ESO 3 B & G 6

Minerals

What to Learn

  • Organic matter vs. inorganic matter. Geochemical elements.
  • Crystalline matter vs. amorphous matter. Crystals vs. glasses. Minerals vs. mineraloids.
  • Physical properties of the minerals: crystal structure, growth habit, density, hardness, tenacity, cleavage, lustre, colour, streak, others.
  • Classification of the minerals. Some important minerals and their physical properties.
  • Obtaining minerals. Beds and mines.
  • Uses of the minerals. Main metallic ores. Main gemstones.

Key Information

Physical Properties of the Minerals
ColourThe colour of a mineral is one of its most obvious attributes and the easiest physical property to determine. Unfortunately, as it results from a mineral's chemical composition and structure, the impurities and structural flaws that may be present can alter completely the colour with regards to the pure mineral. Hence minerals like fluorite and quartz may display a really wide range of colours. This makes colour not the most useful property in helping to characterize a particular mineral.
StreakThe streak refers to the colour of a mineral's powder, which is almost always the same, regardless the impurities and structural flaws of the mineral. Thus, it is much more reliable to characterize a mineral than the colour of the mineral itself. The streak is usually obtained by rubbing the mineral across a plate of unglazed porcelain. The streak and the mineral's typical colour may or may not be the same. Some examples of streaks of common minerals are pyrite (black), magnetite (black), halite (white).
Transparency /
Diaphaneity
A transparent mineral (diamond) allows all light to cross through; a translucent mineral (quartz) allows part of the light to cross through; an opaque mineral (pyrite) does not allow the light to pass at all.
LustreIt refers to the way in wich a mineral's surface reflects light. To some extent it is related to the transparency of a mineral; for instance, metallic minerals are always opaque and vitreous minerals are always translucent.
CleavageIn some minerals, bonds between layers of atoms aligned in certain directions are weaker than bonds between different layers. In these cases, breakage occurs along flat surfaces parallel to those zones of weakness. In some minerals, a single direction of weakness exists, but as many as six may be present. Halite, which forms cubic crystals, presents 3 perfect cleavage directions.
HardnessIt depends on the strength of the chemical bonds and is measured by the ease or difficulty with which a mineral can be scratched. Diamond is the hardest mineral, because it can scratch all others. Talc is one of the softest; nearly every other mineral can scratch it. We measure a mineral's hardness by comparing it to the hardnesses of a standardized set of minerals first established by Friederich Mohs.
TenacityIt is a mineral's physical reaction to stress such as crushing, bending, breaking, or tearing. For example, according to its tenacity, a mineral can be brittle (easy to powder with a hammer), sectil (easy to cut with a knife), malleable (easy to flatten with a hammer, as metallic minerals) or ductile (easy to stretch into a wire, as metallic minerals).
Growth habitRefers to the shape a mineral develops when it is not constricted by lack of available space. For example, quartz forms six-sided prisms capped with pyramid-like faces; halite occur as cubes; and pyrite develop cubes or pentadodecahedrons (polyhedrons with 12 pentagonal faces).
Specific gravityIt's a comparison of the density of a mineral to that of water. For example, quartz has a specific gravity of 2.6 because it is 2.6 greater than that of water. You can also say that it has a density of 2.6 g/cm3
MagnetismWhen a mineral can be attracted by a magnet or act themselves as magnets. The best example is magnetite.
Electrical conductionWhether an electric current can easily pass through a mineral (such as in all metallic minerals and graphite) or not.
FeelWhat you perceive when you touch a mineral. It can be rough, smooth, greasy (talc), cold (diamond)…
TasteWhat you perceive when you lick a mineral. Halite, for instance, tastes salty.
Main Types of Mineral Lustre
[Available in Google Docs]
LustreOpacityExamples
AdamantineTransparentDiamond
VitreousTranslucentQuartz, Halite, Olivine
WetTranslucentFluorite
ResinousTranslucentYellow and red sphalerite varieties
WaxyTranslucentTalc
GreasyTranslucentMilky quartz
SilkyTranslucentFibrous gypsum
PearlyPoorly translucentMica aggregates
Dull metallicOpaqueGraphite
MetallicOpaquePyrite, Galena, Magnetite
EarthyOpaqueKaolinite
Properties of Some Common Minerals
[Available in Google Docs]
CompositionColourStreakTransparencyLustreHardnessGrowth habit
GraphiteCDark greyBlackOpaqueDull metallic1.7Foliated, massive
PyriteFeS2Pale goldenBlackOpaqueMetallic6.5Cubes, pentadodecahedrons
CinnabarHgSVermilion redBright redOpaqueAdamantine / Dull metallic (aggr.)2.2Massive
GalenaPbSGreyGreyish blackOpaqueMetallic2.5Cubes, octahedrons
MagnetiteFe3O4BlackBlackOpaqueMetallic5.7Octahedrons, massive
HematiteFe2O3Reddish greyRedOpaqueMetallic6Tabular
HaliteNaClColourlessWhiteTranslucentVitreous2.5Cubes, etc.
FluoriteCaF2Colourless, yellow, green, blue…WhiteTranslucentWet vitreous4Cubes, etc.
CalciteCaCO3Colourless, white…WhiteTranslucentVitreous3Rhombohedrons
AragoniteCaCO3Reddish greyWhiteTranslucentVitreous4Hexagonal twinning, etc.
GypsumCaSO4·2H2OWhite, grey, redWhiteTranslucentSilky vitreous2Lamellar, fibrous
QuartzSiO2Colourless, white, grey, yellow, violet…WhiteTranslucentVitreous76-sided prisms
OrthoclaseSilicatePinkishWhiteTranslucentVitreous6Massive
Biotite / Black micaSilicateColourless / Black (aggr.)GreyTranslucentVitreous / Pearly (aggr.)2.7Lamellar
Muscovite / White micaSilicateColourless / White (aggr.)WhiteTranslucentVitreous / Pearly (aggr.)2.2Lamellar
OlivineSilicateOlive greenWhiteTranslucentVitreous6.7Granular
TalcSilicatePale green, grey, whiteWhiteTranslucentWaxy vitreous1Massive

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