Calcite, Marble and Limestone

Teacher’s Story

Calc spar, or calcium carbonate, is a mineral and is the material of which marble, limestone and chalk are made. The faces of the calcite crystal are always arranged in groups of three or multiples of three—a three-sided pyramid or two pyramids joined base to base. The pyramids may be obtuse or acute. When acute and formed of three pairs of faces, the crystals are called dog-tooth spar. The crystals appear in a great variety of forms, but they all have the common quality of splitting readily in three directions, the fragments resembling cubes which are oblique instead of rectangular. When these cleaved, or split pieces, are transparent, they are called Iceland spar. When an object is viewed through Iceland spar at least one-quarter inch thick, it appears double. The calcite crystal is often transparent with a slight yellowish tinge, but it also shows other colors; and it has a slightly cloudy or slightly pearly or almost glassy luster like feldspar. It is easily scratched with a knife and will not scratch glass. If a drop of strong vinegar or weak hydrochloric acid falls upon it, it will effervesce.

Limestone—so called because it is burned to make quicklime—was formed on the bottoms of oceans; its substance came chiefly from the skeletons of corals and the shells of other sea creatures, since sea-shells and coral stems are pure calcium carbonate in composition. In the water, the shells and corals were broken down, and then deposited in layers on the bottom of the sea. So wherever we find limestone, we know that there was once the bottom of a great sea. Such layers of limestone are now being deposited off the shores of Florida, where corals grow in great abundance. Limestone is used extensively for building purposes, and in most climates is very durable. The great pyramids of Egypt are of limestone. It is not a good material for making roads, since it is so soft that it wears out readily, making a fine easily-blown dust. It is slowly dissolved in water, especially if the water be acid; thus, in limestone regions, there are caves where the water has dissolved out the rock; and attached to their roofs and piled upon their floors may be large icicle-shaped stalactites and stalagmites, which were made by the lime-bearing water dripping down and evaporating, leaving its burden in crystals behind it. When the roof of a cave falls in, the cavity thus made is called a sink hole and is often dangerous. The famous Natural Bridge in Virginia is all that is left of what was once the roof of such a cavern. The water in limestone regions is always hard, because of the lime which it holds in solution; and in such regions the streams usually have no silt, but have clean bottoms; moreover, the springs are likely to become contaminated because the water has run through long caves instead of filtering through sand.

Chalk is similar in origin to limestone; it is made up of the shells of minute sea creatures, so small that we can only see them with the aid of a microscope. Try and think how many years it must have required for the shells of such tiny beings to build up the beds which make the great chalk cliffs of England!

Marble is formed inside of the earth from limestone, under the influence of heat and pressure; it differs from limestone chiefly in that the grains are of crystalline structure, and are larger; it is usually white or gray in color, and sometimes is found in differing colors. At Cadiz in California, marble is found showing twenty or more quite different colors. The most famous marbles are the Carrara of Italy, the Parian from the Island of Paros, and the Pentelican from the mountain of that name near Athens. The reason why these marbles are so famous is that in ancient times sculptors carved beautiful statues from them, and the architects used them for building magnificent temples. The principal marble deposits in the United States are in Vermont, Georgia, Tennessee and California. Marble deteriorates when it is exposed to air which is filled with smoke and gases. It is also used to make lime. When either marble or limestone is heated very hot, it separates into two parts, one of which is lime, and the other carbonic acid gas—the same that is used for charging soda-water fountains.