Arkosic Sandstones

Arkosic sandstones are of two types. The most common of these is a mixture of quartz, potash feldspar, and granitic rock fragments. Chemically, these rocks are 60–70 percent silica (or silicon dioxide) and 10–15 percent aluminum oxide (Al2O3), with significant amounts of potassium (K), sodium (Na), and other elements. This type of arkosic sandstone, or arkose, can form wherever block faulting of granitic rocks occurs, given rates of uplift, erosion, and deposition that are so great that chemical weathering is outweighed and feldspar can survive in a relatively unaltered state. These rocks are usually reddish, generally immature, very poorly sorted, and frequently interbedded with arkose conglomerate; alluvial fans or fluvial aprons are the main depositional environments.

QFR diagram for Folk's sandstone classification: Q = Quartz, F = Feldspar, R = Rock fragments, F/R = Feldspar/Rock ratio. VRF = Volcanic rock fragment, MRF = metamorphic rock fragments, SRF = sedimentary rock fragments, CFR = carbonate rock fragments, Ss and Sh = sandstone fragments and shale fragments. From Wikipedia

Bibliography

• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore

Photo
Quartz, feldspar and lithic (pale brown-beige) fragments. PPL image, 2x (Field of view = 7mm)	Quartz, feldspar and lithic (brown-beige) fragments. XPL image, 2x (Field of view = 7mm)	Quartz, feldspar and lithic (pale brown-beige) fragments. PPL image, 2x (Field of view = 7mm)
Quartz, feldspar and lithic (brown-beige) fragments. XPL image, 2x (Field of view = 7mm)	Quartz, feldspar and lithic (brown-beige) fragments. XPL image, 2x (Field of view = 7mm)	Quartz, feldspar and lithic fragments (note the metamorphic quartz fragment). XPL image, 10x (Field of view = 2mm)
Quartz and feldspar fragments. XPL image, 10x (Field of view = 2mm)	Quartz and feldspar (with microgaphic texture) fragments. XPL image, 10x (Field of view = 2mm)	Quartz, feldspar and lithic fragments (note the metamorphic quartz fragment). XPL image, 10x (Field of view = 2mm)
Quartz and feldspar fragments. PPL image, 10x (Field of view = 2mm)	Quartz and feldspar fragments. XPL image, 10x (Field of view = 2mm)	Quartz, chlorite and feldspar fragments. PPL image, 10x (Field of view = 2mm)
Quartz, chlorite and feldspar fragments. XPL image, 10x (Field of view = 2mm)	Quartz and feldspar fragments. XPL image, 10x (Field of view = 2mm)	Quartz and feldspar fragments and carbonatic matrix. XPL image, 10x (Field of view = 2mm)