Mineral Properties - Geology

• Determined by the chemical composition of the mineral
• Minerals rich in Al, Ca, Na, Mg, Ba and K are often light coloured
• Minerals rich in Fe, Ti, Ni, Cr, Co, Cu and Mn are often dark coloured
• Determined by the atomic structure of the mineral
• Atomic structure controls which components of white light are absorbed or reflected
• Colour is not particularly useful as a diagnostic property
• Some minerals show a wide variety of colours - quartz can be transparent, white, pink, brown, purple, yellow, orange and even black.
• Many minerals show similar properties.

• When outlines of objects seen through it appear sharp and distinct.
• A good example is Iceland Spar, a variety of calcite that is used for optical lenses.
• Iceland Spar also shows the remarkable property of double refraction.
• Determined by the atomic structure and chemical composition of the mineral.

• The ability for a mineral to let light pass through it
• Many minerals, if cut thin enough, will show some degree of translucency
• Controlled by atomic structure and chemical composition
• All transparent minerals are also translucent

• The way in which a mineral reflects light
• Controlled by the atomic structure of the mineral

Main types of lustre are:

• Vitreous - glassy
  
• Metallic - like metals
• Pearly - like a pearl/mother of pearl
• Silky - like silk (occurs in minerals with a fibrous structure)
• Resinous - like resin (mineral has a grainy appearance)
• Adamintine - like a diamond
• Dull/earthy - does not reflect light and has the same appearance as soil

• The colour of a mineral's powder
• Obtained by rubbing a mineral specimen on an unglazed white porcelain tile
• Useful for identifying metalli ore minerals
• Silicates generally do not mark the tile and have no streak
• White minerals streaked on a white tile will have a white streak
• Any minerals harder than the tile (6) will scratch it

• Measured relative to an equal volume of distilled water at 4 degrees. (1L  = 1kg) (1cm^3 = 1g)
• Controlled by the atomic weight of the constituent atoms (chemical composition) and the atomic structure
• A useful property for identifying metallic ore minerals - these usually have relative densities over 5
• The only non-metallic mineral which is quite dense is barytes (4.5)
• Most of the silicate minerals have densities between 2.5 and 3.2

• Not all minerals have cleavage, but all minerals will have some form of fracture. Fracture describes how a mineral breaks into forms or shapes other than flat surfaces.

Common Fracture Descriptions:

• Conchoidal - describes a curved, nearly rounded, smooth fracture like the inside of a shell. This is best seen in obsidian (igneous), but also in massive pieces of quartz.
• Fibrous - describes minerals (like chrysotile asbestos) that break into fibres.
• Splintery - describes minerals that break into stiff, sharp, needle-like pieces.
• Hackly - fractures that have rough edges
• Uneven/irregular - minerals that break into rough, uneven surfaces.

• How a mineral breaks into flat surfaces.
• Determined by the crystal structyre of the mineral - planes of relative weakness as a result of the regular locations of atoms and ions.

Types of cleavage:

• Basal or pincoidal - only one plane of cleavage. Graphite, mica (can be peeled into thin sheets.)
  
• Cubic - 3 cleavage planes that intersect at 90 degrees. Halite - when broken, will form more cubes.
• Octahedral - 4 cleavage planes. Fluorite, diamond. Common in semi-conductors.
• Rhombohedral - 3 cleavage planes intersecting at angles that are not 90. Calcite.
• Prismatic - 2 cleavage planes. Spodumene.
• Dodecahedral - 6 cleavage planes. Sphalerite.

• Measured on Moh's scale of hardness.
• Atomic structure and bond type control it - covalent bonds are generally stronger than ionic ones.
• Smaller atoms promote greater hardness, generally.
• Larger atoms/ions (e.g. carbonates) care softer.
• Should not be confused with difficulty of breaking - a hard mineral may be very brittle.