Siliceous Rocks - Quartzites

Quartzites can be defined as saccharoidal texture metamorphic rocks, derived from sand sediments and formed by re-crystallized quartz grains involved or not by silicious cement. As much as in marbles, mineralogical re-crystallization occurs under the effect of operating pressure and temperature on the original sand sediments, rendering quartzites normally more cohese and less friable than sandstone.

The most common accessory minerals are the micas (phylosilicates), zircon, ilmenite /magnetite and iron and manganese hydroxides. Accessory minerals determine the quartzites’ aesthetic features, the chromatic standard above all.

Quartzites with small participation of phylosilicates (usually white mica), do not develop metamorphic foliation or preferential plans of partition. Therefore these quartzites are characterized as massive stones of granoblasted saccharoid texture, extracted in blocks from the quarries and then sawed into plates. For example, the quartzites extracted in Bahia, are fit among the massive varieties existing in Brazil.

When there is stronger presence of micas, the quartzites develop granolepidoblasting saccharoid texture, with preferential plans of partition/delamination that make them useful for direct extraction of plates from the mining rock bulk.

The highest degree of water absorption (0,20-0,80%) proportioned by their saccharoid texture, prevents the water soaking and facilitates the draining of external floors, especially the ones at the edge of swimming pools. Moreover, the inexistence of inner reactive minerals makes quartzites inert to alteration agents, such as cleaning products and acid solutions in general.

There is no regulation for commercial designations of those quartzites, but a tendency to use the appellation “São Tomé” for several materials from various origins. The varieties commercialized include stones of whitish, yellowish (champagne), greenish, rosy and grayish coloration, specially presented as non calibrated tiles, shards, slivers and fillets.