Braunite. See [Manganese Minerals].
Calcite
[Calcite], calcium carbonate, is one of the most abundant minerals in Texas. It is the chief mineral in [limestone] and in some [marble]. It also serves as the cementing material in many sandstones. Crystals, grains, and cleavable masses of calcite, which have been deposited by underground water, occur in cracks and cavities in many of the [igneous], [metamorphic], and [sedimentary rocks] of Texas. Calcite also occurs as cave, spring, and stream deposits and as [caliche].
[Calcite] is [transparent] or [translucent], and—depending on the variety—its luster is glassy to earthy. Most calcite is white or colorless, but it can be a shade of pink, blue, green, brown, yellow, or gray. It gives a white [streak] when you rub it across a streak plate.
Two properties of [calcite] to notice are the hardness and the [cleavage]. This mineral cleaves perfectly in three directions that are not at right angles to each other, and some of the [cleavage fragments] are rhombohedrons. Calcite is rather soft—you can scratch it with a copper penny but not with a fingernail. A drop or two of dilute hydrochloric acid also will help you to identify this mineral. The acid will readily fizz and bubble when it is placed on calcite.
[Calcite] has perfect rhombohedral [cleavage]. The three directions of cleavage are not at right angles to each other.
[Calcite] occurs in more different kinds of crystal shapes than any other mineral. Some of these crystals are flat and tabular; some are rhombohedrons; some are prisms. Pointed crystals, called dog-tooth spar, and twinned crystals have been found in the Terlingua area of Brewster County in west Texas. A somewhat unusual occurrence of calcite crystals is in geodes. Some of these are found in Lower [Cretaceous] rocks west of Austin in Travis County.
[Transparent] crystals and [cleavage] fragments of [calcite] show a property called double refraction (other minerals show it, too). To test this property, you can mark a single dot on paper. When you look at the dot through a piece of clear calcite, you will see two dots instead of one. This happens because a ray of light is bent (refracted) and is split into two rays as it enters the mineral. These two rays travel through the calcite in slightly different directions, and each carries an image of the dot through the mineral. The two images that you see are at the points where the two rays leave the calcite.
[Calcite] that is deposited at springs, along river and creek banks, and in caves is known as [travertine]. Cave forms of travertine, including stalagmites and stalactites, occur in several caves in Texas. Another kind of travertine is called calcareous tufa or calcareous sinter. It is a porous and spongy-looking material deposited from water carrying dissolved [limestone] and is found around the openings of some springs and along some creek and river banks.