Marcus Englehart, age 13, of Nogales, Ariz., for his question:

WHAT IS THE WORLD'S MOST COMMON MINERAL?

Quartz is the world’s most common mineral. Composed of silicon dioxide, or silica, it is distributed all over the world as a constituent of rocks and in the form of pure deposits.

Quartz is an essential part of igneous rocks such as granite, rhyolite and pegmatite, which contain an excess of silica. In metamorphic rocks, it is a major constituent of the various forms of gneiss and schist. The metamorphic rock quartzite is composed almost entirely of quartz.

Veins and nodules of quartz are formed in sedimentary rock, principally limestone. Sandstone, a sedimentary rock, is composed mainly of quartz. Many widespread veins of quartz deposited in rock fissures form the matrix for many valuable minerals.

Precious metals, such as gold, are found in sufficient quantity in quartz veins to warrant the mining of quartz to recover the precious mineral. Quartz is also the primary constituent of sand.

Some specimens of quartz are transparent while others are translucent. In the pure form, the mineral is colorless but it is commonly colored by impurities.

There are two general classes of quartz: fibrous and granular. The fibrous varieties include agate, carnelian, heliotrope, onyx and chrysoprase. The granular varieties include chet, flint, jasper and prase.

The different forms of chalcedony and many of the crystalline varieties of quartz are used as gemstones and other ornamental materials. Pure rock crystal is used in optical and electronic equipment.

In the form of sand, quartz is used extensively in the manufacture of glass and silica brick and is also used in cement and mortar. Ground quartz is used as an abrasive in glass grinding, stonecutting and sandblasting.

Powdered quartz is used in making porcelain, scouring soaps, sandpaper and wood fillers.

Large amounts of quartz are used as a flux in smelting operations.

Almost ail natural high grade quartz crystal, which is an important raw material in the electronics industry, is imported from Brazil, the only country with large deposits of the mineral in commercial quantities. Quartz crystals can also be made synthetically.

The coarsely crystalline varieties of quartz are in general transparent and lustrous. Rock crystal, a colorless from of quartz, usually occurs in distinct crystals

Rose quartz is coarsely crystalline but without distinct crystal form and is colored rose red or pink, the color often fading on exposure to the light. Smoky quartz or cairngorm stone occurs in crystals ranging from smoky yellow to dark brown. Amethyst, a semiprecious variety of quartz, is colored purple or violet.