mineralogy

• Minerals


  Homogeneous substances with fixed chemical composition, crystal forms, and other distinctive characteristics such as colour, lustre, and hardness
• Crystals


  Formed when a rock body passes from the liquid to the solid state
• All minerals are solids except metallic mercury and water, which is considered a mineral
• International Mineralogical Association (IMA): 'A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes'
• Mineralogy


  Systematic study dealing with the characteristics of minerals
• Scientific branches of mineralogy

  • Crystallography
  • Physical mineralogy
  • Chemical mineralogy
  • Environmental mineralogy
  • Descriptive mineralogy
• Crystallography

  Studies crystal forms, internal structure, relations, and distribution of atoms, ions or ionic groups in the crystal lattice
• Physical mineralogy


  Studies physical properties of minerals such as cohesion, optical, thermal, magnetic properties, electrical conductivity, and radioactivity
• Chemical mineralogy


  Studies chemical formula and chemical properties of minerals
• Environmental mineralogy

  Studies complex conditions of mineral origin, element behavior in ecosystems, natural and industrial effects of minerals, and mitigates contamination problems
• Descriptive mineralogy

  Deals with the classification of minerals into groups based on their common properties, mostly chemical and structural properties
• Elementary knowledge on symmetry elements of crystallographic systems
• Crystal geometry or crystal morphology

  Deals with symmetry, faces, and forms of a crystal
• A crystal possesses three types of symmetry: Plane of symmetry, Centre of symmetry, Axis of rotation symmetry
• Crystal system is a category of crystals with reference to the position of their crystal faces and the relationship of the intercepts that the planes containing the faces make with three (or four) axes, intersecting at an origin
• Crystal systems

  • Cubic (isometric)
  • Tetragonal
  • Orthorhombic
  • Hexagonal
  • Trigonal
• Crystal systems

  • Orthorhombic
  • Hexagonal
  • Trigonal
  • Monoclinic
  • Triclinic
• Different crystal forms can be referred to the same set of crystal axes and belong to the same crystal system
• Each mineral crystallizes in one of the seven crystal systems
• If no crystal of any geometric shape is visible in the constituent minerals of a rock in its thin section under the microscope, it is called amorphous
• If only some small crystals are detected in the thin section of the rock under the microscope, it is termed cryptocrystalline
• Twinning of crystal: Two or more crystals of the same species can occur intergrown or in contact together in a particularly symmetric manner; such crystals are called twins
• Colour of a mineral is seen on its surface by the naked eye and may depend on impurities present
• Streak is the colour of the powdered mineral, observed by scraping the mineral across a plate of unglazed hard porcelain
• Iridescence is a play of colours characteristic of certain minerals
• Cleavage: Most minerals can be cleaved along certain specific crystallographic directions which are related to planes of weakness in the atomic structure of the mineral
• Some minerals possess no cleavages, whereas others may have one, two, three, or four cleavages
• When a cleavage is poorly developed, it is called a parting
• A surface formed by breaking the mineral along a direction which is not a cleavage is called a fracture
• A fracture may occur in a specimen which is either an aggregate of tiny crystals or glassy
• A curved, rippled fracture is termed conchoidal (shell-like)
• Hardness is defined by scratching minerals against each other and seeing which one is gouged
• Relative hardness is defined by Mohs' scale of hardness, ranging from 1 to 10
• Properties used for mineral identification

  • Hardness
  • Lustre
  • Crystal Habit
  • Specific Gravity
  • Transparency
  • Reaction with acid
  • Tenacity
  • Other properties
• Terms to describe hardness

  • 1 to 10 on Mohs' scale of hardness
• The only common mineral with hardness greater than 7 is garnet
• Most other minerals with hardness greater than 7 are semiprecious or precious stones
• Light is reflected from the surface of a mineral, the amount of light depending on physical qualities of the surface (such as its smoothness and transparency). This property is called the lustre of the mineral
• Terms to describe lustre

  • Splendent
  • Dull
• The development of an individual crystal or an aggregate of crystals, to produce a particular external shape depends on the temperature and pressure during their formation