sandstones. framework fraction –silicate grains 1/16 to 2 mm matrix –cement and very fine-size...
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Sandstones
Framework fraction– Silicate grains
• 1/16 to 2 mm
Matrix– Cement and very fine-size material
• <~0.03 mm
Framework Mineralogy
Major Minerals – Quartz– Feldpars– Clay minerals & fine micas
Accessory minerals Rock fragments Chemical cements
Quartz
Most Stable: – Greatest resistance to chemical decomposition– Multiple recycling
50-60% of framework fraction Monocrystalline Polycrystalline Undulatory extinction Origin: felsic plutonic rocks-granites,
metamorphic rocks, and older sandstones
Feldspars 10-20% of framework grains Alkali feldspars (K-feldspars)
– orthoclase, microcline, sanidine, anorthoclase– More abundant in sandstones– Plutonic or Metomorphic origin
Plagioclase feldspars– Albite (Na), oligoclase, andesine, labradorite,
bytownite, anorthite(Ca)– More abundant in sandstones derived from
volcanic rocks– Felsic or continental crust origin
Clay minerals & fine micas
clay minerals – kaolinite group– illite group– smectite group – chlorite group
fine micas – muscovite – biotite
Accessory Minerals <~1-2% Include: muscovite, biotite, & heavy minerals
(specific gravity > 2.9) Muscovite more stable than biotite, more abundant Easy to concentrate Stable nonopaque-zircon, tourmaline, rutile Metastable nonopaque-amphiboles, pyroxenes,
garnet, apatite, epidote, topaz monazite Stable opaque-hematite, limonite Metastable opaque-magnetite, ilmentite,
leucoxene.
Rock Fragments
10-15% of framework mineral grains Range from 0-95% Igneous: cystalline colvanic rock and volcanic
glass are most common in sandstones Metamorphic: metaquartzie, schist, phyllite, slate,
argillite, and less commonly gneiss clasts Sedimentary: Chert-microcrystalline quartz All preserved as sand-size fragments
Mineral Cements
Most common:– quartz, calcite, clay minerals, and hematite
Also pyrite, gypsum, and barite can also form cements under special geologic conditions
Quartz Cements
form in environments of high energy currents, such as beach deposits, marine bars, desert dunes, and some fluvial sandbars
most of the quartz cements are derived from the sands themselves or quartz sands
Quartz Cements
Overgrowths- rim of cement where the quartz cement is chemically attached to the crystal lattice of existing quartz grains
Syntaxial- when overgrowth retains crystallographic continuity of the grain
Calcite Cements
patchy cement soluble in surface waters Often partially dissolved cements secondary porosity
Hematite Cements
hematite cement indicates an oxidizing environment during diagenesis
most common oxidation state is Fe+2
Fe+2 is brought near the surface where the iron oxidizes to Fe+3 and can be carried away by hydrous fluids
Precipitation of Fe+3 forms hematite (Fe2O3)
Sulfate Mineral Cements
Barite (BaSO4) can form if the fluids are
rich in Ba Gypsum (CaSO4
.H2O) can from if the fluids
are oxidizing and rich in sulfur
Sulfate Mineral Cements
Sand Crystals- crystallographically continuous crystals in the cement when the cements form near the surface
Sands in the Gulf of Mexico I. Eastern Gulf of Mexico - Kyanite + Staurolite
(32%) derived from metamorphic rock in the Appalachian Mountains.
II. Mississippi River Province - Augite (23%), Hornblende (40%), Epidote (16%), and Garnet (3%) derived from glacial deposits in upper Mississippi River drainage.
III. Central Texas Province - Hornblende (58%), Epidote (17%), and Garnet (7%) but no Augite. Mostly from Colorado River of Texas.
Sands in the Gulf of Mexico
VI. Rio Grande Province - Epidote (15%), Hornblende (23%), Augite (24%), and brown hornblende from volcanic rocks (7%).
VII. Mexican Province - There are few studies of these sands, but they are expected to be similar to Rio Grande Province, reflecting a volcanic source.
Characteristics of Sandstones tell us:
Source area – rock type – current directions – weathering environment
Transport – medium, energy – distance
Depositional environment – marine or non-marine – physical environment (beach, river, delta, etc.)
Increased Textural Maturity:
clay removal increased sorting increased rounding breakdown (absence) of unstable
fragments breakdown (absence) of unstable
minerals
Super-mature Sandstones:
Clean (no mud matrix) well-sorted well-rounded grains mostly quartz grains quartz arenites Cratonic, typically recycled, formed in
beach or other high energy environment
References
http://www.science.ubc.ca/~geol256/notes/ch8_sands.html
http://www.tulane.edu/~sanelson/geol212/sandst&cong.htm
http://www.geo.wvu.edu/~lang/Geol185/Pet15-99SsClassn/sld001.htm
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