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Arkose | by James St. John
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Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.


There are three categories of sedimentary rocks:

1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks.

2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms).

3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.


Arkose is a distinctive, potassium feldspar-rich, siliciclastic sedimentary rock. It can be a breccia or a sandstone. Arkoses have obvious clasts of pinkish- to orangish- to salmon-colored potassium feldspar (KAlSi3O8). Some arkoses have an overall dark red color. Arkoses are not exclusively K-feldspar - they have a significant quartz component (= grayish-colored grains in the above rock).


The presence of abundant, unaltered K-feldspar in arkose indicates deposition near the sediment source. Original source areas for arkoses are usually interpreted as landscapes with fairly significant topographic relief with extensive exposures of granites - large K-feldspar crystals are only common in granites.


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Uploaded on March 12, 2015