The Spraberry sands and siltstones consist largely of angular to subangular quartz, mostly of igneous origin. The sands are low in porosity and permeability because of close initial packing, pressurizing and tight cementation. The chief cementing material is opal with impurities of clay, carbonates and iron oxides. The most abundant inclusions in quartz are tourmaline, zircon and futile. Authigenic and metasomatic minerals include dolomite, calcite, opal, chalcedony, quartz and manganese dioxide. Most of the carbonates have been replaced by chalcedony. Photomicrographs and micro-drawings illustrate these findings.