Mineralogy of a Paralithic Horizon (Well Weathered Bedrock) in the Mojave Desert.

Paralithic materials can have important ecosystem functions in arid environments because they can store water for deeply rooted plants. As within the solum, the mineralogy of the paralithic zone is an important determinant of its properties. We studied the mineralogy of a paralithic horizon underlying a Xeric Calciargid on a gently sloping mountain bench in the southern Fry Mountains, Mojave Desert. The area has a mean annual temperature of 18°C and a mean annual precipitation of about 100 mm, occurring mostly as rain in the winter months. The paralithic horizon extended from 38-80 cm, had a pH of 8.1, an EC of 10 dS m^-1, and sodic conditions (SAR=13). The fine-earth fraction had a calcium carbonate equivalent of 15%. The paralithic horizon had two distinct morphologic components: a dark matrix and a greenish-white seam. Both components were easily crushed by hand. As determined by petrographic microscopy, SEM, and XRD, the paralithic horizon had characteristic mineralogy of a biotite-rich diorite with amphibole, biotite, orthopyroxene, and plagioclase dominating the primary mineral fraction. Quartz, chlorite, and titanite also occurred but were rare. Primary minerals were similar in both matrix and seam, but differed in their distribution. Felsic minerals were more common in the seam and mafic minerals were more abundant in the matrix. Primary minerals were weathered and often aggregated. A fibrous smectite was identified in both matrix and seam using XRD and TEM. Chemical weathering was likely initiated during moister pluvial conditions of the Pleistocene, providing base cations and silica that led to formation of the smectite. This smectite persists, however, under conditions of high pH, high salinity, and incomplete leaching that accompanied the change to a more arid climate during the Holocene.