Minerals [Most] rocks are [mostly] made of minerals, so rock identification and interpretation depends on recognizing the types, abundances, and arrangement of mineral grains. Over 4000 mineral types have been described, but only about 20 or so account for the bulk of most rocks. These are the rock-forming minerals.
QUARTZ (SiO 2 ) FROM ARKANSAS
Gypsum (CaSO 4 2H 2 O) from Chihuahua, Mexico. Single crystals are up to eleven meters long.
Quartz crystal Note that the quartz crystals (glassy and grey in appearance) in this granite do not form well-defined pyramidal shaped crystals. Do you think quartz was the first or last mineral to crystallize in granite? Pink is orthoclase. White is plagioclase. Black is hornblende. What does that tell you about the melting temperature of quartz compared to the other minerals comprising granite?
Atoms: The Building Blocks Nucleus: contains protons and neutrons Atomic Number: # of protons in the nucleus Atomic Mass: # of protons + # of neutrons Electrons: orbit the nucleus, responsible for bonds between atoms
Atoms are the smallest particles that define the chemical properties of matter. They are composed of protons (+, yellow) and neutrons (neutral, orange) in the nucleus and electrons (-, white) surrounding the nucleus in defined energy levels.
+1 +2 +3 +4-2 -1 Atoms will tend to form bonds such that their outer shells become full (or empty) by either donating or accepting electrons. Electrons carry a negative charge, so the overall charge of the atoms changes when electrons are transferred. The sizes of the atoms will change as well.
Bonds Ionic: transfer of electron(s) from donor ion (cation, +) to recipient ion (anion, -) Covalent: sharing of electron(s) Metallic: sharing across many atoms, resulting in a cloud of electrons permeating the crystal structure Van der Waals: weak tugging of electron(s)
Ionic bonds may involve more than a pair of atoms, as with CaCl 2.
Adding electrons tends to increase the effective size of the atom (ionic radius) whereas removing the tends to make it smaller. Note the difference between ferric iron (Fe 3+ ) and ferrous iron (Fe 2+ ). This, in part, determines whether and where an element will fit into a particular crystal structure without greatly distorting it.
Other Characteristics of Ionic Bonds Common between elements in the 1 st and 17 th columns (1A and 7A) or in the 2 nd and 16 th columns (2A and 6A). Strong under compression but weak under shear (therefore promoting cleavage planes). Produce highly symmetric crystals (such as cubic) of moderate hardness and density. May dissolve easily in water, but typically have a high melting temperature. Poor conductors of heat and electricity.
Covalent Bonds Covalent bonds arise from the sharing of electron(s) between adjacent atoms. The electrons may be shared equally, as above, or unequally, as with polar bonds. When hydrogen bonds with something other than itself, it usually forms a polar bond.
Other Characteristics of Covalent Bonds High melting temperatures Produce crystals of lower symmetry but high hardness. Relatively insoluble in water. Poor conductors of heat and electricity.
Metallic Bonds Here, the electrons involved in the bonds are not associated with any particular atom but are free to wander throughout the structure. These electrons can absorb and emit across a wide range of energies corresponding to visible light, producing the characteristic metallic luster of such materials (often metals). Metals tend to be excellent electrical conductors due to these non-localized electrons. They also typically conduct heat well. Metals also tend to be malleable (easily shaped by striking with a hammer), since the atoms can readily rearrange themselves in the midst of the swarm of surrounding electrons.
Van der Waals forces do not involve electron transfer, merely a tugging of the electrons of one atom towards a neighboring atom due to the polarization of the atoms themselves. For example, these polarizations may arise from unequal sharing of electrons in covalent bonds. Van der Waals forces are quite weak, producing structures which easily cleave along the intervening planes (graphite shown here). Biotite and muscovite have a similar, but stronger bond between layers, called hydrogen bonding.
Five-Part Definition of a Mineral Two Questions: Is it a mineral? First three parts. Which mineral is it? Last two parts.
Is it a mineral? 1) Naturally occurring. Useful for interpreting geologic phenomena. 2) Inorganic. Organic processes and materials will be considered separately. 3) Solid. Strictly speaking, this means crystalline.
Which mineral is it? 4) Specific crystal structure. A particular regular, periodic, ordered atomic arrangement. 5) Specific chemical composition, within a limited range. Allowable variation is determined case-by-case.
Are these minerals? salt? sugar? cubic zirconia? Zircon? glass? ice?
QUARTZ (SiO 2 ) Crystal faces may grow in differing proportions from sample to sample, but the angles between equivalent faces remain constant, controlled by structure at the atomic scale.
Haüy (1781) proposed that crystals could be constructed from building blocks of identical shape and composition, stacked in a repeating pattern.
A cubic building block (unit cell) can produce several crystal shapes, but only those which are compatible with cubic symmetry.
Within each crystal system (for example, cubic), many forms are possible. All such forms are consistent with the underlying symmetry of the system itself.
Useful Physical Properties Color. Treacherous. Different minerals with same color, same mineral with different color. Streak. Color of powdered mineral. Luster. How the surface reflects light. Hardness. Scratch hardness, from 1 to 10 on the Mohs scale. Cleavage/Fracture. How the mineral breaks. Other. Taste, acid reaction, magnetism, etc.
COLOR: A given mineral can occur in a wide range of colors which may depend only on very minor differences in composition. Also, two different minerals may display identical colors. Color, therefore, is NOT a reliable property for mineral identification, generally speaking. It should be used only as a secondary, confirming characteristic. (Beryl = Be 3 Al 2 Si 6 O 18 )
Streak is tested by rubbing the mineral against an unglazed porcelain plate. The color of the powder is quite consistent from sample to sample. Note that the color of the streak may be different than that of the bulk sample. Many streaks are white or colorless.
Tips for Streak Testing The sample must be softer than the porcelain plate (about 6.5 on Mohs scale). Use a high-contrast background. White plate for dark streaks, black plate for light streaks. If the streak is white or colorless, use a different method of identification most nonmetallic minerals fall into this category.
Luster This refers to the manner in which the surface of the mineral reflects light, but is NOT just a question of how shiny it is. First, consider whether the mineral resembles a piece of metal (metallic luster) or not (nonmetallic luster). Metal can be either bright or dull, however.
Luster describes the quality of reflection from the mineral s surface. Above is galena (PbS) which has a bright metallic luster. It is NOT a piece of metal, it merely reflects light like a piece of metal. A surface which resembles dull metal is also said to have metallic luster.
Nonmetallic Lusters A few, arranged by decreasing reflectivity: adamantine vitreous resinous pearly waxy silky dull earthy
Hardness Mohs hardness scale is based on scratch hardness, not indentation hardness. Scale was developed by Mohs (1812) using minerals as reference points. Mineral hardness can be anisotropic. It varies with the crystallographic direction in which it is measured. Report hardness as a range of possible values, depending on available tools.
Cleavage Planes Many (not all) minerals not only grow with flat faces, but also break along flat surfaces known as cleavage planes. Cleavage planes may or may not be in the same directions as crystal growth faces. Keep track of the number of planes (each plane has two sides), the angles between them (90 or not 90 ), and the quality of the cleavage.
Chemical bond strength and the fixed arrangement of atoms will define whether weakness (cleavage planes) will form in minerals. The halite (salt) crystal shown in the image on the left has three cleavage planes at right angles (90 ) to one another. The weakness planes develop between the ionic bonds between the sodium (Na) and chloride (Cl) atoms in the crystal lattice.
Graphite and diamond have the same composition, but different atomic arrangements. The bonds between the carbon sheets in graphite are weak Van der Waals bonds, therefore it breaks easily along these planes.
Fracture Fracture occurs in directions along which cleavage planes are absent. Some minerals will cleave and fracture, others only will fracture (such as quartz). Common types of fracture include conchoidal (shell-like), splintery (elongate fibers), and earthy (like broken chalk).
Conchoidal fracture. Note the dished (concave) surfaces with curved ridges and grooves. Glass often breaks in this manner, as does quartz. This photo is of obsidian, which is volcanic glass.
Other Useful Physical Properties Many other mineral ID tests are possible, most of which are only useful in a few cases. Examples include taste (not recommended in lab), reaction with dilute HCl (effervescence), magnetism, radioactivity, fluorescence, and so forth.
Mineral Habit Mineral crystals seldom grow within an environment of unlimited resources, so they almost never achieve ideal geometric shapes. Limitations of space, time, and materials will control the appearance in which the minerals are actually found. This appearance is known as the habit of the minerals.
Aspects of Habit 1) Which crystal faces actually form. 2) The relative sizes of these crystal faces. 3) Twinning. 4) Aggregation of multiple crystals.
A cubic mineral, such as garnet, may grow into any of these forms (among others) OR may display a shape that is a combination of faces from more than one form.
When both the cubic and octahedral faces develop, the results may resemble a cube missing its corners, an octahedron missing its corners, or anything in between, depending on the proportions. Note that the shapes at the end of each sequence are identical.
Each of these quartz crystals may be described in terms of a prism (parallel-sided column) which terminates in a dipyramid (comes to a point at both ends). The proportions of the prismatic faces to the pyramidal faces can vary considerably from specimen to specimen.
Some Typical Shapes Acicular: needle-like Bladed: broad, flat, elongate Dendritic: branching, may resemble a plant fossil in the rock Tabular: plate-like Equant: close to the same dimensions in all directions, also called equidimensional
Twinning may develop whenever there is more than one way to follow the rules of crystal growth, for example by switching to the mirror-image of the crystal structure. Twins may be penetrant (joined within a shared volume), contact (joined along a plane) or repeated (multiply-twinned, often at a microscopic or submicroscopic scale).
Contact Twin Penetration Twin Repeated Twin, Striations
Aggregations Adjacent (but not necessarily twinned) crystals of the same mineral may develop distinctive patterns or arrangements. The most common aggregation is massive, which consists of small (often microscopic) interlocking grains. If the specimen just looks like a chunk of rock then it is probably massive. Massive does not mean large.
Some Other Aggregations Botryoidal: like a bunch of grapes Columnar: parallel columns Druse: crystals projecting from a surface, often found in gift shops Radiating: outward from a central point
Types of Minerals The thousands of mineral species are arranged by chemical composition and further subdivided by crystal structure. Earth s crust contains at least some quantity of all the naturally occurring elements, but only a few are abundant. As a result, certain types of minerals are far more common than others.
Crustal Crystal Chemistry For every 100 atoms in Earth s crust, there are approximately 63 O (oxygen) 21 Si (silicon) 6.5 Al (aluminum) 2-3 each Fe, Mg, Ca, Na, K some Ti (titanium) Everything else is minor.
Composition of continental crust BY MASS.
Some of the Mineral Groups Native Elements: composed of only one element, which is unusual. Name is the element (except graphite and diamond). Oxides: metal(s) plus oxygen, such as XO, X 2 O, X 2 O 3, and XY 2 O 4. Hydroxides: contain water as H 2 O or OH. Halides: metal plus halide, usually F or Cl.
Halides (e.g., halite, NaCl, common salt) also form an important mineral group which sustains our lives. Halite often forms as an evaporite deposit in desert lakes (playas).
More Mineral Groups Sulfides: metal(s) plus sulfur. Sulfates: metal(s) plus sulfur AND oxygen. Carbonates: metal plus CO 3. Phosphates: metal plus PO 4. Silicates: metal(s) plus silicon and oxygen.
Galena (PbS) is a mineral in the sulfide group. It is an important source of the world s lead. Hematite (Fe 2 O 3 ) is a mineral in the oxide group. It is an important source of the world s iron.
Other important mineral groups include the carbonates of which calcite belongs. Calcite comprises the marine sedimentary rock, limestone and the metamorphic equivalent marble.
Silicates Oxygen and silicon account for over 80% of the atoms in the crust. About 25% of known minerals are silicates. About 40% of commonly occurring minerals are silicates. About 90% of the crust is made of silicates.
The silicate tetrahedron is the basic building block of the silicate minerals. It is not a unit cell by itself, but silicate crystals are constructed from various arrangements of silicate tetrahedra (and other components) into unit cells and crystal lattices.
Oxygen ions are much larger than silicon ions, so the silicon fits in the space between four clustered oxygens. By itself, this structure is not charge-balanced, since the four O 2- have a net -8 charge and the Si has a +4 charge.
Charge balance is achieved by sharing oxygen atoms either with (a) other silicate tetrahedra or (b) other parts of the crystal structure. The first option is a type of polymerization, a linking of identical units. This may be represented by showing all the atoms or just the shape of the tetrahedra, where adjacent corners represent shared oxygens. Silicates are classified by the extent of silica polymerization in their structures.
Six Classes of Silicates 1) Nesosilicates: isolated tetrahedra 2) Sorosilicates: paired tetrahedra 3) Cyclosilicates: rings of tetrahedra 4) Inosilicates: chains of tetrahedra, either single or double 5) Phyllosilicates: sheets of tetrahedra 6) Tectosilicates: 3-D framework of tetrahedra
In Nesosilicates, the silicate tetrahedra are isolated from each other, therefore other positivelycharged ions (such as iron, magnesium, or other metals) must be present to link the structure together and provide a way of balancing the total charge. Olivine (Mg,Fe) 2 SiO 4 Common examples include olivine (shown) and garnet.
Sorosilicates feature pairs of silicate tetrahedra which are isolated from other pairs. Sorosilicates are not common, but the epidote group is one example.
Cyclosilicates contain rings of tetrahedra which may have either 3, 4, or 6 members apiece. Beryl (above left) and tourmaline (above right) each have sixmembered rings, but in beryl the tetrahedra all point outwards whereas in tourmaline they alternate, as shown at left.
(Mg,Fe,Ca,Na)(Mg,Fe,Al)Si 2 O 6 Single-chain inosilicates include the pyroxene minerals, which are notable for having two directions of cleavage at right angles.
(Na,Ca) 2 (Mg,Al,Fe) 5 (Si,Al) 8 O 22 (OH) 2 Double-chain inosilicates are represented by such minerals as the amphibole group, which has two cleavages NOT at right angles, often said to be at approximately 120 and 60 (actually 124 and 56 ).
Biotite: K 2 (Mg,Fe) 6 Si 3 O 10 (OH) 2 Muscovite: K 2 Al 4 (Si 6 Al 2 O 20 )(OH,F) 2 The phyllosilicates are the sheet silicates, members of which include the micas (such as biotite, muscovite, and chlorite) and the clays. Most are easily cleaved in one direction, as defined by the weak bonds connecting the parallel sheets of silicate tetrahedra in their structures. Thick specimens are sometimes called books because their sheets can be peeled apart like pages.
Tectosilicates Name refers to carpentry (framework) Make up about 75% of the crust Major groups: quartz and the feldspars Minor groups: feldspathoids, scapolite, zeolites Note that quartz (SiO 2 ) is already in charge balance, so making the other groups involves the limited substitution of Al 3+ for Si 4+ in order to fit positive ions elsewhere.
All the oxygens are shared with adjacent tetrahedra in the tectosilicate structure. For quartz (SiO 2 ), this alone is sufficient to achieve charge balance. There are other minerals with the formula SiO 2 which differ from quartz by systematic distortion in the angles at which the tetrahedra are joined.
Plagioclase Feldspar: NaAlSi 3 O 8 to CaAl 2 Si 2 O 8 The feldspar structure is similar to quartz except aluminum (Al 3+ ) replaces silicon (Si 4+ ) within some tetrahedra, thereby requiring additional positive ions elsewhere to satisfy charge. These are commonly calcium (Ca 2+ ), sodium (Na + ), and/or potassium (K + ). Orthoclase Feldspar: KAlSi 3 O 8