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.
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.
[Illustration: Limonite ore is changed to metallic iron in a blast
furnace.]
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.