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Nancy Staff, age 10, of Charlotte, No. Carolina, for her question:

How are geodes formed?

One of nature's favorite games is hide and seek. For example, many useful minerals are hidden in the earth's crust    and it's up to us to go seek them. One of these buried treasures is the geode, a hoard of colorful crystals concealed inside what looks like a plain rock.

Every well stocked rock collection has a geode, a roundish rock sliced in half to reveal its pocket of sparkling crystals. Basically, it is a hollow stone of quartz, sandstone or some other common mineral. Its glassy crystals are clustered together, lining the inside wall of the hollow.

Usually the glassy treasures are tinted crystals of silica in the form of semi precious gem stones. They may be rose tinted or honey¬ tinted quartz crystals. The secret pocket of the loveliest geode is lined with violet tinted crystals of amethyst. Other geodes are lined with rainbow colored bands of agate. All of these surprising pebbles are formed in more or less the same way.

The patient recipe is done by water, drip dripping and seep seeping through pores in the earth's rocky crust. Often the ground is bedrock sandstone, which contains lots of silica. Molecules of this common mineral are used to build tough quartz and gritty grains of sand, also a long list of semi precious stones in colors that rival the rainbow.

Seeping ground water dissolves silica and other minerals from our sandstone bedrock. It seeps faster or slower through the pores, depend¬ing on the dry and rainy seasons. When it evaporates or becomes trapped, it tends to dump its dissolved loot. These deposited molecules patiently arrange themselves to form glassy crystals.

Sandstone is a porous rock, filled with mini pockets. Here and there, it may have holes as big as your fist. In order to create a geode, the earth needs a secret little cave in the bedrock and lots of percolating ground water. The patient recipe takes perhaps a hundred million years or more.

As the ground water seeps through the porous sandstone, it dissolves silica and other minerals. Time after time after time, it seeps into the mini sandstone cave and leaves its loot behind. One by one the deposited molecules arrange themselves in crystal formations. If all of them are silica, the crystals will be glassy clear quartz. If a few impurities, such as iron, are present, the glassy crystals will be tinted with some flowery color.

Sooner or later, the relentless weather wears down the bed of sand¬stone. It falls apart in chunky pebbles and shatters into piles of sand. Our geode has a hard rocky crust and when the bedrock is eroded, it becomes an ordinary looking rock. Because it is hollow, it is lighter than solid rocks of the same size. And where you find one geode, usually there are others.

 

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