Minerals

1. Minerals Geology 115

2. Mineral • Definition: “A naturally-occurring homogenous inorganic solid substance with a definite chemical composition and a characteristic crystalline structure.” • What about ice (frozen H2O)?

3. Mineral • Definition: “A naturally-occurring homogenous inorganic solid substance with a definite chemical composition and a characteristic crystalline structure.” • Ice does not have a “characteristic crystalline structure” -- thus it is a mineraloid.

4. Atomic theory • John Dalton (1800): “Matter is made out of atoms -- the smallest units with distinguishable chemical properties.”

5. Molecules • Atoms form associations called molecules; atoms in molecules are held together by chemical bonds

6. Crystals • Molecules can attract each other due to intermolecular forces -- much weaker than chemical bonds • If the molecules are held together in a regularly spaced lattice, a crystal is formed

7. Ice’s structure • Note large-scale irregularities in the symmetry of the lattice -- ice is a mineraloid

8. Mineral intermolecular force • Molecules in minerals are held together by ionic bonds -- the atoms lose or gain electrons and are thus called ions • Positively charged ions are cations; negatively charged ions are anions

9. Geologists and minerals • Minerals are the components of rocks • Unlike minerals, rocks do not have a single definite chemical composition, and minerals retain their integrity when incorporated in a rock

10. Granite = rock

11. Mineral classification • Basic classification is by chemical composition, typically by the anion it contains: • Carbonates (CO32–) • Halides (e.g., Cl–) • Phosphates (PO43–) • Sulfates (SO42–) • Oxides (O2–) • Sulfides (S2–)

12. Silicates • But no mineral class is as prevalent as those made with the silicate anion (SiO44–) -- 95% by volume of the crust • Tetrahedral shape

13. Silicate structure • Since silicates are all the same shape, it is the arrangement of the silicates, and the cations that electrically balance the silicates that determine the structure of the mineral • Olivine and quartz are examples; in fact, quartz is made only of silicon and oxygen, so is the most common mineral on Earth

14. Silicate structure • Knowing the way the ions stack can tell you some mineral properties • Sheet silicates (phyllosilicates) form thin sheets because there are not many ionic bonds between different levels of molecules • Mica minerals, like biotite and muscovite, are examples

15. Silicate structure • Tectosilicates (framework silicates) have a much more complex structure but have regular planes of weaker forces -- leads to characteristic 60°/120° cleavage of these minerals • Feldspar (=“field stone”) minerals, like plagioclase and orthoclase, are examples

16. The properties of minerals are determined by their chemical composition and crystal habit; there are over 7000 unique minerals

17. Crystal habit • Crystal shapes are determined by local (nearest neighbor) intermolecular attractions

18. Cleavage • Cleavage, on the other hand, relies on the alignment of weak areas held together by only intermolecular forces through the whole crystal

19. Color (and luster) • Mineral color can be due to its intrinsic chemical composition (e.g., copper ores) or the crystal’s refractive properties or “impurities” such as a low concentration of metal ions not in the mineral’s chemical formula • Latter reason explains much of quartz’s color variability

20. Specific gravity • SG is the ratio of the mineral’s density to water’s density; it can be thought of as a “unitless” density. • SG measures how efficiently packed the atoms are in a crystal, and gives some indication of composition (e.g., the lead in galena).

21. Hardness • Hardness is an indication of molecule alignment in crystals (similar to cleavage) but also measures the strengths of the intermolecular forces

22. Strength ≠ Hardness • Carbon nanowire is one of the strongest (tensile) materials known, but is no harder than graphite

23. Sadly, minerals aren’t usually large enough to identify • So what techniques are available? • Optical mineralogy

24. Optical mineralogy • Relies on the behavior of polarized light transmitted through a very thin cross-section of a rock sample

25. X-ray crystallography • Basic premise: shoot X-rays at a crystal, look at the shadow pattern of atoms, determine structure and, eventually, identity