[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].
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.
