Minerals and Crystals: Crystallographic axes and angle, crystal system, Properties of minerals, Rock forming and their engineering significance

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MINERALOGY AND CRYSTALLOGRAPHY

MINERAL

  • A mineral is a naturally occurring chemical compound, usually crystalline form(solid) and organic(abiogenic) in origin. There are many varieties of mineral which are found in earth they are identified by observing their physical, chemical, optical and other properties.
  •  Mineralogy is the branch of geology which does the details study of various aspect related to minerals including properties, occurrence, mode of formation, classification as well as identification.
  • Minerals are broadly classified into ore forming and rock forming minerals.

CLASSIFICATION OF MINERAL

Physical properties of minerals:

  • Hardness
  •  Cleavage
  • Fracture
  • Luster
  •  Color
  • Specific Gravity
  •  Crystal habit
  •  Tenacity
  • Steak
HARDNESS
  • Hardness is defined as the resistance that mineral offers to an external deformation like scratching, rubbing, abrasion etc. It is tested by rubbing the mineral specimen over a fine cut file and noting the amount of powder and degree of noise produced in time of rubbing. The less powder and grater noise produced in time of rubbing, harder the mineral and vice versa.
  • Austrian mineralogist F.Mohs proposed scale of hardness assigning values between 1 to10 which is universally adopted. A series of 10 minerals that are used as scale of hardness developed by mineralogist F. Mohs is known as Mohr’s scale of hardness. The moh’s scale of hardness is given in table:
  • Sometimes other substances beside above minerals are used to find hardness. Hardness of brass (H=3), Glass (H=5.5), Steel (H=6), Fingernail (1-1.5).
CLEAVAGE
  • It is a tendency of mineral to break along the crystallographic planes with formation of smooth surface. Minerals may have no cleavage, 1 to 6 sets of cleavage. For e.g. Quartz- no cleavage, calcite- 3 sets and orthoclase- 2 sets.
FRACTURE
  • It is a surface obtained by breaking the minerals in a direction other than clevage in crystalline minerals and in any direction in massive mineral. On the basis of surface obtain following terms are applied:
  •  Even: The broken surface is smooth and flat. e.g.: Feldspar.
  •  Uneven: The broken surface is rough and entirely irregular. e.g.: chromite.
  • Conchoidal: The broken surface curved concavities more or less deep. e.g.: quartz.
  •  Hackly: The broken surface contains sharp edges and points. e.g.: Gold
  • Splintery: The broken surface is contains Fibers and splinters. e.g.: as in serpentine.
  • Earthy: The broken surface is smooth, soft and porous. e.g.: as in chalk.
LUSTURE
  • It is the way how light reflects from the mineral surface and also called shines of minerals.
  • Mainly there are two types of luster: metallic and Non- metallic luster.
  • Metallic Lustre: If the mineral shine resembles to that of any metal the resulting luster is called metallic luster. Metallic luster are shown by shown by high density, high refractive index and opaque minerals like glena, pyrite etc.
  • Non-Metallic Lustre: If the mineral shines resembles to that of non metal is called Non- Metallic luster. They are of following types:
  • Adamantine: Highly brilliant luster like diamond.
  •  Pearly: Pearl like luster. E.g.: muscovite. Silky: Shines like pure silk. E.g.: gypsum.
  • Vitreous: Reflection like broken glass. E.g.: Quartz, calcite.
  • Resinous: Shines like resin (wax or grease).
  • Earthy or dull: No shines on light. E.g.: clay, bauxite.
COLOUR
  • It is the appearance of the substance in light and depends upon the composition and Structure of the substance. The mineral shows the colour of wavelength which is not absorbed by mineral from white light. Colour may be inherent or of an exotic type but cannot taken as conclusive property in identification of mineral. Example: Magnetite is black in colour, calcite is white in colour.
  •  Idiochromatic minerals are “self colored” due to their composition. The color is a constant and predictable component of the mineral. Examples are blue Azurite, red Cinnabar, and green Malachite.
  • Allochromatic minerals are “other colored” due to trace impurities in their composition or defects in their structure. In this case, the color is a variable and unpredictable property of the mineral. Examples are the blue in Amazonite (orthoclase), yellow in Heliodor (spodumene) and the rose in rose quartz.
  • Pseudochromatic minerals are “false colored” due to tricks in light diffraction. In these cases, color is variable but a unique property of the mineral. Examples are the colors produced by precious opal and the shiller reflections of labradorite.
SPECIFIC GRAVITY
  • In mineralogy, the term specific gravity is used more frequently than density and signifies “the ratio between the density of a mineral and that of water at 4 degree Celsius. ”
  •  Since it is a ratio, it has no units. Specific gravity of quartz is, for instance, 2.65. Specific gravity is also termed relative density.
  •  Sp. Gr. is considered as Very low, low, medium, high and very high.
CRYSTAL STRUCTURE (FORM)
  • Mineral often occur in characteristic body forms or physical shapes. The physical make up of a mineral is expressed by the term structure and is often helpful in identifying a particular mineral. In reality, structure merely shows the habbit in which the crystal or crystalline substance making a mineral tends to occur in nature.
STREAK
  • The streak is colour of mineral in powdered form obtained by scratching or rubbing the mineral over a rough unglazed porcelain plate.
  • The streak may or may not resemble to its body colour.
  • For example: Chromite and magnetite both has colour but streak of chromite is brown and black for magnetite. Also, hematite has body colour black, silver or red but has cherry red to reddish brown colour.
TENACITY:
  • It is the manner in which a mineral reptures or its behavior of minerals towards the force which tends to break, bend, cut or crush.
  • The following term is used to describe Tenacity:
    • Brittle: Minerals break in powder on applying stress. e.g.: quartz.
    •  Flexible: Minerals can be bent. e.g. Talc.
    •  Elastic: Minerals regain their shape on removal of force. e.g.: muscovite.
    •  Malleable: Minerals can be hammered out into thin sheets. E.g.: Gold.
    • Sectile: Minerals can be cut with knife: Gypsum.
    • Ductile: Minerals extend into thin wire on applying force. E.g. cupper

Rock forming minerals:

  • There are only few minerals (less than 100) form the great amount of the rocks of the earth crust. These most common minerals have been grouped as rock forming minerals.
  • Among these rock forming minerals, only about 25 minerals make almost 99.5% of the rocks.
  • There are 10 groups of these most common rock forming minerals.
  • About 8% of the earth crust is made up of silicate minerals.
  • Silicate minerals :
    • The Feldspar Group (Na, K, Ca)AlSi3O8
    • Pyroxene Group (Ca,Na,Al,Li)SiO3
    • Amphibole Group (Ca,Mg,Fe,Mn,Na,K)7(Si4O11)(OH)2
    • Mica Group (K,Na,Li,Mg,Fe)Al3Si3O10(OH)2
  • Oxide minerals:
    • Oxide minerals are second large occurring rock forming minerals.
    • Quartz (SiO2) It is hard and resistant to weathering. It has no cleavage and partings. It has different colored varieties that have economic importance.
    • Corundum (Al2O3) It is very hard and resistant mineral. It has a number of gem varieties.
    • Spinel (MgAl2SiO4) It is also hard as quartz and resistant.
  • Carbonate Minerals:
    • There are a few carbonate minerals, which are very important rock forming minerals:
    • Calcite (CaCO3) Limestone is composed of high percentage of calcite. Marble is recrystallized form of calcite.
    •  Dolomite [Ca,Mg(CO3)2] It occurs in massive forms with extending for kilometers across. It contains a significant amount of magnesium and can be used economically.
    • Magnesite (MgCO3) It is useful as refractory material and source of magnesium.
  • Others Group:
    • Sulphate Group e.g. gypsum
    • Phosphate Group e.g. apatite
    •  Halide Group e.g. Fluorite
    •  Sulphide Group e.g. galena, cinnabar
    •  Native minerals e.g. gold, graphite, diamond sulphur

Engineering significance of rock forming minerals:

Topographical controls:
  • Erosion rate of rock forming minerals
  • Topographical breaks – steepness or smoothness
  • Karst Topography – controlled by Limestone
  • Groundwater control
Strength of Rock Forming Minerals:
  • Hard and soft nature of different minerals
  • Engineering design and site investigation controlled by strength of minerals
  • Quality of construction materials are controlled by strength of minerals
  • Clay minerals like kaolinites swells with water and therefore creates hazardous site for engineering structures.
  • The rock containing silicates are generally resistant towards weathering.

CRYSTALLOGRAPHY

  • Crystallography is branch of geology which deals with various aspects related to crystal such as their geometrical arrangement, external shape, properties, internal or atomic structure, form and their classification into classes and system.
  • A crystal is a homogenous solid regular, polyhedral form bounded by naturally formed smooth geometrical surface. There are thousands of varieties of crystal found in nature. The given substance is crystalline or amorphous depends upon the arrangements of atoms. The crystals are formed by either solidification from liquid or gaseous states or by precipitation from solution. The crystallization is defined as the chemical solid-liquid separation technique, in which mass transfer of a solute from liquid solution to a pure solid crystalline phase occurs.

ELEMENTS OF CRYSTAL

Crystallographic axes and angle :

  •  Crystallographic axes are imaginary line that can be drawn within crystal lattice based on the symmetry of the crystal.
  • They are used to define different crystal systems.
  • All crystal systems except hexagonal have three crystallographic axes, among them two are horizontal (a and b) and one vertical (c). I
  • n hexagonal crystal system there are four crystallographic axes among them three are horizontal axes represented by a1, a2, a3 and one vertical (c). Angle between crystallographic axes are called crystallographic angle. They are α,β & γ

CRYSTAL SYSTEM:

  • It is also called crystal family.
  • Depending upon the different kinds of symmetry exhibited by crystal, six crystal systems are recognized and all the crystal can be grouped into 32 crystal classes.

Isometric or cubic system:

  • In Isometric or cubic system there are three crystallographic axes which are of equal in length and at right angles to each other.
  • Among three, two lies in the horizontal planes and one lies in vertical plane.
  • Symbolically above relation are expressed as: a = b = c α=b=g=90° where a, b, c are crystal axes andα, b, g are the angle between them. Example: Glena, Garnet, pyrite, fluorite

Tetragonal system:

  • In Tetragonal system there are three crystallographic axes among three, two lies in the horizontal planes are equal in length and one lies in vertical plane is not equal to remaining two.
  • All three axes are mutually at right angles to each other.
  • Symbolically above relation are expressed as a = b ≠ c α=b=g=90° Classes: There are 7 classes under Tetragonal system. Example: Zicron, Rutile, Chalcopyrite, Braunite etc.

Orthorhombic system:

  • In this system there are three crystallographic axes among three, two lies in the horizontal planes and one lies in vertical plane and all are of different length equal.
  • All three axes are mutually at right angles to each other.
  • Symbolically above relation are expressed as: a ≠ b ≠ c α=b=g=90° Classes: There are 3 classes under orthorhombic system. Example: barites, Topaz, calamine, Hypersthene etc.

Hexagonal system:

  •  In hexagonal system there are four crystallographic axes among which three lies in the horizontal planes are equal in length, interchangeable and intersect each other at 120°.
  • The fourth one lies in vertical plane is at right angles to all three horizontal axes. This system is further divided into two divisions.
  • Symbolically above relation are expressed as: a1=a2=a3≠c a1 ∩a2= a2∩a3= a3∩a1= 120° a1∩c= a2∩c= a3∩c= 90°
  • Hexagonal division: It is further divided into 7 classes.
  • Trigonal or rhombohedral division: It is further divided into 5 classes. Example: Quartz, calcite, Tourmaline, Corundum.

MONOCLINIC SYSTEM:

  • In monoclinic system has three crystal axes of unequal length and their intersections between axes a and c is not equal to 90 and between a and b, b and c are right angles to each other.
  • Symbolically above relation are expressed as: a ≠ b ≠ c α=g=90°≠ b Classes:
  • There are 3 classes under this system.

Triclinic system:

  •  In triclinic system all three crystal axes have unequal length and are intersects obliquely to each other. Symbolically above relation are expressed as: a ≠ b ≠ c α=g=b≠90° Classes: There are 2 classes under this system. Example: Kyanite, Albie, Microcline, Axinite, Rhodizite etc.

Reference 1: Text book of Engineering Geology , N Chenna Kesavulu

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