minerals for Cam's course 1

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088 Minerals: Physical PropertiesDavid L. Dillon, M.Sc

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Formation of MineralsMinerals are simply made of atoms.These vary in size accordingto species.Many atoms interact by bonding with adjacent atoms.This occurs in several ways:1) one atom donates an electron that is loosely held to it and the two are attracted by opposite electrical charges; 2) the two atoms share an electron or two between them; 3) two atoms share an electron, but one tends to have it more than the other; and 4) weak atomic forces hold them together.

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In the case of the mineral halite, sodium atoms donate an electron to chlorine and the difference in charge binds them.In the case of diamond, carbon atoms share electrons equally.In sphalerite, zinc shares two electrons with sulphur, but the sulphur atom has a stronger hold on the shared electrons.

Formation of Minerals II

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Formation of Minerals IIISo with a little imagination, it might be possible to envision how atoms might be carried in solution and under the right circumstances, bind together to form minerals.

minerals for Cam's course 2

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Physical properties and IdentificationBecause atoms bind together in known ways, it becomes possible to identify minerals by the physical properties that result from bonding.These are:

StreakSpecific gravityCleavageCrystal form

Diagnostic PropertiesReaction to acidStriationsMagnetism Taste

LustreShadeHardnessDiaphaneity (Ability to transmit light)

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LustreThis 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).

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MetallicOther 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.

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VitreousAlthough it is best to look at fresh surfaces as in 1 and 2, crystals can frequently exhibit thelustre characteristic of a mineral. Examples: 1,2 plagioclase; 3 calcite; 4 halite; 5 fluorite; 6 aragonite; 7 quartz.

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minerals for Cam's course 3

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Lustre IIResinous - 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.

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ResinousResinous is the look of tree sap or plastic.Examples: 1 polished amber (fossilised tree sap – not a true mineral.); 2 sulphur; 3 sphalerite. 1

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PearlyFor our purposes, pearly and nacreous are identical terms. Both describe the inside of most mollusk shells. This example is muscovite.

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Greasy/waxy 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.

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minerals for Cam's course 4

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AdamantineAdamantine 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.

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EarthyEarthy means having the look of soil or dry mud.These examples are: 1 uranium ore: 2 hematite and; 3 goethite.

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ShadeThis 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.Shade is more reliable than colour because trace amounts of some chemical elements can alter the colour of a mineral species.

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HardnessA 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 soft2 Gypsum3 Calcite4 Fluorite5 Apatite6 Orthoclase7 Quartz8 Topaz9 Corundum

10 Diamond hard

minerals for Cam's course 5

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Hardness IIHardness is dependent upon how the atoms in the structure are bound together.Minerals with ionic bonds tend to be softer than those bound by polar and covalent bonds.Talc is softest because the structure is made of sheets bound by weak atomic forces.

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DiaphaneityThis 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.

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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.

Diaphaneity II - examples

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StreakA 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.

minerals for Cam's course 6

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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.

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Specific gravitySpecific 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.

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CleavageThe tendency of minerals to break

along predictable planar directions is called cleavage. At the atomic scale, if atomsare bound together by ionicor 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.

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The number andquality of cleavages are characteristic of particular minerals and mineral groups.1 cubic cleavageinvolves three cleavages each at 90o to the others. 2 rhombohedralcleavage also involves three directions, but none of these is at90o to the others. 3 octahedral cleavage involves four cleavage. 4 prismatic cleavage involves two equally developed directions. 5 dodecahedral involves six directions. 6 basal cleavage occurs when one direction is developed better than others.

Cleavage II

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minerals for Cam's course 7

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CubicGalena (1) and halite (2) exhibit cubic cleavage.

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RhombohedralRhombohedral cleavage is exhibited by the carbonate minerals including calcite, shown here.

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OctahedralOctahedral 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.

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PrismaticTypically, prismatic minerals tend to form elongated fragments due to intersection of the two cleavage directions.Examples: 1 augite; 2 hornblende; 3 orthoclase. 1

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minerals for Cam's course 8

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DodecahedralSphalerite 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 thatif you can count four and there are no equilateral trianglespresent, then it’s likely that there are six.

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BasalWhen there is one direction of cleavage, fragments tend to break off as flakes and sheets.Examples of basal cleavage: 1 muscovite and 2 biotite.

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Basal IIBasal 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 withcovalent bonds with weaker forces holding the sheets together.In the case of both muscoviteand biotite, single potassium atoms lying between the sheets of silicate tetrahedra bind the structure together and make it slightly stronger.

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Crystal formIn 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.

minerals for Cam's course 9

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Diagnostic PropertiesThere are 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 plagioclasefeldspars.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.

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Your AssignmentYou’ll be expected to answer questions about mineral specimens that you’ll find in display cases in the Department of Earth Sciences.

These areto be found in the southeast part of the first floor of Biological and Geological Sciences Building.

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Your Assignment IIIn each of the questions, you’ll see Roman numerals as well as Arabic numerals. The Roman numerals indicate which mineral cabinet houses the specimen you need to see. So for example: IV is cabinet four, the fourth one from the south door along the west wall of the hallway.The Arabic numerals indicate the catalogue number of the mineral specimen in the collection. Be sure you answer the question using the suggested mineral specimen. Some exhibit properties we want you to see better than others.

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Submitting Your AssignmentDeposit your assignment prior to next Wednesday in order to maximize your marks.Do so in the drop box that includes the initial of your last name. I.e. Dillon goes in box A-L; Tsujita goes in M-Z.Assignments delivered late or somewhere other than the proper drop box will be penalized.Late assignments lose 10% per day.If your assignment is received by me in another fashion, penalties will begin with a one day late loss and increase from there. For example: a paper one day late and in the wrong drop box will lose 20% of its value.Simply put, discipline pays off!