Transcript Minerals

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 2013-08-28 12:59:08
LastSavedBeforeThis 2012-08-30 10:26:25
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 [14], and
surprisingly few possible crystal symmetry classes [230].
–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 SiO2 Always this composition or very close to it.
–Varying: Olivine (Fe,Mg)2SiO4 Fe and Mg freely substitute for one
another. Allowed compositions include Mg2SiO4 , Fe2SiO4 , and
Mg0.5Fe1.5SiO4 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.
Properties of minerals, Essential
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, 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.
–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.
–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 likeglass (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.
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 (CaF2)
–Hydroxides contains OH- Most of these are too fine-grained to identify easily
–Sulfides contain sulfide ions S-2 galena PbS, pyrite
–Carbonates contain CO3-2 (carbonate ion). Common minerals are Calcite CaCO3, Aragonite CaCO3,
and Dolomite CaMgCO3
–Sulfates contain SO4-2 anhydrite CaSO4, gypsum CaSO4·2H2O, barite BaSO4
–Phosphates contain PO4-3 apatite Ca5(PO4) 3 (OH,F,Cl) essential to plant growth and found in teeth
–Oxides contain O-2 that isn’t in a complex ion. Hematite Fe2O3, magnetite, corundum
–Silicates/Silicate minerals contain SiO4-4 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.
–CaCO3 (calcite and aragonite) is unstable in the presence of acidic water, due to the tendency of CO 3-2 to
dissolve in water, while
–Fe2O3 (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.
Mineral
Abbrev
Ideal Formula
Where formed
Plagioclase
Plag
(CaAl, NaSi)AlSi2O8
Ign, Met
K-Spar
K-Spar
KAlSi3O8
Ign, Met, Sed
Quartz
Qtz
SiO2
Sed, Met, Ign, soil
Olivine
Ol
(Fe,Mg)2SiO4
Ign
Pyroxene/Augite
Pyr
(Fe,Mg,Ca)SiO3 TrashCan
Ign
Amphibole/Hornblende
Amph
Simplified: (Fe,Mg,Ca)7Si8O22(OH)2 TrashCan Recognize Si8O22(OH)2.
Ign, Met
Biotite
Biot
K(Fe,Mg)3(AlSi3O10)(OH)2 TrashCan
Met, Ign
Muscovite
Musc
K(Al)2
Ign, Met
Garnet
Grn
(Ca,Mg,Fe,Mn)3(Al,Fe,Cr)2(SiO4)3 TrashCan
Met, very rare ign.
Calcite
Calc
CaCO3
Sed, soil, Met, rare Ign
Dolomite
Dol
CaMg(CO3)2
Sed, Met
Clays
Kaolinite Al2Si2O5(OH)4
Montmorillonite (Al,Mg)8(Si4O10)(OH)10·12H2O
Illite can include muscovite K(Al)2(AlSi3O10)(OH)2
Soil, Sed
Fe oxides
Magnetite Fe3O4
Hematite; Fe2O3
Goethite FeO(OH)
Ign, Met, Sed, soil
Sed, Soil
Al oxides
Corundum Al2O3
Diaspore AlO(OH)
(AlSi3O10)(OH)2
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.