Sandstone Framework Mineralogy

Introduction

Although sandstones contain a wide range of possible minerals, the actual variety that appears in most sandstones is surprisingly small. Only a few principal kinds of minerals and rock fragments make up the bulk of all sandstones. This simplicity reflects the combined filtering action of chemical weathering, mechanical abrasion during transport, and the selective preservation of durable minerals. Understanding the full inventory - which minerals are present, in what proportions, and in which stability class - is the foundation of sandstone classification, provenance analysis, and reservoir characterisation.

Major Minerals (>~1-2% Abundance)

The major minerals in siliciclastic sedimentary rocks divide into stable and less stable groups based on resistance to chemical decomposition. ^[1]

Quartz (SiO₂) is the most stable major mineral. It makes up approximately 65% of framework grains in the average sandstone and about 30% of minerals in the average shale. ^[1]

Feldspars are the less stable major minerals. They include K-feldspars (orthoclase, microcline, sanidine, anorthoclase) and plagioclase feldspars (albite, oligoclase, andesine, labradorite, bytownite, anorthite), and make up about 10-15% of framework grains in the average sandstone and about 5% of minerals in the average shale. ^[1]

Clay minerals and fine micas include the kaolinite group, illite group, smectite group (with montmorillonite as a principal variety), and chlorite group. Fine micas are principally muscovite (sericite) and biotite. They occur in minor abundances in sandstones as matrix minerals but compose a much larger fraction of the minerals in average shale. ^[1]

Accessory Minerals (<~1-2% Abundance)

Coarse micas - principally muscovite and biotite - occur as accessory framework grains, separate from the fine-grained matrix micas. ^[1]

Heavy minerals are minerals with specific gravity greater than about 2.9. They subdivide into stable and metastable varieties. Stable non-opaque heavy minerals include zircon, tourmaline, rutile, and anatase. Metastable non-opaque heavy minerals include amphiboles, pyroxenes, chlorite, garnet, apatite, staurolite, epidote, olivine, sphene, zoisite, clinozoisite, topaz, and monazite, plus about 100 others of minor volumetric importance. Stable opaque heavy minerals include hematite and limonite; metastable opaque heavy minerals include magnetite, ilmenite, and leucoxene. ^[1]

The stability classification here reflects chemical durability during weathering, transport, and diagenesis. Stable minerals can survive multiple erosion-transport-deposition cycles; metastable ones are destroyed more readily. A sandstone dominated by stable heavy minerals like zircon and rutile has been multiply recycled. One rich in metastable minerals like pyroxene and amphibole is younger in a sedimentary sense - closer to its igneous or metamorphic source.

Rock Fragments

Rock fragments make up about 10-15% of the siliciclastic grains in the average sandstone and most of the gravel-size particles in conglomerates; shales contain few rock fragments. Igneous rock fragments can include clasts of any kind of igneous rock in conglomerates, but fragments of fine-crystalline volcanic rock are most common in sandstones. Metamorphic rock fragments may include clasts of any metamorphic rock, but metaquartzite, schist, phyllite, slate, and argillite clasts are most common in sandstones. Sedimentary rock fragments can include clasts of any sedimentary rock in conglomerates, but clasts of fine sandstone, siltstone, shale, and chert are most common in sandstones; limestone clasts are comparatively rare in sandstones. ^[1]

Chemical Cements

Chemical cements in siliciclastic rocks occupy variable proportions of the total rock volume. Silicate cements are predominantly quartz, but may also include microquartz (chert), opal, feldspars, and zeolites. Carbonate cements are principally calcite, with less common aragonite, dolomite, and siderite. Iron oxide cements include hematite, limonite, and goethite. Sulfate cements include anhydrite, gypsum, and barite. ^[1]

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