Minerals. [Most] rocks are [mostly] made of minerals, so identification and interpretation depends on recognizing

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1 Minerals [Most] rocks are [mostly] made of minerals, so identification and interpretation depends on recognizing Over mineral types have been described, but only about account for the bulk of most rocks. These are the rock-forming minerals.

2 Atoms: The Building Blocks Nucleus: contains and Atomic Number: The number of in the Atomic Mass: # of + # of in the nucleus. Electrons: orbit the nucleus, responsible for between atoms

3 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.

4 Bonds : transfer of electron(s) from donor ion (cation, +) to recipient ion (anion, -) : sharing of (s) : sharing of valence electrons across many atoms, resulting in a cloud of electrons permeating the crystal structure Van der Waals: weak tugging of electron(s)

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6 electrons tends to increase the of the atom (ionic radius) whereas removing the tends to make it. Note the difference between ferric iron (Fe 3+ ) and ferrous iron (Fe 2+ ). How will the size of an ion affect its ability to fit into a crystal structure?

7 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). Give examples: Strong under but weak under (therefore promoting cleavage planes).. Produce highly symmetric crystals (such as cubic) of moderate and. May dissolve in water, but typically have a melting temperature. conductors of heat and electricity.

8 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. melting temperatures Produce crystals of lower symmetry but. Relatively in water. conductors of heat and electricity.

9 Metallic Bonds Here, the electrons involved in the bonds are not associated with any particular atom but are to wander about the structure_. These electrons can and 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 due to these nonlocalized electrons. They also typically heat well. Metals also tend to be (easily shaped by striking with a hammer), since the atoms can readily rearrange themselves in the midst of the swarm of surrounding electrons.

10 Van der Waals Forces Van der Waals forces (also known as bonds) 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 in covalent bonds. Hydrogen bonds are quite, producing structures which easily along the intervening planes (graphite shown here).

11 Five-Part Definition of a Mineral Two Questions: 1) Is it a mineral? First three parts. 2) Which mineral is it? Last two parts. Question 1: Is it a mineral 1). Useful for interpreting geologic phenomena. 2). Organic processes and materials will be considered separately. 3). Strictly speaking, this means crystalline.

12 Question 2: Which mineral is it? 4). A particular regular, periodic, ordered atomic arrangement. 5), within a limited range. Allowable variation is determined case-by-case. Are these minerals (circle yes ones): Salt, sugar, cubic zirconia, glass, ice

13 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.

14 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.

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16 Within each crystal system (for example, cubic), many forms are possible. All such forms are consistent with the underlying of the system itself.

17 Useful Physical Properties. Treacherous. Different minerals with same color, same mineral with different color.. Color of powdered mineral.. How the surface reflects light.. Scratch hardness, from 1 to 10 on the Moh s hardness scale.. How the mineral breaks. Other., etc.

18 COLOR: Color, is a reliable property for mineral identification, generally speaking. It should be used only as a, confirming characteristic. (Beryl = Be 3 Al 2 Si 6 O 18 )

19 is tested by rubbing the mineral against an unglazed porcelain plate. The color of the powder is quite consistent from sample to sample. 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.

20 Luster This refers to the manner in which the surface of the, 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). can be either bright or dull, however.

21 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 : adamantine, vitreous, resinous, pearly, waxy, silky, dull, earthy

22 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. It varies with the crystallographic direction in which it is measured. Report hardness as a range of possible values, depending on available tools.

23 Tall, Green Cumquats Fall All Over Queen Theresa s Cute Dress

24 Cleavage Planes Many (not all) minerals not only grow with flat, but also break along flat known as cleavage planes. Cleavage planes may or may not be in the same directions as crystal. Keep track of the of planes (each plane has two sides), the between them (90 o or not 90 ), and the of the cleavage. Cleavage will often appear as a series of planes, while crystal faces will not.

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26 Fracture Fracture occurs in directions along which are. Some minerals will cleave and fracture, others only will fracture (such as quartz). Common types of fracture include (shell-like), (elongate fibers), and (like broken chalk). Earthy Fracture Conchoidal Fracture Splintery Fracture

27 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 (effervescence),, radioactivity, fluorescence, and so forth.

28 Mineral Habit (Appearance/Growth Shape) Mineral crystals seldom grow within an environment of unlimited resources, so they almost never achieve ideal geometric shapes. Limitations of,, and will control the appearance in which the minerals are actually found. This appearance is known as the habit of the minerals.

29 Aspects of Habit 1) Which crystal faces actually form. 2) The relative sizes of these crystal faces. 3). 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.

30 Some Typical Shapes : needle-like : broad, flat, elongate : branching, may resemble a plant fossil in the rock : plate-like : close to the same dimensions in all directions, also called equidimensional

31 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 (joined within a shared volume), (joined along a plane) or (multiply-twinned, often at a microscopic or submicroscopic scale).

32 Aggregations Adjacent (but not necessarily twinned) crystals of the same mineral may develop distinctive patterns or arrangements. The most common aggregation is, 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.

33 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

34 Crustal Crystal Chemistry For every 100 atoms in Earth s crust, there are approximately O (oxygen) Si (silicon) Al (aluminum) each Fe, Mg, Ca, Na, K And a few Ti (titanium) Everything else is.

35 Some of the Mineral Groups : composed of only one element, which is unusual. Name is the element (except graphite and diamond). A few are: : metal(s) plus oxygen, such as XO, X 2 O, X 2 O 3, and XY 2 O 4.Example: : contain water as H 2 O or OH. For example: : metal plus halide, usually F or Cl. For example:

36 Look on P 42 in your book for examples. More Mineral Groups : metal(s) plus sulfur. : metal(s) plus sulfur AND oxygen. : metal plus CO 3. : metal plus PO 4. : metal(s) plus silicon and oxygen.

37 Silicates Oxygen and silicon account for over % of the atoms in the crust. About % of known minerals are silicates. About 40% of commonly occurring minerals are silicates. About % of the crust is made of silicates. The silicate tetrahedron is the basic building block of the silicate minerals.

38 ions are much larger than ions, so the silicon fits in the space between four clustered oxygens. By itself, this structure is, since the four O 2- have a net -8 charge and the Si has a +4 charge.

39 Charge balance is achieved by sharing oxygen atoms either with (a) other silicate tetrahedra or (b) other parts of the crystal structure.

40 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 Name refers to carpentry (framework). Make up about % of the crust! Major groups: quartz and the feldspars (see online notes for pictures of all classes of silicates)