Minerals
The rocks which formed the Earth, the Moon and the planets are made up of minerals. Everyone has a certain familiarity with minerals for they are present in the rocks of the mountains, the sand of the sea beach, and the soil of the garden. Minerals are naturally occurring solid inorganic substances composed of atoms having an orderly and regular arrangement. The orderly arrangement is the criterion of the crystalline state so that it is possible to express the composition of a mineral as a chemical formula. Minerals have played a major role in humanity’s life and standard of living. They have become increasingly important, and today we depend on them in countless ways, from the construction of skyscrapers to the manufacture hi-tech electronic products. Without minerals, modern society would not exist. They provide the basic framework of industrial production.
The minerals are bounded by crystals’ face when they are free to grow without constraint (Photo 1, 2 and 4). They are invariably disposed in a regular way such that there is a particular relationship between them in any one mineral species. A crystal is bounded by a naturally formed plane faces, and its regular outward shape is an expression of its regular atomic arrangement (Photo 3, 8 and 9). Crystal can be grouped into seven crystal systems, such as cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and trigonal. Crystal can show remarkable differences in shape according to crystal form or combination of forms is developed.
Most minerals occur as aggregates of crystals that rarely show perfect crystal shapes. The form of the aggregate are useful in identification of the minerals. Some crystals grow outwards from a center, and the aggregate so formed is said to be internally radiating. Some grow outwards but are rounded and nodular (Photo 5, 7 and 10) resulting in form which resembles a bunch of grapes called botryoidal (Photo 6). Physical properties are also important in identification of mineral. This properties include density, hardness, cleavage and style of fracture.
Minerals are also classified by major chemical component such as oxides, sulfides, silicates, carbonate, phosphate and so forth. The careful description and identification of minerals often requires highly specialized techniques such as chemical analysis and measurement of physical properties. Minerals may be given names on the basis of some physical property or chemical aspect, or they may be named after a locality, a public figure, a mineralogist, or almost any other subject considered appropriate. Some examples of mineral names and their derivations are as follows :
- Albite (NaAlSi3O8) from the Latin, albus (white), in allusion to its colour.
- Sarabauite (CaSb10O10S6) after a locality, Bau, Sarawak, where it occurs as the by-product of antimony ore.
- Malayaite (CaSnSiO5) after the first discovery in Malaysia.
Rocks
The Earth, and indeed the Moon and planets, are built of material we call rock. Rocks are aggregates of minerals, and can be grouped into igneous rocks, metamorphic rocks and sedimentary rocks. Igneous rocks are formed by the solidification of molten rock material; metamorphic rocks are formed through the alteration of igneous and sedimentary rocks by heat and pressure; while sedimentary rocks are produced by the accumulation of rock waste at the Earth’s surface.
Igneous rock : Most of the igneous rocks are generated within the upper parts of the mantle. This material, which is called magma, migrates upwards into the Earth’s crust and forms rock masses which are called igneous intrusion. If the magma reaches the Earth’s surface and flows out over it, it is called volcanic rocks. The majority of volcanic rocks consist of black, rather dense rock called basalt. However granite is the commonest of the intrusive igneous rock. The recognition and naming the igneous rocks involves an assessment of grain size and estimation of the relative amounts of the constituent minerals. Additional information is obtained from colour index, texture, structure and sometimes from field relationship (Photo 11 – 14).
Metamorphic rocks : Rocks derived from the pre-existing rocks by mineralogical, chemical and structural changes within the Earth in a solid state. Metamorphic rocks display a wide range of texture, structure and mineralogy because of the range of temperatures and pressures. The terms low, medium and high grade refer to the degree of metamorphism that has affected the rocks (Photo 15 – 19).
Sedimentary rocks : Sedimentary rocks are formed by erosion and deposition processes, which are active at the Earth’s surface. The surface of land is continually being attack by the agents of weathering and erosion, such as rain and rivers. These physical agents are helped by chemical decay from percolating waters and together they break up even the toughest rocks and produce rock waste. These sediments are transported mainly by river and deposited at river mouths, in lake or in the sea. The accumulation of materials is often deposited many kilometers thick to make up sedimentary rocks. The main features to be considered when studying sedimentary rocks in the field are texture, structure, mineralogy, field relationships and colour. Bedding is of almost universal occurrence in sedimentary rocks, and is a layering expressed by variation of texture and mineralogy (Photo 20 – 22).
