops at right angles to the compressive…
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Color of Slate
Most slates are gray in color and range in a continuum of shades from light to dark gray. Slate also occurs in shades of green, red, black, purple, and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock.
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Composition of Slate
Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals.
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What is Slate?
Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.
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Epidosite
Epidosite is the name of a rock that is composed mainly of epidote and quartz. It looks similar to unakite, but with little or no pink feldspar. Epidosite is also an attractive material used to make beads, cabochons, tumbled stones, and other items. Its pistachio-green color and crystalline texture cause many people to call it unakite,…
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Gemology of Unakite
Unakite is not seen in fine jewelry and is seldom seen in commercial jewelry, but it is a common stone used in craft and lapidary jewelry. It is usually cut into cabochons, beads, pendants, or freeform shapes. Unweathered, fine-grained unakite with mineral crystals less than a few millimeters in size is relatively easy to work.…
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Unakite Localities
Unakite is named after the Unaka mountain range of western North Carolina and eastern Tennessee, where it was first discovered and described. Similar material is found in many other locations around the world. Unakite is known as the Pompton Granite in a 1/2-square-mile outcrop in the Piedmont physiographic province of New Jersey. That small area…
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Geologic Occurrence
Unakite is a metamorphic rock that forms when granite (an igneous rock) is altered by hydrothermal metamorphism. During metamorphism, plagioclase in the granite is replaced by epidote to produce a rock composed primarily of green epidote, pink orthoclase, and clear to milky quartz. Unakite can also contain minor amounts of magnetite, chromite, ilmenite, apatite, zircon, and other minerals. Unakite is found in…