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

Transcription

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 Gypsum (CaSO 4 2H 2 O) from Chihuahua, Mexico. Single crystals are up to eleven meters long.

3 Quartz crystal How many different minerals do you see in this photo? How did you recognize them as different? Quartz is gray, when did it crystallize?

4 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

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6 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.

7 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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9 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?

10 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. Produce highly symmetric crystals of. May dissolve in water, but typically have a melting temperature. conductors of heat and electricity.

11 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.

12 Other Characteristics of Covalent Bonds melting temperatures Produce crystals of. Relatively in water. conductors of heat and electricity.

13 Metallic Bonds Here, the electrons involved in the bonds are not associated with any particular atom but. These electrons can across a wide range of energies corresponding to visible light, producing the characteristic metallic luster of such materials (often metals). Metals tend to be due to these non-localized electrons. They also typically 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.

14 do not involve electron transfer, merely a tugging of the electrons of one atom towards a neighboring atom due to the of the atoms themselves. Polarity arises from of electrons in covalent bonds. bonds are quite.

15 Five-Part Definition of a Mineral Two Questions: Is it a mineral? First three parts. Which mineral is it? Last two parts.

16 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.

17 Which mineral is it? 4). A particular,, atomic arrangement. 5), within a limited range. Allowable variation is determined case-by-case.

18 Are these minerals? salt? sugar? cubic zirconia? glass? ice?

19 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.

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21 Haüy (1781) proposed that crystals could be constructed from building blocks of identical shape and composition, stacked in a repeating pattern.

22 A cubic building block (unit cell) can produce several crystal shapes, but only those which are compatible with cubic symmetry.

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

25 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 hardness scale.. How the mineral breaks. Other., etc.

26 COLOR: Color, is NOT a reliable property for mineral identification, generally speaking. It should be used only as a characteristic. (Beryl = Be 3 Al 2 Si 6 O 18 )

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

28 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.

29 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.

30 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 (like the penny) is also said to have metallic luster.

31 Nonmetallic Lusters A few, arranged by decreasing reflectivity:

32 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.

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34 Planes Many (not all) minerals not only grow with flat faces, but also break along flat surfaces known as. Cleavage planes may or may not be in the same directions as. Keep track of the of planes (each plane has two sides), the between them (90 or not 90 ), and the of the cleavage.

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36 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).

37 Conchoidal fracture. Glass often breaks in this manner, as does quartz. This photo is of obsidian, which is volcanic glass.

38 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), with (effervescence),, radioactivity, fluorescence, and so forth.

39 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 will control the appearance in which the minerals are actually found. This appearance is known as the habit of the minerals.

40 Aspects of Habit 1) Which crystal faces actually form. 2) The relative sizes of these crystal faces. 3). 4).

41 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.

42 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.

43 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.

44 Contact Twin Penetration Twin Repeated Twin, Striations

45 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..

46 Some Other Aggregations : 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

47 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.

48 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:

49 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.

50 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.

51 The silicate tetrahedron is the basic building block of the silicate minerals.

52 are much than, 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.

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

54 Six Classes of Silicates 1) Nesosilicates: 2) Sorosilicates: tetrahedra 3) Cyclosilicates: of tetrahedra 4) Inosilicates: of tetrahedra, either single or double 5) Phyllosilicates: of tetrahedra 6) Tectosilicates: of tetrahedra

55 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.

56 Sorosilicates feature pairs of silicate tetrahedra which are isolated from other pairs. Sorosilicates are not common, but the epidote group is one example.

57 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.

58 (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.

59 (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 ).

60 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.

61 Tectosilicates Name refers to carpentry (framework) Make up about % of the crust! Major groups: quartz and the feldspars

62 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.

63 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