Part 17

Limonite has no cleavage and no crystal shape of its own. But crystals of other iron minerals, such as pyrite and magnetite, alter to form limonite. It then occurs with a crystal shape that originally belonged to one of these other minerals. (Such false forms of minerals are called pseudomorphs.) Limonite also occurs as layers in rocks, as hollow or solid concretions, or as coatings on other minerals. It is found mixed with minerals such as clays and serves as the cementing material in some sandstones.

Limonite is found in many localities in Texas including Blanco, Brewster, Burnet, Llano, and San Saba counties. The most important limonite deposits in Texas, however, are in the eastern part of the State, particularly in Anderson, Cass, Cherokee, Henderson, Marion, Morris, Nacogdoches, Smith, and Upshur counties.

The east Texas limonite deposits occur mainly in Weches sedimentary rocks. These rocks, which were deposited in the sea during Eocene Tertiary time, contain clay along with greensands. (Greensands are small, soft grains that contain glauconite, a mineral composed of iron, silicon, and several other elements.) Later, as the sea retreated, these sediments became a part of the land. Waters seeping through the sediments changed into weak solutions of carbonic and sulfuric acid that dissolved the iron out of some of the greensands. When conditions were favorable, this iron was re-deposited as an iron-carbonate mineral called siderite. Siderite was changed to limonite by weathering. Some siderite is still found in east Texas, and it is also mined along with the limonite as an iron ore.

East Texas iron ore has been mined from time to time ever since about 1855, and records show that a number of local iron furnaces once operated. The brown iron ore (as the limonite is also called) now is mined from open pits in Cass, Cherokee, and Morris counties.

This ore, after being washed, goes into blast furnaces at Lone Star (near Daingerfield) and at Houston. In the blast furnaces the ore is changed into metallic iron by mixing it with coke (made from coal) and limestone and blowing in blasts of hot air.

To make steel, the iron from the blast furnace (called pig iron) is put into open-hearth furnaces together with scrap iron, limestone, and other materials. This mixture is heated and melted together to get rid of unwanted substances. Then other elements, such as molybdenum, manganese, or nickel, are added to make steel with the right strength and toughness.

Skip car Hot Gases Blast Furnace Iron Ore Limestone Coke Fire Brick lining Steel Plate covering 3550°F Hot Air Blast Slag Slag Ladle Molten Iron Iron Ladle

Steel mills alongside the furnaces in Texas turn out many products, such as steel plates for oil tanks, ships, and tank cars and steel beams for framework in buildings and bridges. Some of their other products include pipes for the oil and chemical industries and wire for nails and fencing material.

Lithographic Limestone. See Limestone.

Llanite

Llanite is a unique rock that is found only in Llano County in central Texas. This intrusive igneous rock is made up of easily seen crystals and grains of quartz and feldspar that are scattered through a brown-colored mass of extremely small mineral grains. The quartz is beautiful, sky-blue, and opal-like; the feldspar has a rusty pink color. (Because the quartz looks like opal, this rock often is called opaline granite.) The mineral grains that make up the brown-colored mass are so tiny that they can be identified only with a microscope. They are quartz, feldspar, mica, fluorite, and apatite.

Llanite formed during Precambrian time. Molten rock material forced its way upward into cracks that cut across granite and schist rocks while the rocks were still far underground. This hot magma remained in the cracks where it cooled and hardened to form long, narrow, wall-like masses (called dikes) of llanite. We can see some of the llanite dikes exposed at the earth’s surface to the north and northeast of Llano in Llano County because the overlying rocks have weathered away.

Llanite has been quarried from one of the dikes west of Babyhead in northern Llano County. Because llanite is both attractive and strong, it has been used as an ornamental stone and as a monument stone.

Magnetite

Magnetite, iron oxide, is a black, metallic mineral with an outstanding physical property: it is magnetic—fragments of magnetite readily cling to a magnet. It also leaves a black streak when rubbed across a streak plate. Although this mineral is too hard to be scratched by the average pocket knife, a steel file will scratch it. Magnetite is fairly heavy—it has a specific gravity of 5.18.

Magnetite occurs as compact or granular masses, as scattered grains, and as crystals. Most of the crystals are octahedrons, but some dodecahedrons are found. Magnetite helps make up a part of many metamorphic and igneous rocks, and it also occurs as tiny crystals and grains in some sands, sandstones, and other sedimentary rocks.

Open-Hearth Furnace Scrap metal Alloying Elements Limestone Furnace Interior Live Fuel Burner Air pre-heated

Most of the magnetite that has been found in Texas occurs in Precambrian gneiss and schist rocks of the Llano uplift area of central Texas, particularly in Llano County and in eastern Mason County. It occurs as thin layers, as thick lens-shaped deposits, and as scattered grains in the rocks. Probably at least a billion years ago these gneisses and schists were sedimentary rocks, such as shales and sandstones. Some geologists believe that these rocks could have contained iron sediments (perhaps in the form of glauconite). Great forces below the earth’s surface crumpled and squeezed the sedimentary rocks and changed them into the metamorphic schist and gneiss rocks we see today. As this happened, the iron sediments in the rocks were changed into magnetite.

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.

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.