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.
[Illustration: Metallic iron, after leaving the blast furnace, is made
into steel in an open-hearth furnace.]
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.
[Illustration: 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.