The weathering of granite, Llano region, Central Texas




Begle, Elsie Alkin

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The weathering of granite in central Texas yields tremendous amounts of granitic detritus to adjacent rivers. The first step in the production of such sediment is the formation of grus--chemically unaltered yet totally disintegrated granite. It had long been thought that grus formed solely from the hydration and expansion of biotite which shattered the rock along fractures that radiated outward from biotite flakes. However, in central Texas the first fractures to form as granite disintegrates are closely spaced parallel sub-horizontal fractures which cut through quartz grains, microcline phenccrysts, and even biotite books. They have no spatial relationship to biotite and could not have formed by its simple expansion. The parallel fractures that initiate granite weathering may have formed by laterally confined or buttressed expansion. For example, if a body of granite expands but that expansion is confined laterally, then it must accommodate the expansion by arching upward and fracturing parallel to the arching. This would produce fractures roughly parallel to the weathering surface. The cause of the initial expansion may be solar insolation for fractures close to or at the surface, pressure release, or incipient biotite expansion at the weathering front. Once the parallel fractures form, more surface area is exposed to water moving through the rock. This speeds the expansion of biotite which ultimately shatters the rock along fractures that follow grain bound aries and radiate outward from biotite books. The formation of grus in central Texas is accompanied by the break-up of hornblende crystals along cleavage planes and, of course, the expansion of biotite parallel to the c-axis producing thin gaps along cleavage planes. Grus formation, however, was not accompanied by significant chemical alteration. Biotite did not alter to vermiculite or hydrobiotite, and neither hornblende nor feldspar were chemically altered in the grus zone. In the B horizon of soils developed on granite, biotite altered to iron oxide and either to trioctahedral illite in alkaline K+-rich waters or to vermiculite in Mg²+-rich acidic conditions. At the same time plagioclase crystals show evidence of weathering. This weathering, however, did not attack the outer surface of the grain and work uniformly inward to produce a weathering rind. Instead, plagioclase feldspars dissolved along selective discontinuous and irregular planes within crystals. These irregular planes were probably sites of previous hydrothermal or deuteric alteration, for the end result of solution is a fragile network of crystal-clear feldspar with no remnants of the once abundant fluid inclusions and sericite flakes. Microcline, too, dissolved in soil environments but only under conditions of either prolonged weathering or poorly drained acidic soils. It is much more stable than plagioclase. Sites of hydrothermal alteration also control solution of microcline in the weathering environment. Quartz was chemically unaltered in soils developed on granite. It was, however, reduced in size in the soil zone by the spalling of thin slivers from grain surfaces. The surface of quartz grains was also altered in soils. Excess silica derived from the solution of feldspar was precipitated on quartz surfaces producing a frosted appearance. Silica was also precipitated as a botryoidal crust on the under sides of slabs of granites and platy grains in grus as pendulous drops of water evaporated during dry periods. These opal crusts precipitated low in the weathering profile and represent an incipient silcrete