[Granular] [magnetite] fragments from northwest of Llano, Llano County, Texas, are attracted to a magnet.
At least some of the [magnetite] in this area (such as the deposit at Iron Mountain in Llano County) probably had a different sort of origin. Molten [igneous] rock material containing iron could have moved up into cracks in the ancient [sedimentary rocks]. Then the magnetite formed from this iron material when the igneous and sedimentary rocks were changed into the schists and gneisses of today.
None of the Llano and Mason County [magnetite] deposits is really very large. Nevertheless, prospecting and a little mining have been carried on from time to time at several deposits in this area. At Iron Mountain, which is about 12 miles northwest of Llano in Llano County, magnetite has been mined from open pits. Although magnetite is commonly used as a source of iron, the magnetite from this deposit was used as a heavy concrete aggregate.
Malachite. See [Copper Minerals].
Manganese Minerals ([Braunite], [Hollandite], [Pyrolusite])
Although manganese does not occur alone in nature as a native [element], it makes up a part of many minerals and compounds. This element has an important use in steel making, where it helps rid the steel of unwanted substances, such as oxygen and [sulfur], and, in addition, it is used to make tough, hard, manganese steel for armor plate, railroad tracks, safes, and steam shovels. Manganese has various uses outside the steel industry. It is added to copper and nickel to make alloys, it is used in the manufacture of dry-cell batteries, and (as manganese sulfate) it is used as a fertilizer.
[Manganese minerals] and compounds, such as [braunite], [hollandite], [pyrolusite], and [wad], occur in several counties in Texas. No large, commercial deposits have been found here.
Some manganese compounds and minerals are covered with a soft, sooty black material that will soil your fingers. This can help you recognize these minerals; however, a few non-manganese minerals, such as some [chalcocite], also have a black coating that soils your fingers in a similar way.
One of the [manganese minerals], [braunite], is a complex oxide of manganese that contains silica. It has a submetallic luster and is dark steel-gray or black. When rubbed across a [streak] plate, it leaves a steel-gray or a black streak. This mineral is too hard to be scratched by a pocket knife, but a piece of [quartz] or a steel file will scratch it. Braunite has a [specific gravity] of 4.75 to 4.82. It has four directions of [cleavage] that are parallel to the faces of a pyramid.
In the Spiller mine, about 15 miles northeast of Mason in Mason County, masses of [braunite] occur as lens-shaped layers in [Precambrian] [gneiss] and [quartzite] rocks. This braunite may have formed from another manganese mineral (possibly manganese [garnet]) that was exposed at the earth’s surface after the overlying rocks eroded away. As this other mineral weathered, it may have altered into braunite, or the braunite could have been deposited from solutions emanating from hot magmas before the great thickness of overlying rock was removed.