Rock Forming Minerals
All the minerals constituting the rocks can be classified into two main types:
Engineering Significance of Rock Forming Minerals
All the minerals constituting the rocks can be classified into two main types:
- Essential Minerals: These are the minerals which form more than 50% of the rocks.
- Accessory Minerals: These minerals occur in limited quantities as small crystals.
Engineering Significance of Rock Forming Minerals
- The civil engineers need to know the properties of rocks precisely to enable them to consider different rocks for any required purpose, i.e., as foundation rocks, as road meta, as concrete aggregate, as building stones, as flooring or roofing material, as decorative material etc. All properties of rocks are, in turn, depended on the properties of their constituent minerals. Thus, properties of civil engineering importance such as their strength, durability and appearance of rocks can be assessed only with the knowledge of the minerals that form rocks i.e. rock-forming minerals.
- The economic minerals, since they are scare, do not influence the properties of rocks and are hence irrelevant from the civil engineering point of view. However, if they happen to occur in large quantities, their economic value will not permit them to be used either as construction materials or as foundation sites.
Quartz
Quartz is a glassy looking, transparent or translucent mineral which varies in color from white and grey to smoky. When there are individual crystals they are generally clear, while in larger masses quartz looks more milky white. Quartz is hard - it can easily scratch a steel knife blade. In many rocks, quartz grains are irregular in shape because crystal faces are rare and quartz does not have a cleavage (i.e. it does not break on regular flat face).
Quartz is a glassy looking, transparent or translucent mineral which varies in color from white and grey to smoky. When there are individual crystals they are generally clear, while in larger masses quartz looks more milky white. Quartz is hard - it can easily scratch a steel knife blade. In many rocks, quartz grains are irregular in shape because crystal faces are rare and quartz does not have a cleavage (i.e. it does not break on regular flat face).
Feldspar
Feldspar is the other common, light-colored rock forming mineral. Instead of being glassy like quartz, it is generally dull to opaque with a porcelain-like appearance. Colour varies from red, pink and white (orthoclase) to green, grey and white (plagioclase). Feldspar is also hard but can be scratched by quartz. Feldspar in igneous rocks forms well developed crystals which are roughly rectangular in shape, and they cleave or break along flat faces. The grains, in contrast to quartz, often have straight edges and flat rectangular faces, some of which meet at right angles.
Feldspar is the other common, light-colored rock forming mineral. Instead of being glassy like quartz, it is generally dull to opaque with a porcelain-like appearance. Colour varies from red, pink and white (orthoclase) to green, grey and white (plagioclase). Feldspar is also hard but can be scratched by quartz. Feldspar in igneous rocks forms well developed crystals which are roughly rectangular in shape, and they cleave or break along flat faces. The grains, in contrast to quartz, often have straight edges and flat rectangular faces, some of which meet at right angles.
Mica
Mica is easily distinguished by its characteristic of peeling into many thin flat smooth sheets or flakes. This is similar to the cleavage in feldspar except that in the case of mica the cleavage planes are in only one direction and no right angle face joint occurs. Mica may be white and pearly (muscovite) or dark and shiny (biotite).
Mica is easily distinguished by its characteristic of peeling into many thin flat smooth sheets or flakes. This is similar to the cleavage in feldspar except that in the case of mica the cleavage planes are in only one direction and no right angle face joint occurs. Mica may be white and pearly (muscovite) or dark and shiny (biotite).
Pyroxene
The most common pyroxene mineral is augite. Augite is generally dark green to black in co lour and forms short, stubby crystals which, if you look at an end-on section, have square or rectangular cross-sections.
The most common pyroxene mineral is augite. Augite is generally dark green to black in co lour and forms short, stubby crystals which, if you look at an end-on section, have square or rectangular cross-sections.
Amphibole
The most common amphibole is hornblende. Hornblende is quite similar to augite in that both are dark minerals, however, hornblende crystals are generally longer, thinner and shinier than augite and the mineral cross-sections are diamond-shaped.
The most common amphibole is hornblende. Hornblende is quite similar to augite in that both are dark minerals, however, hornblende crystals are generally longer, thinner and shinier than augite and the mineral cross-sections are diamond-shaped.
Olivine
Olivine, or peridot, in the jewelry trade, is yellow-green, translucent and glassy looking. Crystals are not common; it usually occurs at rounded grains in igneous rocks or as granular masses. Olivine is almost as hard as quartz; it does not have a well-developed cleavage.
Olivine, or peridot, in the jewelry trade, is yellow-green, translucent and glassy looking. Crystals are not common; it usually occurs at rounded grains in igneous rocks or as granular masses. Olivine is almost as hard as quartz; it does not have a well-developed cleavage.
Quartz and Feldspar are light-coloured minerals; mica, pyroxene, amphibole and olivine are dark-coloured. The color of a rock will be determined by the proportions of light and dark-coloured minerals present. If most of the grains are quartz and feldspar, then the overall appearance of the rock will be light, while the opposite will be true if the minerals are mainly mica, pyroxene, amphibole or olivine. The color of a rock with between 25 and 50% dark minerals is intermediate.