The mineral of ordinary marble is calcite; dolomite marble contains dolomite. The cementing material in sandstone and a common accessory mineral in shale are calcite. It is truly a wide-spread and abundant mineral. Even the lime deposit in the bottom of the tea-kettle, the water heater, boiler, or automobile cooling system is calcite, or aragonite, a twin brother to calcite. The use of calcite in the form of limestone is treated under limestone. As for the use of large calcite crystals, they are sold as ornaments and curiosities. Visitors to the Missouri State Fair may recall the exhibit of a beautiful, reconstructed crystal cave which was lined with large calcite crystals. Calcite crystals have been shipped in car-load lots to beautify grottos, notably some in Iowa and Illinois, and are displayed in almost all prominent museums. Water-white (clear), optical-quality calcite crystals, which command a high price, are relatively rare and have not been found in Missouri. The optical property of calcite which accounts for its high value is its ability to separate, or refract, every single ray of light passing through it into two widely separated, easily distinguishable rays, hence doubling their number. This is called double refraction, and is shown by the double image of an object viewed through the calcite. Instruments which polarize light may contain calcite crystals. The artificial product, “Polaroid”, is used for a similar purpose. [Illustration: Calcite cleavage rhomb, characteristic rhombohedral shape. Note the double image due to high double refraction of calcite.] Dolomite Dolomite mineral occurs in Missouri as a constituent of dolomitic limestone or as a vein and cavity filling in the rocks of the Joplin mining district and as a lining in cavities in the dolomitic limestones of the southern and eastern parts of the state. Dolomite when _powdered_ (by scraping the surface of the specimen, for dolomite is softer than steel or glass) effervesces freely in cold dilute hydrochloric (muriatic) acid, but the lump dolomite effervesces _very slowly, if at all_. Calcite effervesces freely in the lump with cold dilute acid. This acid test is the one certain test for dolomite, and works with the thick-bedded formations as well as with the showy, crystal-faced material from veins. See page 11. Dolomite crystals have a pearly luster and are usually pale pink in the Joplin district. Their faces are commonly curved but where broken show glistening to pearly cleavage faces. These properties assume more significance in mineral determination as one becomes familiar with mineral collections, but the non-technical person can rely on the acid test. [Illustration: Typical dolomite crystals from Joplin region.]