The mineral variety [hollandite] is a rare manganate of manganese and barium. It has a metallic luster, and its color is silvery gray or black. When you rub it across a [streak] plate, hollandite leaves a black streak. It has a [specific gravity] of 4.7 to 5. Hollandite is rather hard, but a steel file will scratch it.
[Hollandite] from Jeff Davis County, Texas.
[Hollandite] occurs in western Jeff Davis County in west Texas at what is called the Mayfield prospect. Here, it is found as rounded masses that occur in a vein near a large [fault] in Lower [Cretaceous] [limestone] rocks.
Other manganese compounds, [pyrolusite] and [wad], are found in several important deposits near the Pecos River in western Val Verde County. Pyrolusite is a manganese dioxide mineral. It is black, [opaque], and so soft that it rubs off on your fingers like soot. Pyrolusite may be [granular] and [massive] or may be powdery. It also occurs as a fern-like coating on rocks. Wad is not really a mineral but is an impure, dull-black or brownish-black mixture of manganese oxide, water, and other substances. It can be soft enough to soil your fingers, or it can be too hard to scratch with a pocket knife. Wad occurs in earthy or compact masses or in crusts or stains on rocks.
In Val Verde County, the [wad] and [pyrolusite] are found mixed with soil, [clay], [gravel], [sand], and plant remains. This material fills cracks in Lower [Cretaceous] limestones, it is scattered through gravels, and it is deposited in low places at the surface. The manganese in these deposits came from [limestone] rocks that have since weathered away. Rainwater trickled into these rocks and dissolved the [manganese minerals] they contained. This manganese was washed down toward the Pecos River and was deposited as wad and pyrolusite.
Marble
[Marble] is a [metamorphic rock] made up chiefly of sparkling grains of [calcite] or [dolomite], but other minerals may be present. The marble may be fine grained, medium grained, or coarse grained; commonly, all the mineral grains are about the same size.
[Marble] may be of uniform color, banded, spotted, or streaked. If it is made up only of pure [calcite] or [dolomite], the marble is white. If, however, it contains carbonaceous material, such as [graphite], it is grayish or black. [Limonite] impurities cause the marble to be yellowish brown, and manganese oxides and [hematite] give it a brownish, pinkish, or reddish color.
[Marble] is a rather soft rock, and you can scratch it easily with a pocket knife. A few drops of dilute hydrochloric acid will bubble and fizz readily on [calcite] marble; on [dolomite] marble, it may fizz slightly.