Presentation on theme: "Minerals Minerals are the basic constituents of rocks, so understanding minerals is the foundation of understanding rocks. v 0055 of 'Minerals' by Greg."— Presentation transcript:
Minerals Minerals are the basic constituents of rocks, so understanding minerals is the foundation of understanding rocks. v 0055 of 'Minerals' by Greg Pouch at :59:08 LastSavedBeforeThis :26:25 02Minerals.ppt Corrected formula for hematite, a
Minerals 3 Definition of mineral Properties 4 Properties of minerals > Essential 5 Properties of minerals > Accidental 6 Mineral Groups 7 Mineral Groups: Silicate Figures 8 Mineral occurrence 9 Composition of the earth and the universe 10 Major rock-forming (common) minerals
Definition of mineral A naturally-occurring, inorganic crystalline substance whose chemical composition is fixed or varies systematically within a range of compositions Usually, inorganic and naturally-occurring are not important. Crystalline: a solid whose atoms are arranged in a regular, repeating pattern. –The regular, repeating pattern is referred to as a lattice. A lattice is a space-filling, three-dimensional orderly arrangement of unit cells. –The smallest group of atoms that can be used to represent a lattice is a unit cell. Unit cells are equivalent to molecules, but often consist of fractional atoms shared between unit cells. The formula for a mineral is the formula for a unit cell and is electrically neutral. –There are a small number of possible unit cell shapes , and surprisingly few possible crystal symmetry classes . –If you think of this in terms of Legos, a unit cell is a type of block, and a lattice is a particular way of connecting the blocks to fill a volume. Definite composition: fixed composition or a definite range of compositions. There are sometimes substitutions of one element for another, usually between ions with the same charge and similar ionic radii, like Fe +2 and Mg +2 –Fixed: Quartz SiO 2 Always this composition or very close to it. –Varying: Olivine (Fe,Mg) 2 SiO 4 Fe and Mg freely substitute for one another. Allowed compositions include Mg 2 SiO 4, Fe 2 SiO 4, and Mg 0.5 Fe 1.5 SiO 4 This is known as a solid solution series. All three chemical compositions shown have the same arrangement of atoms in the crystal lattice, just different amounts of Fe and Mg atoms.
Mineral properties are determined largely by the crystal structure and the chemical composition of the particular specimen. Particularly influential is strength of bonding between atoms (covalent, ionic, polar metallic). Essential properties (controls identity) define the mineral's identity. In order for a specimen to be an example of a particular mineral, it must have the right symmetry/structure and composition. If the specimen does not have the right composition and lattice, then it is a specimen of some other mineral. –Composition is determined by the elements available "Conditions of formation", mainly –temperature (T) –pressure (P) –pH (acid/base) –eH (oxidizing/reducing). –Lattice Structure, Symmetry Each mineral belongs to one of a small number of symmetry classes, such as hexagonal or cubic. The symmetry class is a reflection of the internal symmetry of the mineral’s unit cell. Sometimes, the same chemical composition can crystallize in several symmetries, in which case the various minerals are polymorphs. –Density is mass per unit volume, so density is controlled by composition+arrangement –All quartz crystals/grains have a quartz crystal lattice and a composition of SiO 2 (or close with some substitutions) Accidental properties do not define a grain/crystal/chunk's identity. An accidental property may or may not be displayed in a given specimen/example. Some quartz crystals show the hexagonal pyramid crystal shape (habit), but other pieces of quartz do not. Properties of minerals, Essential
Properties of minerals, accidental Accidental properties may or may not be exhibited in a particular specimen. Consequent Properties depend closely on the mineral identity. –Habit, or Crystal form, is the external shape of a crystal and is usually only expressed if the mineral grew with a lot of open space (first minerals solidifying from a melt, or grew in a cavity). The shape indicates the symmetry group of the mineral, or of the original mineral. –Breakage is how a specimen breaks in non-preferred directions, or when there is no cleavage. –Hardness is resistance to abrasion (scratching). Geologists use an ordered set of minerals to express hardness: the Moh’s hardness scale. (Memorize it.). Hardness is diagnostic, if you can measure it. –Taste, Smell, Magnetic, Fizzes in weak acid, Fizzes in weak acid when powdered, … Non-consequent properties –Luster is the quality of the light reflected from a specimen, in the sense of like-metal (metallic) or like- glass (glassy, vitreous), greasy, pearly, resinous….It is a very diagnostic property. Luster is best observed by rotating the specimen in sunlight or incandescent lighting (fluorescent lights are hard to work with.) –Streak is the color of the crushed form of the mineral, as observed by sliding it across a ceramic plate. Many minerals, especially non-silicates, have a distinctive streak color. Only minerals softer than a ceramic plate can leave a streak. –Color is often controlled by minor impurities, and is rarely a useful diagnostic property. Except for a few distinct colors, you should ignore it. –Cleavage If a specimen splits/cleaves apart in preferred directions, that direction is a cleavage plane. Cleavage planes are formed by the alignment of weak bonds in the crystal structure. The number and arrangement of cleavage is one of the most diagnostic properties a mineral has.
Mineral Groups Minerals are often classified by the major anion, which determines most of the structure because it is large and occupies most of the volume. –Halides contain a halogen (F -, Cl - ) Common minerals are halite (NaCl) and fluorite (CaF 2 ) –Hydroxides contains OH - Most of these are too fine-grained to identify easily –Sulfides contain sulfide ions S -2 galena PbS, pyrite –Carbonates contain CO 3 -2 (carbonate ion). Common minerals are Calcite CaCO 3, Aragonite CaCO 3, and Dolomite CaMgCO 3 –Sulfates contain SO 4 -2 anhydrite CaSO 4, gypsum CaSO 4 ·2H 2 O, barite BaSO 4 –Phosphates contain PO 4 -3 apatite Ca 5 (PO 4 ) 3 (OH,F,Cl) essential to plant growth and found in teeth –Oxides contain O -2 that isn’t in a complex ion. Hematite Fe 2 O 3, magnetite, corundum –Silicates/Silicate minerals contain SiO 4 -4 The most common minerals. The most complex group of minerals, because the silica tetrahedra can share oxygens and form even more complex ions. Compositional groups –Ferromagnesian; mafic; basic Silicate minerals rich in iron (ferrum) and magnesium. Almost always dark, often black or dark green, and dense. Includes olivine, augite, hornblende, biotite. –Sialic: Silicate minerals comparatively rich in silica and aluminum. Often contain sodium and potassium as well. Usually light colored and lower density. Includes quartz, feldspar, muscovite. Structural groups based on arrangement of silica tetrahedra –Isolated Olivine, garnet –Cycles beryl –Chain-single Pyroxene –Chain-double Amphiboles –Sheet Micas, clays –Framework Quartz, feldspars
Mineral Groups: Silicate Figures Structural groups Based on arrangement of silica tetrahedra –Isolated Olivine, garnet –Chain-single Pyroxene –Chain-double Amphiboles and has hydroxyls –Cycles beryl –Sheet Micas, clays (also has hydroxyls) –Framework Quartz, feldspars
Mineral occurrence Formation of minerals is controlled by –the elements available (if you only have Si and O, you can only get Si and O minerals) –and the "conditions of formation". These are mainly temperature, pressure, pH (acid/base) and eH (oxidizing/reducing). Stability of a mineral is controlled by its chemical environment. Minerals formed under one set of conditions may be unstable under another. –NaCl (halite, used as road salt and table salt) is very unstable in the presence of water. –CaCO 3 (calcite and aragonite) is unstable in the presence of acidic water, due to the tendency of CO 3 -2 to dissolve in water, while –Fe 2 O 3 (hematite, rust) is chemically stable at low-temperature, oxidizing conditions. What minerals are present (easy to determine) and were present (harder to determine) are clues to the history and origin of a rock, so by knowing the mineralogy, you know a lot about where the rock came from and what it has been through. Knowledge of the mineralogy is important by itself, sometimes, like when you're prospecting for ores.
Composition of the earth and the universe –The universe as a whole is composed mainly of hydrogen and helium. (or Dark Matter, but we'll ignore that.) The solar system is also mainly composed mainly of hydrogen and helium, but with more heavy elements, due to a pair of supernovae right before the formation of the solar nebula. The relative amounts of heavy elements in the earth closely matches the relative amounts of these elements in the sun, meteorites, and other planets, suggesting a common origin. –Eight elements (O, Si, Al, Fe, Ca, Na, K, Mg) account for more than 95% of the mass of the earth, and of the crust: –Minerals made of these elements plus H and C account for the vast majority of rocks.
Major rock-forming (common) minerals There are around 3000 to 4000 naturally occurring minerals. Of those, a geologist only needs to be familiar with ~300 common enough to be likely seen in a career. (learnt in Mineralogy) Most of the volume of most rocks is a combination of the minerals in the following table, called the rock-forming minerals. You will occasionally see rocks that have minerals not in this table: the only way to avoid that would be to learn to identify all the known minerals. Memorize these formulae (flashcards are good), since we'll need them when talking about rocks and ores. MineralAbbrevIdeal FormulaWhere formed PlagioclasePlag(CaAl, NaSi)AlSi 2 O 8 Ign, Met K-Spar KAlSi 3 O 8 Ign, Met, Sed QuartzQtzSiO 2 Sed, Met, Ign, soil OlivineOl(Fe,Mg) 2 SiO 4 Ign Pyroxene/AugitePyr(Fe,Mg,Ca)SiO 3 TrashCanIgn Amphibole/HornblendeAmphSimplified: (Fe,Mg,Ca) 7 Si 8 O 22 (OH) 2 TrashCan Recognize Si 8 O 22 (OH) 2.Ign, Met BiotiteBiotK(Fe,Mg) 3 (AlSi 3 O 10 )(OH) 2 TrashCanMet, Ign MuscoviteMuscK(Al) 2 (AlSi 3 O 10 )(OH) 2 Ign, Met GarnetGrn(Ca,Mg,Fe,Mn) 3 (Al,Fe,Cr) 2 (SiO 4 ) 3 TrashCanMet, very rare ign. CalciteCalcCaCO 3 Sed, soil, Met, rare Ign DolomiteDolCaMg(CO 3 ) 2 Sed, Met Clays Kaolinite Al 2 Si 2 O 5 (OH) 4 Montmorillonite (Al,Mg) 8 (Si 4 O 10 )(OH) 10 ·12H 2 O Illite can include muscovite K(Al) 2 (AlSi 3 O 10 )(OH) 2 Soil, Sed Fe oxides Magnetite Fe 3 O 4 Hematite; Fe 2 O 3 Goethite FeO(OH) Ign, Met, Sed, soil Sed, Soil Al oxides Corundum Al 2 O 3 Diaspore AlO(OH) Gibbsite Al(OH) 3 Met, Ign Soil, Sed TrashCan "Garbage can" mineral that can accept lots more substitutions than the formula shows, usually because it has a loose lattice.
Summary Rocks are made of minerals, so understanding minerals is key to understanding rocks. Each mineral has fixed or systematically-varying chemical composition (substitutions based on ion-size and charge) and crystal lattice. These are the essential properties of the mineral. Consequent/accessory properties derive from the essential properties and include crystal shape (form), cleavage pattern (or absence of cleavage), luster, hardness (resistance to abrasion), and color (least diagnostic). One mineral often has many different appearances. Minerals are classified based on the major anion (negatively charged, large ion that dominates crystal structure). Silicate minerals are further classified on how the silica tetrahedra are arranged. The initial minerals are determined by conditions of formation and available chemicals. The final minerals are determined by the initial minerals and their stability in reference to conditions the rock been through. The eight+two common elements account for most of the material, and there are a small number of important minerals that account for most rocks.