Barite (“Tiff” in south_east_ Missouri, Heavy Spar, Barytes) occurs in Missouri predominantly as a white, quite heavy, soft, non-metallic mineral which has a high luster on a freshly broken surface. Slightly bluish “glass” barite or “glass tiff” has been found in smaller quantity with the more abundant, opaque white material. The glassy barite may superficially resemble calcite or selenite gypsum, but in distinction, barite breaks or cleaves to surfaces joining at right angles and does not effervesce with acid, whereas calcite does effervesce in acid and cleaves at oblique angles (rhombohedral cleavage). Gypsum is so soft that it can be scratched very easily with the thumb-nail, whereas barite is scratched with difficulty, if at all, by the thumb-nail. Notably, again, barite is “heavy,” with a specific gravity of about 4.5, whereas calcite, gypsum, limestone, and chert are “lighter,” with a specific gravity of about 2.6 to 2.7. Barite has the composition barium sulphate, BaSO₄, of which barium oxide constitutes 65.7 per cent. [Illustration: Three pieces of barite, the crested and bladed form at the left, “glass” barite in the center, and a small crystal at the right.] Barite occurs in abundance in the Jefferson-Washington counties district, which furnishes about 80% of Missouri production. Other production comes from near Houston, Texas county, and from the central district—Miller, Moniteau, Morgan, Cole counties, and adjacent territory. In the Jefferson-Washington counties district, it is dug from residual clay over dolomite and is run through washing and concentrating mills which remove the clay and lighter waste rock. Most of the central district production comes from old sinkhole deposits, the ore being also crushed, washed, and concentrated in preparation for shipment. Missouri barite which was produced during 1941 had a value of over $1,300,000 and constituted about 40% of the total United States productions. Barite is used as a paint pigment and extender, as a flux, as a source of barium in the chemical industry, as a filler in rubber, paper, oil cloth, textiles, and leather, and as a heavy substance in oil well drilling mud. The largest single use is in the manufacture of lithopone paint. Gypsum Gypsum is a soft mineral which can be scratched easily with the finger or thumb-nail. It may be glassy or transparent, or may grade into an opaque white body, possibly stained by iron oxide, but it is always very soft. Of the three varieties of gypsum—selenite, alabaster, and satinspar—only the first two have been found in Missouri by the writer. The chemical composition of gypsum is CaSO₄·2H₂O. [Illustration: Transparent, flexible variety of gypsum (selenite).] [Illustration: Fine-grained, white, opaque gypsum (alabaster).]