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Chemical weathering of granite occurs when dilute carbonic acid, and other acids present in rain and soil waters, alter feldspar in a process called hydrolysis. [ 50 ] [ 51 ] As demonstrated in the following reaction, this causes potassium feldspar to form kaolinite , with potassium ions, bicarbonate, and silica in solution as byproducts.
Alkali feldspar granite, some varieties of which are called 'red granite', [1] is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar (K-spar). It is a dense rock with a phaneritic texture. The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals. [2] [3]
S-type granites are over-saturated in aluminium, with an ASI index greater than 1.1 where ASI = Al 2 O 3 / (CaO + Na 2 O +K 2 O) in mol percent; [1] [2] [3] petrographic features are representative of the chemical composition of the initial magma as originally put forth by Chappell and White are summarized in their table 1. [1] [4]
[20] [21] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. [ 24 ]
Different specific granite types have differing propensities to weather, and so differing likelihoods of producing decomposed granite. The parent granite material is a common type of igneous rock that is granular, with its grains large enough to be distinguished with the unaided eye (i.e., it is phaneritic in texture); it is composed of ...
Pegmatitic granite with pink potassium feldspar crystals, surrounding a finer-grained cumulate-filled enclave, Rock Creek Canyon, eastern Sierra Nevada, California. Most pegmatites have a simple composition, often being composed entirely of minerals common in granite, such as feldspar, mica, and quartz. [3]
Tonalite–trondhjemite–granodiorite (TTG) rocks are intrusive rocks with typical granitic composition (quartz and feldspar) but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. [1]
The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. [3] [4] It is distinguished from gabbro on the basis of the composition of the plagioclase species; the plagioclase in diorite is richer in sodium and poorer in calcium. [5] [6] [7]