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Proposed solutions to the anorthosite problem have been diverse, with many of the proposals drawing on different geological subdisciplines. It was suggested early in the history of anorthosite debate that a special type of magma, anorthositic magma, had been generated at depth, and emplaced into the crust.
The typical cap rock is a salt, topped by a layer of anhydrite, topped by patches of gypsum, topped by a layer of calcite. [8] Interaction of anhydrite with hydrocarbons at high temperature in oil fields can reduce sulfate (SO 2– 4) into hydrogen sulfide (H 2 S) with a concomitant precipitation of calcite. [9]
Feldspars make up about 60% of the Earth's crust [3] and 41% of the Earth's continental crust by weight. [5] [6] Feldspars crystallize from magma as both intrusive and extrusive igneous rocks [7] and are also present in many types of metamorphic rock. [8] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. [9]
Except these last, practically all rocks contain felspars or feldspathoid minerals. In the acid rocks, the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the mafic rocks labradorite, anorthite, and bytownite prevail, being rich in lime and poor in silica, potash, and soda.
Calculated εNd value of rocks can be correlated to source rocks to perform provenance studies. In addition, Sr and Nd isotopes have been used to study both provenance and weathering intensity. [32] Nd is mainly unaffected by weathering process but 87Sr/86Sr value is more affected by chemical weathering. [36] [37]
Sepiolite can be identified in hand specimen by applying a drop of a saturated solution of methyl orange on the sample. A positive test result for sepiolite turns purple. [8] This can distinguish calcite from sepiolite in the field: sepiolite reacts to change the methyl orange to a shade of purple where calcite remains orange. [9] [10]
Galena is also an important ore mineral in the silver mining regions of Colorado, Idaho, Utah and Montana. Of the latter, the Coeur d'Alene district of northern Idaho was most prominent. [2] Australia is the world's leading producer of lead as of 2021, most of which is extracted as galena.
Diagenesis (/ ˌ d aɪ. ə ˈ dʒ ɛ n ə s ɪ s /) is the process of physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a role as sediments become buried much deeper in the Earth's crust. [1]