Dolomitic marble (10.3 cm across at its widest)
Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.
Dolomitic marbles are crystalline-textured, ~monomineralic metamorphic rocks composed of dolomite (CaMg(CO3)2 - calcium magnesium carbonate). They form by metamorphism of dolostones. These rocks are identified by effervescing (bubbling) in acid when powdered. Some dolomitic marbles have a noticeable tremolite component (Ca2Mg5Si8O22(OH)2). A very similar metamorphic rock is dolomitized marble, which is a calcitic marble (metamorphosed from limestone) that has been subsequently dolomitized by the chemical addition of magnesium, introduced by hydrothermal fluids.
The dolomitic marble shown above is a clast derived from an Upper Pleistocene glacial till. It ultimately comes from the Precambrian of Canada.
Locality: Granville, Licking County, east-central Ohio, USA
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