Original Characteristics of Clastic Quartz Grains

Abstract

Granular gravel and sand-sized quartz fragments in untransported naturally disintegrated detritus were examined from nineteen massive plutonic rocks, sixteen gneisses, and six schists located in the desert areas of southern Arizona and southern California. The mean size and standard deviation of each quartz type derived from each rock group reveal: (1) the mean size of total quartz released from massive plutonic rocks and gneisses is identical (coarse sand) and is one and one-half to two times courser than total quartz released from schists; (2) monocrystalline quartz grains with undulatory extinction are courser in size than non-undulatory grains. Therefore, grain size must be specified when the percentages of non-undulatory quartz in sandstones are compared. The angular difference between c-axes in medium sand-sized quartz grains with undulatory extinction averages four to five degrees with standard deviation of two to three degrees and is independent of the type of rock from which the grain was derived. Measurements of this feature are probably useless as indicators of provenance. Polycrystalline quartz grains are yielded in abundance by each of the groups of rocks examined. Higher proportions of these grains occur in the coarser size fractions. Therefore, the proportion of polycrystalline quartz released from each type of rock depends, in part, upon the size frequency distribution of total quartz released from the rock. Polycrystalline quartz grains of granular gravel to medium sand size are not derived almost entirely from foliated metamorphic rocks. Polycrystalline quartz grains from the several types of rocks examined possess some distinguishing features. (1) For any grain size other than very fine sand, polycrystalline quartz from gneisses will be formed of a greater number of quartz crystals than will grains from massive plutonic rocks or schists. (2) Quartz crystals in polycrystalline grains from gneisses and schists commonly show preferred crystallographic orientation. (3) The grain size distribution of quartz crystals forming polycrystalline grains from gneisses and schists is frequently bimodal. Intragranular suturing and moderate grain elongation are not reliable as indicators of provenance. Monocrystalline quartz grains between granular gravel and medium sand size are derived in ranch greater abundance from massive plutonic rocks than from gneisses or schists. However, a terrain exposing foliated rocks (gneisses plus schists) will yield an amount of monocrystalline quartz approximately equal to that yielded by a terrain of massive plutonic rocks. Comparison of the character of first-cycle quartz, as observed in this study, with quartz as seen in many mineralogically mature sandstones provides strong evidence that quartz grains are reduced in size by sedimentary processes. Size for size, the more mature sediments contain lower proportions of polycrystalline quartz in their total quartz, higher proportions of non-undulatory quartz, lower proportions of grains in the 1-4 mm size range, and fewer elongate quartz grains.