Julie Sherman, age 13, of Peoria Heights, Illinois, for her question:

What is trona?

This rocky material is not easy to trace down in the usual books on minerals. However, one of its main ingredients happens to be sodium    the 6th most plentiful element in the earth's crust. Sodium is a very active and energetic element, always eager to combine with other elements. We never find it in nature in its pure form, but we do find a vast assortment of sodium compounds. One of these is sodium chloride, ordinary table salt. There are sodium salts dissolved in the sea and countless sodium compounds among the rocky minerals. One of these is trona. The mineral trona is a grayish white or yellowish white rock, about 2 1/2 times heavier than water. It tends to form in layers, crystals and fibrous masses around mineral springs and in the dry beds of old salt water seas. Basically, trona is a sodium carbon compound with an acid quality.