Physical properties of crystals minerals

Geologist à University of Mysore
24 May 2017

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Physical properties of crystals minerals

  3. • A crystal may be defined as a solid polyhedral form of a substance bounded by smooth geometrical surface called faces • Crystal word comes greeks word crystallos meaning clear ice • Crystal may be natural or synthetic . Natural crystals occurring in thousands of varieties are formed in the earth form natural fluids by their cooling under different conditions of temperature and pressure. • Most minerals occur in the form of crystals or crystalline aggregates • A mineral is a naturally occuring inorganic solid substence that is characterized with a definite atomic structure • Deals with most important physical properties of minerals which are studied megascopically and microscopically for their identification • Includes explanation of essential features of crystallography with special reference to minerals
  4. Physical Properties of Minerals
  5. • Minerals are composed of atoms, arranged in a specific order, with a well defined chemical composition. It might expect the microscopic variations will also be manifested in macroscopic physical and chemical properties.
  6. • Crystal form or habit : - The external morphology of crystals generally reflect the internal arrangement of their constituent atoms. This can be obscured, however, if the mineral crystallized in an environment that did not allow it to grow without significant interaction with other crystals (even of the same mineral).
  7. Chrysotile Asbestos Belongs to the Serpentine mineral family - hydrated ferromagnesian silicate.
  8. Crystal Forms: Quartz
  9. Feldspar
  10. Intergrown cubic crystals of fluorite
  11. • Luster : - This property describes the appearance of reflected light from the mineral's surface. Nonmetallic minerals are described using the following terms: vitreous, pearly, silky, resinous, and earthy.
  12. • Color : - Although an obvious feature, it is often unreliable to use to determine the type of mineral. – Color arises due to electronic transitions, often of trace constituents, in the visible range of the EM spectrum. For example, quartz is found in a variety of colors. • Color of a mineral may be quite diagnostic for the trace element and coordination number of its bonding environment.
  13. • Streak : - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate. – Streak is usually less variable than color. – Useful for distinguishing between minerals with metallic luster.
  14. Density and Specific Gravity • Density :- Defined as the mass divided by the volume and normally designated by the Greek letter, row, mass/volume; SI units: kg/m3 or kg m-3 , but geologists often use g/cm3 as the unit of choice. • Specific Gravity: - Ratio of the mass of a substance to the mass of an equal volume of water. Note that water = 1 g cm-3 . S.G. is unitless. • Examples: - quartz (SiO2) has a S.G. of 2.65 while galena (PbS) has a S.G. of 7.5 and gold (Au) has a S.G. of 19.3.
  15. • Hardness - This is the resistance of the mineral to abrasion or scratching. This property doesn't vary greatly from sample to sample of the same mineral, and thus is highly diagnostic. It also is a direct reflection of the bonding type and internal atomic arrangement. A value is obtained by comparing the mineral to a standard scale devised by Moh, which is comprised of 10 minerals ranging in hardness from talc (softest) to diamond (hardest).
  16. Mohs’ Hardness Scale
  17. Fingernail Hardness (2.5) Scratches Gypsum (2)
  18. Hope Diamond: 44.5 carats
  19. Polymorphism and polymorphs • Substances having the same chemical composition but different crystal structures. – e.g. diamond and graphite • Both minerals are composed of pure carbon, but diamond is the high pressure polymorph of graphite. • This gives rise to extremely different physical properties.
  20. Polymorphism Graphite & CalciteNatural Octahedral Diamond 3 mm From:
  21. Diamond vs. Graphite Crystal Structures From: Hardness: 10 Hardness: 1-2
  22. • Cleavage : - Orientation and number of planes of weakness within a mineral. Directly reflects the orientation of weak bonds within the crystal structure. This feature is also highly diagnostic. • Fracture : - This describes how a mineral breaks if it is not along well defined planes. In minerals with low symmetry and highly interconnected atomic networks, irregular fracture is common.
  23. Planer Cleavage in Mica
  24. Weak Bonding Yields Planer Cleavage
  25. Amphibole Cleavage ~120/60°
  26. Rhombohedral Cleavage in Calcite
  27. Conchoidal Fracture in Glass
  28. Special and Other Properties • Striations - Commonly found on plagioclase feldspar. Straight, parallel lines on one or more of the cleavage planes caused by mineral twinning. • Magnetism - Property of a substance such that it will spontaneous orient itself within a magnetic field. Magnetite (Fe3O4) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe2O3). • Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images.
  29. Plagioclase striations
  30. Calcite Double Refraction
  31. CONCLUSIONS • Mineral have different physical properties due to different internal atomic arrangement and are impurites • Based on the physical properties and minerals classified in to different groups
  32. REFERENCE • Crystal chemistry (2001) JAK Tareen TRN Kutty {ISBN 8173712972} • Basic course in crystallograpy [2001] JAK Tareen TRN Kutty •