David Hum, age 11, of Fredericton, New Brunswick, Canada, for his question:

How  many minerals are in the earth?

If you wander far and wide, it is possible to collect samples of more than 100 different rocks. The common rocks of the earth's crust are made of assorted minerals. Most minerals are chemical compounds made from molecules    packages of assorted atoms. Gold and several other minerals found in the ground are chemical elements made from atoms of one kind. A dozen different elements can combine to form hundreds of different minerals. Only 92 natural elements are used to build all the solids, liquids and gases of our planet. In various combinations they form thousands of different chemical com¬pounds.

Most minerals in the earth's crust are limited to chemicals that are solid at ordin¬ary temperatures. However, water rates as a mineral and the earth's oceans are full of it. About 1500 different minerals are used to build the solid crust. The most plentiful samples are silicates, chemical compounds of the elements silicon and oxygen. Most rocks and stones are mixtures of several mineral compounds. For example, opal is silica, the silicon oxygen compound, mixed with water    a compound of hydrogen and oxygen.