Rock types likely to be encountered in Jaipur
(and how to recognize them)
The following is a brief description of the rocks and minerals you are likely to encounter in and around Jaipur, the area selected for a Geological Excursion in January 2019, For a more complete description of common rocks and minerals, visit Alex Strekeisen’s website.
Orthoquartzite: A clastic sedimentary rock that is made up almost exclusively of quartz sand (with or without chert), and relatively free of or lacks a fine-grained matrix, derived by secondary silicification; a quartzite of sedimentary origin, or a pure quartz sandstone. The term generally signifies a sandstone with more than 90% to 95% quartz and detrital chert grains that are well-sorted, well-rounded, and cemented primarily with secondary silica (sometimes with carbonate) in optical and crystallographic continuity with the grains. The rock is characterized by stable but scarce heavy minerals (zircon, tourmaline, and magnetite), by lack of fossils, and by prominence of cross-beds and ripple marks. It commonly occurs as thin but extensive blanket deposits associated with widespread unconformities (e.g., an epicontinental deposit developed by an encroaching sea), and it represents intense chemical weathering of original minerals other than quartz, considerable transport and washing action before final accumulation (the sand may experience more than one cycle of sedimentation), and stable conditions of deposition (such as the peneplanation stage of diastrophism); e.g., St. Peter Sandstone (Middle Ordovician) of midwestern United States.
Quartzite: A granoblastic metamorphic rock consisting mainly of quartz and formed by recrystallization of sandstone or chert by either regional or thermal metamorphism; metaquartzite.
A very hard but unmetamorphosed sandstone, consisting chiefly of quartz grains that are completely cemented with secondary silica that has grown in optical continuity around each fragment. The silica grains are so firmly cemented that the rock breaks across or through the grains rather than around them; an orthoquartzite.
Rock composed of silica grains so firmly cemented by silica that fracture occurs through the grains rather than around them.
Synaresis Cracks: In quartzites and metaquartzites found around Jaipur, you are likely to find syneresis cracks, which at first glance, look like mud cracks. These are polygonal cracks filled with relatively more siliceous and finer material than the surrounding material. The individual cracks are variable in length and width. They are observed only on the surface and do not penetrate downwards.
Syneresis cracks (also known as subaqueous shrinkage cracks) are a sedimentary structure developed by the shrinkage of sediment without drying. Syneresis is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a deposit. The cracks can occur, for example, in mudstones deposited between two beds of sandstone. They commonly occur in thin mudstones interbedded with sandstones, as positive relief on the bottom of the sandstone, or as negative relief on the top of the mudstone. Syneresis cracks in some shales and lime mudstones may initially be preserved as small cavities, which then usually fill with silt and sand from either the overlying or underlying beds.
Conglomerate: A coarse-grained clastic sedimentary rock, composed of rounded to subangular fragments larger than 2 mm in diameter (granules, pebbles, cobbles, boulders) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay; the consolidated equivalent of gravel. The rock or mineral fragments may be of varied composition and range widely in size, and are usually rounded and smoothed from transportation by water or from wave action.
Mylonite: Mylonite is a metamorphic rock formed by ductile deformation caused by intense shearing during folding and faulting – a process termed cataclastic or dynamic metamorphism. This process involves pulverisation of the parent rock so the original minerals are almost completely broken down and recrystallise as smaller grains which are tightly intergrown, forming a dense, hard rock. As a result of heta and pressure during shearing, minerals recrystallize and grow preferentially along the direction of shearing, lending a prominent foliation to the rock. Mylonites may also be characterised by the smearing, flattening or rotation of any porphyroblasts formed during metamorphism.
For a more complete description of mylonites and their recognition, visit Alex Strekeisen’s website.
Schistose Sericite Quartzite: A schist is a strongly foliated crystalline rock, formed by dynamic metamorphism, that can be readily split into thin flakes or slabs due to the well developed parallelism of more than 50% of the minerals present, particularly those of lamellar or elongate prismatic habit. The individual mineral grains can be seen with the naked eye. Schists are named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, muscovite schist, biotite schist, chlorite schist and glaucophane schist. The mineral composition schistose sericite quartzite is essentially quartz, with varying degrees of sericite.
Amphibolite: A coarse-grained metamorphic rock that is composed mainly of green, brown, or black amphibole minerals (especially the species hornblende and actinolite) and plagioclase feldspar. The amphiboles are usually members of the hornblende group. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Phyllite: A metamorphic rock, intermediate in grade between slate and mica schist. Minute crystals of sericite and chlorite impart a silky sheen to the surfaces of cleavage (or schistosity). It is primarily composed of quartz, sericite mica, and chlorite, and represents a gradation in the degree of metamorphism between slate and schist. Phyllites commonly exhibit corrugated cleavage surfaces.
Calc-silicate rocks: Are rocks that contain calcium-bearing silicate minerals or calc-silicate minerals. They are most often formed as a product of metasomatism in the contact zone between granitic magma intrusives and carbonate sedimentary rocks such as limestone and/or dolostone. They are also called skarns, and tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals.
The calc-silicate minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with faults or shear zones. The hydrothermal fluids associated with the metasomatic processes can originate either from magmatic, metamorphic, meteoric, marine, or even a mix of these. Skarns often contain a respectable amount of ore mineralization that can be mined for a profit.
Marble: A hard crystalline metamorphic form of limestone, typically white with coloured mottlings or streaks. It is composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated, although there are exceptions. In geology, the term "marble" refers to metamorphosed limestone. Marble is a typically granoblastic rock. Granoblastic rocks have uniformly sized, equidimensional, anhedral grains produced by recrystallization in a solid state (metamorphism). Grain boundaries are usually straight or slightly curved and triple junctions between the adjacent grains define angles of about 120°
Background Information - Aravalli
Craton and the Delhi Fold Belt
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