Pam Turner, age 11, of Mullins, South Carolina, for her question:
How do gypsum roses grow?
These everlasting roses are mineral crystals that do not grow on bushes in a flower garden. They may be called gypsum rosettes or gypsum flowers, perhaps because some of the samples resemble glassy bows and others are clusters of long pointed petals. Others are shaped like fish tails or arrowheads. Some of the loveliest glassy crystals have round or tubular cavities, and sealed inside them there may be drops of water or bubbles of air.
Crystallized gypsum flowers are too lovely for severe chemical terms. But the everyday gypsum from which they are formed is classed as a hydrate of calcium sulphate, the chemical term for a mineral compound of water, calcium and sulfate. Gypsum is one of the softest minerals, and countless tons of it are dissolved in sea water. It is thought that large deposits on the land were formed when stretches of salty water were sealed off from the sea. The gypsum was separated from the other dissolved ingredients and deposited when about 37 per cent of the water evaporated.
Most of this gypsum formed massive beds of soft rock, either snowy white or tinted with delicate amber, grey or pink. This common gypsum has a silky or 'Pearly luster, and a thin sliver is transparent around the edges. It is soft enough to scratch with a fingernail, and weighs only 2.3 times more than water. Fine grained gypsum is soft, waxy alabaster. Another form of fibrous gypsum is satin spar, a soft stone that can be polished to glow like a pearl.
Under certain conditions, these ordinary gypsums can be converted intot glassy crystal called selenite. Its name was borrowed from an older word for the Moon because only moonbeams can compare with its delicate beauty. (Its radiance was also once thought to change with the phases of the moon.) The crystals grow when molecules of gypsum are able to arrange themselves in their particular lattice formations. The structure of these molecules tend to form prisms or flat, table top structures.
Sometimes they build twin crystals joined together, forming fish tails or arrowheads. Sometimes long crystals grow in graceful curves. When conditions are right, selenite crystals tend to grow in groups. The lovely gypsum roses are clusters of curved selenite crystals.
The growth of selenite crystals requires gypsum molecules and moisture, plus a long period of peace and quiet. Huge geodes lined with selenite crystals grew in the salt lakes of Utah. Others grew patiently in deposits of salt, sulpher or clay. The flowery types tend to grow in moist, quiet caves. The loveliest gypsum roses are formed in the age old Mammoth Caves of Kentucky.