Properties of Rocks

Rocks, normally comprising a mixture of minerals, not only combine the properties of these minerals, but also exhibit properties resulting from the way in which the rocks have been formed, or perhaps subsequently altered by heat, pressure and other forces in the earth's crust. It is comparatively rare to find rocks forming a homogeneous mass, and they can exhibit hard-to-predict discontinuities such as faults, perhaps filled with crushed material, and major jointing and bedding unconformities. These discontinuities can be important in mining, not only for the structural security of the mine, and for gaining access to mineral deposits, but also as paths for fluids in the earth's crust which cause mineral concentrations.

In order for mining to be economic, the required minerals have to be present in sufficient concentration to be worth extracting, and within rock structures that can be excavated safely and economically. As regards mine development and production employing drilling, there must be a correct appraisal of the rock concerned. This will affect forecast drill penetration rate, hole quality, and drill-steel costs, as examples.

One must distinguish between microscopic and macroscopic properties, to determine overall rock characteristics. As a rock is composed of grains of various minerals, the microscopic properties include mineral composition, grain size, the form and distribution of the grain, and whether the grains are loose or cemented together. Together, these factors develop important properties of the rock such as hardness, abrasiveness, compressive strength and density. In turn, these rock properties determine the penetration rate that can be achieved, and how heavy the tool wear will be. In some circumstances, certain mineral characteristics will be particularly important to the means of excavation. Many salts, for example, are particularly elastic, and can absorb the shocks of blasting without a second free face being cut, thereby directly influencing mining method. The drillability of a rock depends on among other things, the hardness of its constituent minerals, and on the grain size and crystal form, if any. Quartz is one of the commonest minerals in rocks. Since quartz is a very hard material, a high quartz content in rock can make the rock very hard to drill, and will certainly cause heavy wear, particularly on drillbits.This is known as abrasion.

Conversely, a rock with a high content of calcite can be comparatively easy to drill, and cause little wear on drillbits.

As regards crystal form, minerals with high symmetry, such as cubic galena, are easier to drill than minerals with low symmetry, such as amphiboles and pyroxenes.

A coarse-grained structure is easier to drill, and causes less wear of the drill string than a fine-grained structure. Consequently, rocks with essentially the same mineral content may be very different in terms of drillability. For example, quartzite can be fine-grained (size 0.5-1.0 mm) or dense (grain size 0.05 mm). A granite may be coarse-grained (size >5 mm), medium-grained (1-5 mm) or fine-grained (0.5-1.0 mm).

A rock can also be classified in terms of its structure. If the mineral grains are mixed in a homogeneous mass, the rock is termed massive, as with most granite. In mixed rocks, the grains tend to be segregated in layers, whether due to sedimentary formation or metamorphic action from heat and/or pressure. Thus, the origin of a rock is also important, although rocks of different origin may have similar structural properties such as layering.

The three classes of rock origin are:

- igneous or magmatic formed from solidified lava (at or near the surface) or magma (underground).

- sedimentary formed by the deposition of reduced material from other rocks, organic remains or by chemical precipitation (salts etc.).

- metamorphic formed by the transformation of igneous or sedimentary rocks, in most cases by an increase in pressure and heat.

Igneous Rock

Igneous rocks are formed when magma solidifies, whether deep in the Earth's crust (plutonic rock), as it rises to the surface (in dykes cutting across other rock or sills following bedding planes), or on the surface (volcanic, as lava or ash). The most important mineral constituents are quartz and silicates of various types, but mainly feldspars. Plutonic rocks solidify slowly, and are therefore coarse-grained, whilst volcanic rocks solidify comparatively quickly and become fine-grained, sometimes even forming glass.

Depending on where the magma solidifies, the rock is given different names, even if its chemical composi­tion is the same. A further subdivision of rock types depends on the silica content, with rocks of high silica content being termed acidic, and those with lower amounts of silica termed basic. The proportion of silica content can determine the behaviour of the magma and lava, and hence the structures it can produce.

Sedimentary Rocks

Sedimentary rocks are formed by the deposition of material, by mechanical or chemical action, and its consolidation under the pressure of overburden. This generally increases the hardness of the rock with age, depending on its mineral composition. Most commonly sedimentary rocks are formed by the breaking down of another rock by mechanical action such as weathering or abrasion, its transportation by a medium such as flowing water or air, and subsequent deposition, usually in still water. Thus, the original rock will partially determine the characteristics of the sedimentary rock. Weathering or erosion may proceed at different rates, as will the transportation, affected by the climate at the time and the nature of the original rock. These will also affect the nature of the rock eventually formed, as will the conditions of deposition. Special cases of sedimentary rock include those formed by chemical deposition, such as salts and limestones, and organic material such as coral and shell limestones and coals, while others will be a combination, such as tar sands and oil shales.

Another set of special cases is glacial deposits, in which deposition is generally haphazard, depending on ice movements.

Several distinct layers can often be observed in a sedimentary formation, although these may be uneven, according to the conditions of deposition. The layers can be tilted and folded by subsequent ground movements. Sedimentary rocks make up a very heterogeneous family, with widely varying characteristics.

Metamorphic Rocks

The effects of chemical action, increased pressure due to ground movement, and/or temperature of a rock formation, can sometimes be sufficiently great to cause a transformation in the internal structure and/or mineral composition of the original rock. This is called metamorphism. For example, pressure and temperature may increase under the influence of up-welling magma, or because the strata have sunk deeper into the Earth's crust. This will result in the recrystallization of the minerals, or the formation of new minerals. A characteristic of metamorphic rocks is that they are formed without complete remelting, or else they would be termed igneous. The metamorphic action often makes the rocks harder and denser, and more difficult to drill. However, many metamorphic zones, particularly formed in the 'contact zones' adjacent to igneous intrusions, are important sources of valuable minerals, such as those concentrated by deposition from hydrothermal solutions in veins.

As metamorphism is a secondary process, it may not be clear whether a sedimentary rock has, for example, become metamorphic, depending on the degree of extra pressure and temperature to which it has been subjected. The mineral composition and structure would probably give the best clue.

Due to the nature of their formation, metamorphic zones will probably be associated with increased faulting and structural disorder, making the planning of mine development, and efficient drilling, more difficult.

Date: 2015-12-24; view: 814