Marcia Pleming, Age 11, Of Columbus, Ind., for her question:
How do rocks differ from minerals?
Geology books often mention rocks and minerals as though they were two names for the same ob3ects. In some cases this is true, for many rocks are minerals and many minerals are rocks. But this general rule does not hold true for all solid substances of the earth's crust.
The rocky ground on which we live is a shell enfolding the round globe. It is between 10 and 40 miles thick, and it is made of the lightest substances of our solid planet. These lightest layers of assorted rocks rest or float on a denser layer some 600 miles thick. But this are still denser materials, and the core at the center of the solid globe is thought to be a dense, dense blend of heavy nickel and. Iron.
The thin crustal layers contain the greatest assortment of rocky solids, for here they are made and remade by wind and weather, by rushing rivers and weighty glaciers. They are pummeled and powdered, shifted and sifted, melted and crushed by heaving mountains and seething volcanoes.
Geology, also called earth science, is the over all study of the earth and its amazing history. Mineralogy the study of minerals and petrology the study of rocks are specialized branches of earth science. The mineralogist must be a chemist for his subject is related to elements and compounds the world of miniature molecules and atoms. The minerals from which the rocks of the earth's crust are made are inorganic or non living chemicals gold is a mineral, and the chemical nature of every nugget and flake of gold in the earth's crust is the same. For gold is a chemical elrment made from atoms of one type. The major rock making minerals are compounds made from molecules of one type. The most plentiful compounds are silicates, composed of molecues made from atoms of Silicon and Oxygen.
The silicates include milky white quartz and glossy black obsidian, rainbow striped agate and pansy pink amethyst. There are many families of mineral compounds, such as feldspars and. Calcites, phosphates and sulphates.
A rock made entirely of silica or feldspar is also a mineral. But the restless earth tends to mix and blend, seal and cement assortments of minerals together in solid lumps. These mineral mixtures are just plain rocks. A good example is granite. It is a rocky mixture of the minerals feldspar and quartz with perhaps a smidgen of mica or hornblende.
Most of the earth's crusty skin is made from rocks of assorted minerals. The next most plentiful solids are mineral compounds, such as silica and calcium. The rarest substances are elements, such as tin and copper, gold and silver. In a world wide treasure hunt you could find 100 different rocks, and if you broke every sample into fragments you would have about 1500 different minerals.