Physical Properties of Minerals

Colour and Streak One mineral can have different colours Colours are therefore not always diagnostic for minerals Minerals with metallic luster have less colour variations than minerals with non-metallic luster Streak = colour of fine powder can be different from crystal colour, Tested with unglazed porcelain plate (hardness = 7)

Luster Metallic Non-metallic Appearance of a mineral surface which reflects light Non-metallic Metallic Vitreous Resinous Pearly Greasy Silky Adamantine

Metallic luster Metallic luster in pyrite

Vitreous luster = broken glass Vitreous luster in quartz Vitreous luster = broken glass

Resinous luster = piece of resin Resinous luster in sphalerite

Pearly luster = like a pearl Pearly luster in talc

Greasy luster = thin layer of oil

Silky luster = looks like silk Silky luster in chrysotile, a type of asbestoes

Adamantine luster = reflectance of light from a diamond

Other properties depending on light Transparent = transmitting light Translucent = transmitting light diffusely Opaque = impermeable regarding light Fluorescence = emission of light during exposure with UV-light or X-rays (f) Phosphorescence = emission of light after the exposure with UV-light or X-rays

Transparent

Translucent

Opaque

Fluorescence

Minerals in form of aggregates

Parting and Fracture Parting = breaking along planes of structural weakness, produced by pressure, twinning, or exsolution Fracture: the way minerals break which do not have cleavage or parting Conchoidal: smooth curved fracture (interior of a shell) Fibrous: after fibers Hackly: jagged fractures with sharp edges Uneven or irregular: rough and irregular faces

Cleavage Tendency of minerals to break parallel to atomic planes Octahedral Cubic Dodecahedral (12) Prismatic Basal Rhombohedral

Hardness Fingernail: 2.2 Copper penny: 3.2 Pocket knife: 5.1 ..the resistance that a smooth surface of a mineral offers to scratching Fingernail: 2.2 Copper penny: 3.2 Pocket knife: 5.1 Glass plate: 5.5 Steel file: 6.5 Porcelain plate: 7.0

Specific Gravity ..or relative density expressed the ratio between the weight of a material versus the weight of an equal volume water at 4ºC (density = 1g/cm3 in cgs units) The denser the structure and the higher the number of heavy elements, the higher the relative density e.g. test for gold vs gold-coloured minerals

But g/cm3 are not SI units! The SI units for density, as opposed to specific gravity which is a unitless ratio, are: kg/m3 So, converting from g/cm3 to kg/m3 ,

So, for example, water at 4ºC has a density of 1 So, for example, water at 4ºC has a density of 1.000 g/cm3 , but in SI units the density is 1000 kg/m3 ! Specific gravity is numerically equal to density only in cgs units, but since it is a ratio, S.G. does not have units.

Magnetism, Radioactivity, Solubility and Piezoelectricity Ferromagnetic minerals: possesses magnetic order; attracted strongly.to a mganetic field (hand magnet); Paramagnetic minerals: attracted to a magnetic field Diamagnetic minerals: do not response to magnetic field Radioactivity: decay of radioactive isotopes (Geiger counter) Solubility-test with HCl: proof of CO3-containing minerals (fizz test= release of CO2) Piezoelectricity: Generation of an electric current in a crystal through directed pressure

Physical Optics Effects Colour based on dispersion So-called “fire”, which can be seen in diamond, zircon, rutile and some amethyst, is caused by significantly differing refractive indice in the mineral producing different absorptions of wavelength of light; e.g. red and blue in amethyst.

Pleochroism – Dispersion caused absorption (absorption pleochrism) or reflection (reflection pleochroism) of different wavelengths of light in different orientations of a mineral. Reflection pleochroism in covellite

Colour based on scattering Chatoyancy – Refers to the appearance of a cat’s eye (from French oeil de chat), which is caused by scattering of light in bands perpendicular to the fibrous structure present in some minerals; e.g. satin spar, a variety of gypsum. It is also seen in the gemstones tiger eye and cat’s eye. Polished tiger eye, in which the fibres are asbestos which have been silicified ;i.e. permeated by quartz.

Asterism – Scattering yields three-pronged or six-pronged star patterns caused inclusion in some minerals with hexagonal symmetry. This is particularly impressive in the gem varieties of corundum, star rubies and star sapphires. Star sapphire

Aventurescence – Scattering produced by numerous small inclusions as seen in aventurine quartz. In this case, the effect is produced by platy Inclusions of the mica,fuchsite, and hematite.

Opalescence – Scattering caused by atom-sized structures that comprise the mineral as seen in pearls and opal. Precious opal in rock matrix

Colour based on interference Iridescence – Colour produced by thin films such as tarnish. This can be produced on bornite, chalcopyrite or pyrite Iridescent goethite

Colour based on diffraction Labradorescence and Schiller – Diffraction gratings are produced by periodic lamellar structure. In labradorite (Ca-rich plagioclase), exsolution lamellae at low temperatures produced this effect.