Minerals and Mineralogical Analysis

Minerals are the building blocks for soils and rocks present in the Earth, and they have distinctive physical properties namely; color, streak, hardness, cleavage, fracture, luster, habit (or form), tenacity, specific gravity, magnetism, odor, taste and feel.

The streak of a mineral is the color of its powder.

The hardness of a mineral is its abrasion resistance.

The cleavage of a mineral is its tendency to break down along a particular direction; it is described as one set of cleavage, two sets of cleavage, and so on.

Fracture is the character of the broken surface of the mineral in a direction other than the cleavage direction.

Luster is the appearance of the mineral in reflected light.

Habit (or form) of a mineral refers to the size and shape of its crystals.

Tenacity describes the response of a mineral to hammer blows, to cutting with a knife, and to bending.

Hardness and specific gravity are the most useful diagnostic physical properties of a mineral. Hardness is tested by scratching the minerals of known hardness with a specimen of the mineral of unknown hardness. In practice, a standard scale of 10 minerals, known as the Mohs scale of hardness (see Table 1.1), is used for this purpose. The hardness of minerals listed in Table 1.1 increases from 1 for talc to 10 for diamond.

The specific gravity of a mineral is the ratio of its weight to the weight of an equal volume of water at a standard temperature, generally 4°C. The specific gravity of the common silicate minerals forming soils and rocks is about 2.65. For minerals forming the ores, the specific gravity may be as high as 20, for example, native platinum has a specific gravity of 21.46. 

Most minerals have a specific gravity in a range of 2–6. Table 1.2 provides the specific gravity values of some common minerals. Minerals are basically naturally occurring inorganic substances; however, coal and petroleum, though of organic origin, are also included in the list of minerals. Almost all minerals are solids; the only exceptions are mercury, water, and mineral oil (oil petroleum).

It is important to have knowledge of the minerals that form the rocks; such minerals are called rock-forming minerals. Silicates and carbonates, as listed in Table 1.3, are the essential rock-forming minerals. Silicate minerals form the bulk (about 95%) of the Earth’s crust. Silica and feldspars are the most common silicate minerals in the crust. Silica is found in several crystalline forms such as quartz, chalcedony, flint, opal, and chert; quartz is one of the most common forms of silica. High quartz content in a rock indicates that it will have high strength and hardness. Feldspars form a large group of minerals; orthoclase or K-feldspar (KAlSi₃O₈), albite (NaAlSi₃O₈), and anorthite (CaAlSi₂O₈) are the main members. The mixtures (solid solutions) of albite and anorthite in different proportions form a series of feldspars called plagioclases. A plagioclase containing 40% albite and 60% anorthite is called labradorite and denoted as Ab₄₀An₄₀. K-feldspars alter readily into kaolinite, which is one of the clay minerals. Hornblende is a major mineral of the amphibole group of minerals. Enstatite (MgSiO₃), hypersthene [(MgFe)SiO₃] and augite

[(CaMgFeAl)₂(SiAl)₂O₆] are the major minerals of the pyroxene group of minerals. There are two common types of micas: muscovite (white mica) [KAl₂(Si₃Al)O₁₀(OH)₂], which is rich in aluminum and generally colorless, and biotite (black mica) [K(MgFe)₃(Si₃Al)O₁₀(OH)₂], which is rich in iron and magnesium and generally dark brown to nearly black. Both types occur in foliated form and they can be split easily into thin sheets. The composition of common olivine is [(MgFe)₂SiO₄]. Since olivine crystallizes at a high temperature (higher than 1000°C), it is one of the first minerals to form from the molten rock material called magma. Garnets occur both as essential and as accessory minerals in rocks. Clay minerals are hydrous aluminum silicates. Kaolinite [Al₄Si₄O₁₀(OH)₈], illite [KxAl₄(Si₈-xAlx)O₂₀(OH)₄, x varying between 1 and 1.5], and montmorillonite [Al₄Si₈O₂₀(OH)₄] are the principal clay minerals. Calcite (CaCO₃) and dolomite [CaMg(CO₃)₂] are carbonate minerals present in some rocks.

In addition to essential minerals, there are accessory minerals such as zircon, andalusite, sphene, and tourmaline, which are present in relatively small proportions in rocks. Some minerals such as chlorite, serpentine, talc, kaolinite, and zeolite result from the alteration of pre-existent minerals, and they are called secondary minerals. Since these minerals have little mechanical strength, their presence on joint planes within the jointed rock mass can significantly reduce its stability.

The common rock-forming minerals can be identified in the hand specimen with a magnifying glass, especially when at least one dimension of the mineral grain is greater than about 1 mm. With practice, much smaller grains can also be identified. This task is easily done by experienced geologists. If it is difficult to identify minerals by physical observations and investigations, X-ray diffraction, and electron microscopic analyses make the identification task easy. Figure 1.3 shows a typical X-ray diffractogram of an air-dried clay fraction (<2 μm) collected from a shear surface of a recent landslip in South Cotswolds, United Kingdom, where clay minerals (kaolinite- K; illite- I and montmorillonite- M) are easily identified based on a series of peaks of different intensities of X-rays reflected from the minerals corresponding to different angular rotations (2θ) of the detector of the X-ray diffractometer.