Randy D. Vallier, age l5, of Tulsa, Okla., for his question:
How is marble formed in the earth?
The formation of marble begins with busy little animals of the sea and ends with a waxy stone. Almost every deposit has its own subtle shadings and colors caused by various impurities. But its basic minerals are calciums and its basic formation is a step by step crystallization of molecules.
The ancient seas swarmed with shell building creatures, most of them too small to be seen without a microscope. Many of them built their shells from calcium carbonate and magnesium carbonate extracted from the salty water. As their generations lived and died, the tough shells sank to the bottom and deposited thick carpets of calcium chemicals on the bed of the ocean.
Some of these deposits finally became waxy layers of handsome marble, and since marble originally was made by living animals it is a true fossil mineral. The first stage of the transformation began when the oozy deposits of calcium were deserted by the seas and left high and dry to become rocky layers of land. They became layers of limestone and many of them have endured as limestone through millions of years.
However, some limestone deposits were later covered with newer layers of rock and some were crushed by growing mountains and other upheavals in the earth's crust. These molecules were rearranged in tight crystalline formations. The crumbly limestone was transformed into waxy marble. This transformation is called metamorphosis, and marble is a metamorphic rock. Its special crystalline formation makes it soft enough to carve and gives a shiny gloss to its polished surface.
Pure marble is as white as a summer cloud but it is rare. Almost always traces of various minerals Were mixed with the original calciums in the oozy mud or added later from the earth's crust. Traces of iron add red streaks and the marble looks like frozen bouquets of pink petals. Traces of chlorite add green tints and the marble seems to be permeated with ferny foliage. Other impurities form marbles blended with blues and pansy purples or mottled with golds and candy colors.
Most marbles are white with traces of black or charcoal gray. Their dark streaks, veins and blotches are caused by carbon impurities that were added by fragments of decaying plants.
In the dry Mediterranean countries marble statutes have stood outdoors for centuries. But the handsome stone does not endure in the moist climate of North America. It tends to dissolve in rain water, and for this reason we do not leave marble statutes outdoors or use marble on the outsides of our buildings. As a building stone we use it on walls and floors and we go indoors to admire our marble statuary.