Topic 2 - Minerals Outline Introduction Atoms and Elements Bonding and Compounds Physical Properties of Minerals Mineral Groups Silicate Mineral Family Carbonate, Phosphate and Sulphate Minerals Ore Minerals Mineral Resources and Reserves
Introduction Rock Any naturally formed, non-living, firm and coherent aggregate mass of solid matter that constitutes part of a planet. Or defined as an aggregate of minerals. Mineral Naturally occurring, inorganic, crystalline solids that have definite physical and chemical properties. More than 3500 minerals have been identified. The bulk of the Earth is made up of 14 minerals or mineral groups. Includes gemstones and other collectables.
Granite (igneous rock) is made up of a variety of minerals, such as potassium feldspar, quartz, plagioclase, biotite, and hornblende. Rock vs. Minerals
British Crown Jewels. Tower of London. Set with 2868 diamonds, 17 sapphires, 11 emeralds, 5 rubies and 273 pearls. Gemstones are defined as precious or semiprecious mineral or rock used for decorative purposes, especially jewelry.
Turquoise. Blue-green mineral . CuAl 6 (PO 4 ) 4 (OH) 8 x 4H 2 O. Hydrated copper aluminum phosphate. Semiprecious stone used in jewelry.
Requirements for a mineral . What is meant by the term mineral? To be called a mineral, a substance must meet the following four requirements: (a) Naturally Formed (b) Solid (c) Specific Chemical Composition (d) Characteristic Crystal Structure (a) Naturally Formed Excludes a vast number of substances produced in the laboratory. (b) Solid Excludes liquids and gases. (c) Specific Chemical Composition Excludes solids like glass, which has a continuous composition range that can not be expressed by an exact chemical formula. Specific ratio of cations to anions. (d) Characteristic Crystal Structure Excludes amorphous materials.
Victoria Transvaal diamond from South Africa. Mineral . Precious gemstone used in jewelry.
Black pearls from the Cook Islands, South Pacific. Composed mostly of the mineral aragonite: CaCO 3 . Biological activity of mollusk. Mineral ? Semi-precious gemstones used in jewelry.
Insect preserved in amber. Amber is hardened resin (sap) from trees. An organic substance. Semi-precious gemstone used in jewelry. Mineral ?
Atomic mass number = number of protons and neutrons.
Isotopes are forms of the same element with different atomic mass numbers. Most isotopes are stable (e.g. carbon 12 and 13). Some isotopes are unstable (e.g. carbon 14). By convention, the mass number is noted as a superscript preceding the chemical symbol of an element, and the atomic number is placed beneath as a subscript. e.g. 14 6 C. Carbon for example has an atomic number of 6 and an atomic mass number of 12, 13 or 14, depending on the number of neutrons present.
Adjacent atoms share electrons by overlapping their electron shells
Form a compound
No electron sharing or transfer
Weak attractive force between atoms/molecules
Form a compound
diamond graphite van der Waals Bonding
Metallic Bonding Electrons move about easily from atom to atom. “ Extreme” type of sharing. e.g. Galena PbS
Physical Properties of Minerals Will discuss seven key physical properties (general) of minerals. Presented with a slightly different emphasis in comparison to your lab. (A) Crystal Form (B) Growth Habit (C) Cleavage (D) Luster (E) Colour and Streak (F) Hardness (G) Density and Specific Gravity Above are mostly referred to as “ general ” physical properties. Other general properties that will be discussed in the lab include the following: e.g. tenacity: resistance to breaking, crushing, bending, etc. e.g. diaphaneity: ability to transmit light (transparent, translucent, opaque).
Physical properties of minerals are determined by composition and crystal structure . We can use physical properties to identify the minerals. Properties most often used are the most obvious: colour, external shape and hardness. Other less obvious properties are luster, cleavage and SG. Many other properties. Specific physical properties include: - magnetism - feel - taste - odor - reaction with dilute HCl acid - etc. Some of these specific physical properties will be discussed in Lab 1.
Some minerals can grow two completely different types of crystal forms. e.g. graphite and diamond (both made of carbon). However they will always have the same angles between the crystal faces. Crystal form reflects the internal order of a mineral. Crystals can only form when mineral grains can grow freely in an open space . e.g. geode or amethyst. Crystals are uncommon in nature because most minerals do not form in open, unobstructed spaces. However, the internal organization of the mineral is still ordered, but the faces are “rough and uneven” due to the obstructed growing space. Cubic: halite, galena, pyrite. Dodecahedron (12 sides): garnet. Octahedral (8 sides): diamond. Prism capped by a pyramid: quartz.
Well shaped crystal of smoky quartz (a), versus specimen of rose quartz (b) with no obvious crystals.
Giant gypsum crystals . Cavern in Mexico. Up to 15.2 m long.
(CaSO 4 * 2H 2 O)
(B) Growth Habit Every mineral has a characteristic crystal form. Some have such distinctive forms we can use the property as an identification tool, without having to measure the angles between the crystal faces. Crystalline aggregates. Conditions of growth. How the crystals grow together. Commonly found as inter grown cubes. e.g. mineral fluorite (CaF 2 ). e.g. mineral pyrite (FeS 2 ). e.g. asbestos. Variety of the mineral serpentine. Fine elongate threads.
(C) Cleavage Tendency of a mineral to break in preferred directions along bright, reflective planar surfaces. Do not confuse a crystal face ( growth surface ) with a cleavage surface ( breakage surface ). Cleavage planes are parallel to planes of weak bonding . Governed by crystal structure. Angles between crystal planes are the same for all grains of a given mineral. Cleavage is therefore a valuable guide for the identification of minerals. Most common minerals have distinctive cleavage planes. e.g. mica. Cleavage planes are flat sheets.
Quality and intensity of light reflected from a mineral.
Five most important lusters:
like that on a polished metal surface
Vitreous/Glassy like that on glass
Resinous like that of resin
Pearly like that of pearl
Greasy like a surface covered by a film of oil
glassy resinous pearly greasy metallic
(E) Colour and Streak caledonite hornblende Colour of a mineral is often a striking property. Colour determined by chemical composition. Unfortunately, colour is not a very reliable means of identification. Small amounts of Fe can drastically change the colour of a mineral. Colour will change as the % of Fe increases or decreases. sulfur
Tourmaline (multi-coloured) and quartz (colourless). Himalaya Mine, California.
Streak is defined as a thin layer of powdered mineral made by rubbing a sample on a non-glazed porcelain plate (streak plate). Powder gives a reliable colour effect because all the grains are small. e.g. a red streak characterizes hematite (Fe 2 O 3 ). Even though the specimen itself looks black and metallic. You will be shown an example in your lab.
(G) Density and Specific Gravity Obvious physical property. How heavy it feels. Density or mass per unit volume. Units are grams per cubic centimeter (g/cm 3 ). Gold has a high density (19.3 g/cm 3 ). Gold panning: gold works it way to the bottom and sides of the pan. Atoms are closely packed. Ice has a low density (< 1.0 g/cm 3 ). Atoms are loosely packed. Minerals can be divided into a heaviness or density scale. Many common minerals have densities of 2.5 to 3.0 g/cm 3 . Heavy minerals include gold (19.3 g/cm 3 ), galena (PbS; 7.5 g/cm 3 ) and magnetite (Fe 3 O 4 ; 5.2 g/cm 3 ).
Specific gravity is defined as the ratio of the weight of a substance to the weight of an equal volume of pure water. Ratio of two weights. Does not have any units. Density of water is 1.0 g/cm 3 . Specific gravity of a mineral is therefore equal to it’s density. Practical method for measuring density or specific gravity. Could drop the mineral into a container of water to get the volume. Weigh on a balance to get the weight. Can also be approximated by holding different minerals in each hand. Metallic minerals feel heavy. Nearly all other minerals feel light.
Approximately > 3500 minerals. Twelve elements make up 99.23 % of the crust mass. These elements constitute most minerals . T op five elements by weight: O, Si, Al, Fe, Ca . O and Si make up 74 % of the crust (by weight). Silicate minerals are most abundant of all naturally occurring, inorganic compounds. Oxides are the second most important.
S ulphate Anion: (SO 4 ) 2- : Gypsum and Anhydrite. Silicate Anion (SiO 4 ) 4- : Variety of Silicate Minerals. Carbonate Anion: (CO 3 ) 2- : Calcite and Dolomite. Hydroxyl Anion (OH) 1- : Amphibole.
Silicate Mineral Family Silicate minerals are the most abundant of all naturally occurring, inorganic compounds. Four O atoms are tightly bonded to the single Si cation. Bonding is largely covalent (share electrons, strong bond). Oxygen is a large anion. The four oxygen atoms pack tightly into a tetrahedron with the silicon atom (cation) in the center: Si +4 , O -2 . Structure and properties of silicate minerals are determined by the way in which the silicate tetrahedra pack together in the crystal structure. Following mineral groups comprise the Silicates Mineral “Family”: Olivine Group Garnet Group Pyroxene and Amphibole Group Clay, Mica and Chlorite Group Quartz Feldspar Group
Silica Tetrahedron Each quartz crystal is made of lots of silica tetrahedra with silicon in the middle and oxygen at the corners: (SiO 4 ) -4 . One oxygen atom (2 6) needs a share of two electrons to give it a full shell. Therefore four oxygen atoms would need to share eight electrons altogether. Silicon only needs to share four electrons. Electron sharing goes on between tetrahedra as well as within tetrahedra. Many of the common rock forming minerals are based on silica tetrahedron. These kinds of minerals are silicate minerals. O 2- Si 4+ O 2- O 2- O 2- silicate tetrahedron (SiO 4 ) -4
fayalite forsterite Fe 2+ 2 (SiO 4 ) Mg 2+ 2 (SiO 4 ) Olivine Group One of the most abundant mineral groups in the Earth. Common constituent in igneous rocks in the oceanic crust and upper part of the mantle. Glassy-looking. Pale green in colour. Isolated silicate tetrahedra . Group formula (Mg,Fe) 2 SiO 4 . Fe +2 can substitute readily for Mg +2 .
Mg 3 Al 2 (SiO 4 ) 3 Pyrope Fe 2+ 3 Al 2 (SiO 4 ) 3 Almandine Garnet Group Characteristically found in metamorphic rocks of the continental crust. Can also be found in certain igneous rocks. Range of compositions due to ionic substitution. Isolated silicate tetrahedra . Forms beautiful crystals: gemstones. Hard minerals, useful as an abrasive for grinding or polishing. Complex formula A 3 B 2 (SiO 4 ) 3 . A can be any of the cations Mg +2 , Fe +2 , Ca +2 and Mn 2+ , or any mixture. B can be either Al 3+ , Fe 3+ , Cr 3+ or a mixture of them.
Feldspar Group Most common mineral group in the Earth’s crust. Accounts for 60 % of all minerals in the continental crust. Wide range of composition. Continuous 3D networks of SiO 4 and AlO 4 tetrahedra. Most common feldspars are: potassium feldspar (“K-spar”) K(Si 3 Al)O 8 (orthoclase) plagioclase (“plag”) (Na,Ca)(Si,Al) 4 O 8 Si 2 Al 2 or Si 3 Al Common part of formula: (Al, Si 3 )O 8 KAlSi 3 O 8 Orthoclase Albite NaAlSi 3 O 8
Most important ionic substitution is in the plagioclase mineral group. Na 1+ and Si 4+ substitute for Ca 2+ and Al 3+ . Range in composition from albite mineral (Na-rich) to anorthite mineral (Ca-rich). Albite: NaAlSi 3 O 8 Anorthite: CaAl 2 Si 2 O 8 Brackets with a comma (e.g. (Na, Ca)) indicates that these cations can readily substitute for one another. No comma with brackets indicates no mutual substitution. KAlSi 3 O 8 Orthoclase Albite NaAlSi 3 O 8
Examples of common non-ferromagnesian silicate minerals.
Carbonate, Phosphate & Sulphate Minerals Calcite CaCO 3 Complex carbonate anion (CO 3 ) 2- . Forms three important common minerals: Calcite (hexagonal) Aragonite (orthorhombic) Dolomite Calcite and aragonite have formulas CaCO 3 . Dolomite is CaMg(CO 3 ) 2 . Calcite and dolomite look very similar (vitreous luster, distinct cleavage, soft). Distinguish calcite and dolomite by using dilute HCl (hydrochloric acid). Calcite reacts vigorously (bubbling and effervescing). Dolomite reacts very slowly (little or no effervescence). Carbonates are common in sedimentary rocks. Oil production from SW Manitoba is mostly hosted in carbonate rocks. Oil in Mississippian carbonate rocks: Virden, Daly and Tilston fields, SW Manitoba.
Ca 5 (PO 4 ) 3 (OH,F,Cl) Apatite Apatite is the most important phosphate mineral . Contains complex anion (PO 4 ) 3- . Bones and teeth are made out of apatite. Common in many igneous and sedimentary rocks. Main source of phosphorous used for making phosphate fertilizers. Phosphorite Deposits
CaSO 4 · 2(H 2 O) Gypsum CaSO 4 Anhydrite All sulphate minerals contain the sulphate anion (SO 4 ) 2- . Two common minerals: - anhydrite (CaSO 4 ), and - gypsum (CaSO 4 * 2H 2 O). Both form when sea water evaporates. e.g. potash deposits in Saskatchewan (Rocanville, Esterhazy). Old sea that dried up. Prairie Evaporites (Devonian). Gypsum is used in plaster. “ gyp-rock”. Why use gypsum? Water in structure (hydrous mineral) therefore a fire retardant.
Ore Minerals pyrite hematite cassiterite Minerals that have valuable metal contents. Elements, sulphides and oxides. Most common sulphide minerals have metallic luster and a high specific gravity. Most common are pyrite (FeS 2 ; “fool’s gold”) and pyrrhotite (FeS). Lead is from galena (PbS). Most of the zinc is from sphalerite (ZnS). Most of the copper from chalcopyrite (CuFeS 2 ). e.g. Flin Flon (Cu-Zn sulphides). Most common oxide minerals are magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ). Main Fe-ore minerals. e.g. Thompson (nickel oxides). Other oxide minerals include: Rutile (TiO 2 ): titanium source. Cassiterite (SnO 2 ): tin. Uraninite (U 3 O 8 ): uranium.
Mineral Resources and Reserves Most minerals that are abundant in the Earth’s crust have neither commercial value or any particular use. Ore minerals are rare and hard to find. Non-renewable. Therefore, need to recycle. resource A concentration of naturally occurring solid, liquid or gaseous material in or on the Earth’s crust in such form and amount that economic extraction of a commodity from the concentration is currently or potentially feasible. Includes the following (many of these are not minerals): metallic resources (metals), non-metallic resources(sand, gravel, crushed stone, sulfur), energy resources (uranium, coal, oil, natural gas). Therefore, by definition, not all resources are minerals. Need to distinguish between a resource (total amount of a commodity whether discovered or undiscovered) and a reserve . reserve Part of the resource base that can be extracted economically.
Magnesite (MgCO 3 ) mine, Gabbs, Nevada. Used to manufacture high-T resistant construction materials, refractory bricks, oxychloride cement, medicines, and cosmetics.
Many uses for mineral resources : - metals for machines - industrial products: computers, appliances, etc. - construction and building materials - fertilizers - chemicals - energy Without the needed supply of mineral resources our industries would falter and our living standards would decline. Limited supply. Most of the largest and richest mineral deposits have already been discovered. Most are also depleted. Getting tougher and tougher, and more costly to find additional resources.
Non-renewable resource. We are using mineral resources at a much faster rate than they can form. Therefore, new supplies or suitable substitutes must be found. No way to tell how long the supplies will last. Recycling is important. As mineral resources become depleted, low grade/high cost mineral deposits will become profitable to mine. New mining (extraction) and milling (processing) technologies will help. Geologists are using increasingly sophisticated geophysical and geochemical mineral exploration techniques. How will society cope and respond to mineral limitations? Big challenges lie ahead. Some minerals have significant economic interest. Two examples: diamonds and gold .
Diamond Hardest known substance. Made out of carbon (C). Used in drills and saws to cut through all known rocks and metals. Valuable gem. Very rare. Form in the mantle at depths of 150 km: “explosive”. Hosted in an igneous rock called kimberlite. Kimberlite rock is unstable at the Earth’s surface (rock weathers). Could be beneath a lake or swamp. Kimberlite pipes are a few hundred meters in diameter. Diamond mining is becoming very hot in Canada.
First mine opened in October 1998 (BHP Ekati diamond mine) in the North West Territories. Second mine began production in early 2003 (Diavik Diamonds Project). Both mines are in the Lac de Gras region, 350 km NNE of Yellowknife. Other mines include Snap Lake, NWT and Victor Mine, Ontario. Canada is ranked #3 in the world in diamond production: Botswana #1, Russia #2.
Gold Mined for at least 6000 years. California gold rush (1848-1853). Klondike gold rush, Yukon (1897-1899). Most people that arrived did not strike it rich. Precious metal. Mostly used in jewellery. Not many practical applications. Gold prices plummeted in the early-to-mid 1990s. Resource industries suffered. Many central banks sold gold (e.g. Great Britain). Gold has bounced back very strongly in the early 21 st century. Resource industries booming through the early 21 st century. Gold at US$1000 per ounce.