Minerals: context and value David L. Dillon, M.Sc
Minerals have both geological significance and cultural value. • What is the geological significance? • What is the cultural significance?
What is the geological significance? • Minerals, for the most part, are the building blocks from which rocks are formed. • Rocks tend to be formed from suites of minerals.
Minerals, for the most part, are the building blocks from which rocks are formed
What is a mineral? • Each is a naturally occurring, inorganic, crystalline chemical compound that varies with established limits.
What is the cultural significance? • Minerals have been used for a number of purposes: • Sources of metals; • Building materials; • Electronics applications: • Industrial applications: • Pharmaceuticals; • Cosmetics; • Jewelry; • Medical applications. • Over exposure to some minerals can lead to deteriorating health issues.
Sources of metals • Hematite, and magnetite are the prime natural sources of iron • Sphalerite is the prime source of zinc • Galena is the major source of lead
Building materials • Calcite as limestone and marble are used as building materials. It is also processed to make “quick lime” for the production of Portland cement. • Quartz in the form of sandstone is used as a building material – including the facing material for several buildings on campus. • Drywall was traditionally made from the mineral gypsum.
Electronics applications • Quartz is a component in modern time pieces due to its piezoelectric properties. • Muscovite and biotite have electret properties valued in condenser microphones.
Industrial applications • Calcite in the form of limestone goes into the production of furnace flux in the steel industry. • Halite is used for de-icing roads, and for water softeners. It is also a prime source for chlorine gas and sodium metal. • Corundum, garnet and quartz are used in the creation of sandpaper.
Pharmaceutical and Hygiene applications • Calcite is used in the preparation of antacids and in calcium supplements. • Plagioclase is used in toothpaste as an abrasive. • Talc is used as a filler in tablets of some medications.
Cosmetics • Hematite has long been used for red pigments. • Biotite and muscovite provide a sparkle or sheen in some types of make up. • Talc is used in bath powder.
Jewelry • Garnet is a prized gemstone when transparent. • Corundum is valued for its red variety, ruby, and for its blue, sapphire.
Over exposure • Over exposure to some metallic minerals leads to toxic reactions: • Hematite – iron poisoning. • Sphalerite – zinc poisoning. • Galena – lead poisoning.
Toxicity from minerals • Cinnabar was used as a red pigment in paints and inks. Misuse can lead to mercury poisoning.
Toxicity from minerals II • Arsenopyrite is an uncommon mineral, but much more common than native arsenic. It contributes arsenic to groundwater and rivers when it weathers.
Over exposure II • Over exposure to short fiber asbestos (crocidolite and amosite) can lead to asbestosis and silicosis. These have also been connected with some forms of cancer. Chrysotile, shown here, is considered safer.
Physical properties and Identification • Streak • Specific gravity • Cleavage • Crystal form • Lustre • Shade • Hardness • Diaphaneity(Ability to transmit light) • Diagnostic Properties • Reaction to acid • Striations • Magnetism • Taste
Lustre • This is the quality of the appearance of the mineral. Of what is it reminiscent? Some examples: • - like metal, including coins, mirrors, gunmetal and other things made of metal. E.g. hematite (below left). • Vitreous - like glass, including stained glass, and glazed porcelain. E.g. quartz (below right).
Metallic 1 • Other examples of metallic lustre: 1galena; 2 hematite; 3 bornite; 4 pyrite; 5 native copper; 6 native platinum. • As you can seen colour is independent of lustre. 6 2 5 4 3
Vitreous • Although it is best to look at fresh surfaces as in 1 and 2, crystals can frequently exhibit the lustre characteristic of a mineral. Examples: 1,2 plagioclase; 3 calcite; 4 halite; 5 fluorite; 6 aragonite; 7 quartz. 7 1 6 2 3 5 4
Lustre II • Resinous - like tree sap, amber or plastic. E.g. sphalerite. • Pearly – like pearls or mother of pearl. E.g. muscovite. • Greasy/waxy – like cold animal fat or candle wax. E.g. talc and some forms of serpentine. • Adamantine – brilliant, like diamond. • Earthy – dull, or dirt-like. E.g. limonite and some forms of hematite.
Resinous • Resinous is the look of tree sap or plastic. • Examples: 1 polished amber (fossilised tree sap – not a true mineral.); 2 sulphur; 3 sphalerite. 1 3 2
Pearly • For our purposes, pearly and nacreous are identical terms. Both describe the inside of most mollusk shells. This example is muscovite.
Greasy/waxy 4 • This is the look of cold animal fat or candle wax. • Examples not shown here are serpentine and microcrystalline talc. • Mineral examples here are: 1,2 chalcedony; 3 corundum and: 4 smithsonite. 1 3 2
Adamantine • Adamantine by definition is having the hardness or appearance of diamond. It also implies a high refractive index. • The examples shown here are: 1 diamond and; 2 rutile crystals. 1 2
Earthy • Earthy means having the look of soil or dry mud. • These examples are: 1 uranium ore: 2 hematite and; 3 goethite. 3 2 1
Shade • This refers to whether a mineral is light or dark and not to a specific colour. • For example with the micas, biotite is always dark whereas muscovite tends to be light.
Hardness • A mineral’s hardness is its resistance to being scratched. • Testing is done by comparing the hardness of a mineral with that of tools: fingernail = 2.5; penny = 3; knife = 5; glass plate = 5.5; porcelain streak plate = 7. Mohs’ Hardness scale: 1 Talc soft 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite 6 Orthoclase 7 Quartz 8 Topaz 9 Corundum 10 Diamond hard
Diaphaneity • This the ability of a material to transmit light. The terms used to describe the degree of diaphaneity are transparent, translucent or opaque. • Transparent minerals allow light to pass through and you can see well enough to read through them. • Translucent minerals allow light to pass through but you cannot see objects through them. • Opaque minerals do not allow light to pass through them and you cannot see through them.
Diaphaneity II - examples • 1 calcite. This variety allows light to pass through without any diffusion. Transparent. • 2 biotite. This samples shows that depending on thickness, light may pass through or be absorbed. Transluscent to opaque. • 3 galena. • Opaque. 1 2 3
Streak • A mineral’s streak is seen when it is powdered. • To test for streak rub the sample across a streak plate and observe the colour of the powder that is formed. • It is best to lightly brush away the coarsest powdered matter to see the streak. • Most metallic minerals will give a coloured streak. • Most non-metallic minerals lack a coloured streak.
Streak II - Examples • 1: a sample of hematite being streaked on a porcelain plate. • 2: streak colours of different minerals. From left to right - cinnabar, hematite, goethite and magnetite. 1 2
Specific gravity • Specific gravity is a measure of the density of a mineral with respect to water. • It is generally true that minerals with a metallic lustre are denser that others. There is at least one exception: the barite, which is vitreous to pearly is 4.5, whereas graphite is 2.09-2.23 times the mass of the same volume of water.
Cleavage • The tendency of minerals to break along predictable planar directions is called cleavage. At the atomic scale, if atoms are bound together by ionic or polar bonds, or Van der Waal’s forces, a cleavage might be expected in the direction indicated by the orange plane. • In minerals where bonds are covalent, cleavages are a bit harder to predict. Often a fracture develops.
Cleavage II • The number and quality of cleavages are characteristic of particular minerals and mineral groups. • 1 cubic cleavage involves three cleavages each at 90o to the others. 2 rhombohedral cleavage also involves three directions, but none of these is at 90o to the others. 3octahedral cleavage involves four cleavage. 4prismatic cleavage involves two equally developed directions. 5dodecahedral involves six directions. 6basal cleavage occurs when one direction is developed better than others. 5 3 1 4 2 6
Cubic • Galena (1) and halite (2) exhibit cubic cleavage. 2 1
Rhombohedral • Rhombohedral cleavage is exhibited by the carbonate minerals including calcite, shown here.
Octahedral • Octahedral cleavage is typically seen in fluorite. • The specimen on the right is a fluorite crystal that shows an internal reflection in the upper right that is an incipient break along a cleavage plane. The specimen on the left is a cleavage octahedron produced by cleaving a larger crystal.
Prismatic • Typically, prismatic minerals tend to form elongated fragments due to intersection of the two cleavage directions. • Examples: 1 augite; 2 hornblende; 3 orthoclase. 1 2 3
Dodecahedral • Sphalerite is the only common mineral to exhibit dodecahedral cleavage. • It is often difficult to count all six cleavage directions. As a consequence, it is generally held that if you can count four and there are no equilateral triangles present, then it’s likely that there are six.
Basal • When there is one direction of cleavage, fragments tend to break off as flakes and sheets. • Examples of basal cleavage: 1 muscovite and 2 biotite.
Basal II • Basal cleavage exists because bonding within the crystal structure is weakest in one direction. • In the case of graphite and talc, the atoms are arranged in sheets bound together with covalent bonds with weaker forces holding the sheets together. • In the case of both muscovite and biotite, single potassium atoms lying between the sheets of silicate tetrahedra bind the structure together and make it slightly stronger.
Crystal form • In some cases, the crystal form can be especially helpful in identifying a mineral species. • However, in some minerals there can be a wide variety of crystal shapes within a particular class of crystals.
Diagnostic Properties • There are a a few properties that help determine the identity of a mineral species. These are: • Reaction to acid - effervescence in dilute hydrochloric acid indicates calcite. • Striations – twinning striations are valuable for distinguishing between orthoclase and plagioclase feldspars. • Magnetism – there are a small number of magnetic minerals. Strong attraction to a magnet indicates magnetite. • Taste – this is something to do as a last resort, since there are a number of minerals that are poisonous. Halite has a salty taste. Smell can also be used. A garlic smell indicates the presence of arsenic (as in arsenopyrite) while sulphur smells chiefly indicate native sulphur and sphalerite as well as a few other sulphide minerals.