[Limestone] has many important uses. Much Texas limestone is crushed and used as a road-building material and as an aggregate that is mixed with cement to make concrete. Farmers in some areas improve their crops by adding limestone to the soil. Limestone also is sent to the iron furnaces in east Texas to be used in the production of pig iron and steel.
Some of the Texas limestones are heated to a fairly high temperature in order to change them into lime (calcium oxide). Industry uses a large amount of lime in making chemicals, steel, glass, paper, and other products. Builders use it to make plasters, mortars, and stuccos. At plants in Comal, Johnson, Travis, and Williamson counties, lime is made from [Cretaceous] limestones.
Another important use of [limestone] is in making portland cement. The limestone is mixed with [clay] or [shale], and the mixture is burned in a kiln until it just begins to melt. Then it is allowed to cool. Next, it is finely ground and in order to keep the finished cement from hardening or setting too quickly when it is used, a retarder, such as [gypsum], is added. A number of cement-manufacturing plants in Texas use [Cretaceous] limestones, shales, and clays.
Many of the Texas limestones make excellent building stones. Some of them are quarried from [Pennsylvanian] and [Cretaceous] [formations] in north-central Texas and from Lower Cretaceous formations in counties near the [Llano uplift] of central Texas. A large quarry on the Williamson-Travis County line near Cedar Park in central Texas has supplied Cretaceous [limestone] for many buildings and monuments in the United States and Canada.
Limonite
[Limonite] is not really a definite mineral but is a mixture of iron oxides containing water. It is believed to be closely related to an iron mineral called goethite. Some limonite may be dull and earthy with the appearance of brownish-yellow or rusty brown [clay]. This variety is so soft that a fingernail will scratch it easily.
Other [limonite] has a dark brown or black color and a metallic or almost metallic luster. A copper penny will not scratch it, but a steel file will. This kind of limonite may have a shiny black surface that resembles glossy lacquer. The property that will help you most in identifying limonite is the rusty, yellowish-brown [streak] it leaves when rubbed across a streak plate.
[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.