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Stone Definitions
- SLATE
- Slate is a hard, dense, and fine-grained low-grade metamorphic rock that splits into
thin slabs along planar surfaces. This splitting, known as slaty cleavage, results from
recrystallization under pressure and commonly develops at an angle to the bedding planes.
Slates are formed from clays, shales, volcanic ash, and other fine-grained rocks. Minerals
present are quartz, sericite, chlorite, some graphite, titanium oxide, and iron oxides.
Slate is commonly found in areas where shale has been subjected to heat and pressure
during mountain building. Because of its physical characteristics and its cleavage, slate
has been used for a great variety of surfaces in different forms of construction.
- QUARTZITE
- The metamorphic rock quartzite is formed from sandstone or chert that has been heated
and recrystallized during metamorphic episodes such as regional deformation or contact
with molten rock. Exposed quartzites are resistant to weathering and commonly project as
high hills and ridges.
Some of the textural features of the original sandstone or chert, such as the shape and
size of the grains and the type of bedding, are retained. Quartzite derived from sandstone
shows dust-like inclusions that can show the outlines of the original grains. Quartzite
from impure sandstones (containing mica or feldspar) may grade into other metamorphic
rocks, such as schist or gniess. A mosaic of small grains is typical of quartzite derived
from chert. If the chert had altering thin layers of silica and shaley material, the
quartzite typically preserves this feature. Quartzite formed during regional deformaion
shows the pressure or stress of metamorphism in elongated and parallel quartz grains or in
a preferred orientation of the crystallographic axes of quartz grains of any shape.
- SANDSTONE
- Sandstone is a sedimentary rock made of sand-sized grains of minerals, mostly quartz
and feldspar, or older rocks held together by one of several types of cement or a
fine, muddy matrix. Sandstones vary greatly in color, composition, texture of grains,
degree of cementation and layering. Common colors include grey, tan and red. Sandstones
are deposited on relatively flat surfaces and are stratified or bedded. In most cases the
stratification is parallel to the deposition surface, but subordinate layering may occur.
Ripple marks and animal trails may appear on bedding surfaces.
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