Jack Brown, age 10, of Fountain Valley, California, for his question:
What sort of stone is marble?
This glamorous stone cores in waxy white, often streaked with dark lacy lines, and also in several pastel colors. In olden tines marble eras the favorite stone of master sculptors, though nowadays most of it is used in smooth slices for tables and walls. It can be chiseled and cut smoothly in any direction. The finished work can be polished with a tool that reveals some of the lovely mottled designs inside the stone.
Marble is classed as a metamorphic stone, which means that its original minerals have been remodeled. Slate is a metamorphic rock that has been changed so drastically that you could not guess it started out as muddy clay. The changes in marble were less severe. You still might guess that it is a remodeled form of limestone or dolomite.
Most rocks form patiently through millions of years. But drastic changes occur as the earth remodels its massive crust a job that never ends. Limestones and dolomites are called sedimentary rocks because they were dregs or sediments that sank, to the bottom of seas or fresh craters. Zillions of mini shell builders live and die, discarding limy fragments. Through the ages, these fossilized fragments were cemented to form thick layers of rock.
Sometimes ancient seas receded and submerged sediments were lifted high and dry above the water. They became massive layers of limestones in the continents. These chunky fossilized rocks come in chalky whites.and shades of broom and red, depending upon their impurities. Their main ingredient is calcite, alias calcium carbonate. Dolomite limestones have an extra 'helping of magnesium carbonate.
In some places, limestones were altered, or metamorphisized, by changes in the earth's crust. In regions where new mountains were building, they suffered through heat and pressure. In other regions, percolating groundwater carried acid type chemicals. Both these events caused basic alterations. The fossilized rains were impacted and the gritty old sedimentary limestones became smooth new metamorphic marbles.
Calcite is snow white, the color of pure marble. In many cases, strings of algae and other organic material were embedded in the old deposits. They were carbonized and became marblized streaks of black or grey graphite. Iron impurities added candy shades of golds and browns or petal tints of rosy pinks. The mineral chlorite added leafy greens to the finished marble.
Polished marbles, plain or tinted, are handsome stones. But sad to say, they do not last forever. Their calcite minerals tend to dissolve in rains and moist air. When left outdoors in our climate they generally wear away. In warm dry regions they last much longer. This explains why outdoor marble statues are more common in Mediterranean countries and why the works of old masters have been preserved there through many centuries.