How to study minerals?!

Transcription

1 How to study minerals?! ü What tools did scientists have from pre-history to Renaissance? Eyes and measuring devices Calcite

2 Crystal faces! ü One of the most spectacular aspect of minerals ü NOTE: No mention of crystal faces in the definition!!! ü Minerals do not commonly develop faces but any mineral, under the right conditions, will develop crystal faces ü Conditions? Free (unimpeded) growth in a liquid or a fluid or a hard mineral growing a softer matrix. Mineral collectors look for faceted minerals multiply faceted minerals are much more valuable

3 Faceted garnets! Grossularite showing the development of dodecahedral faces Pyrope showing the development of dodecahedral and trapezohedral faces

4 Consistency of interfacial angle! ü Nicolas Steno (1669): The angle between two faces of a crystal is identical for all forms of the same mineral that exhibit the corresponding faces ü Crystals are built according to a pattern from building blocks that are small relative to the size of a macroscopic crystal

5 End of 19 th century! MINERALOGY

6 Nobel Prize in 1901 Discovery of x-rays! ü 1895 by Wilhelm Roentgen Roentgen s wife hand and purse!!!

7 Nobel Prize in 1914 X-ray Crystallography! ü In 1912, Max von Laue used x-rays to study crystals by passing X-rays through a cleavage fragment of sphalerite (ZnS) and observing the resultant diffracted beams on a photographic plate. ü He showed that the crystal acted like a 3D diffraction grating essentially proving that the crystals had an ordered internal arrangement of atoms.

8 X-ray diffraction: Laue photographic method! Set-up for Laue x-ray diffraction experiment: Fixed beam of X-radiation, fixed crystal

9 Laue X-ray photograph of Vesuvianite! From:

10 Vesuvianite: Contact Metamorphic Mineral! Formula: Ca 10 (Mg,Fe) 2 Al 4 Si 9 O 34 (OH) 4 System: Tetragonal Hardness: 6.5 Also called Idocrase Found in skarns

11 In 1914, William and Lawrence Bragg determined the crystal structure of halite (NaCl) using x-rays. Nobel Prize in 1915 where! n is an integer determined by the order given,! λ is the wavelength of x-rays, and moving electrons, protons and neutrons,! d is the spacing between the planes in the atomic lattice, and! θ is the angle between the incident ray and the scattering plane!

12 Halite (NaCl) crystal showing the external form of the crystal faces. Halite crystals form cubes reflecting the regular internal arrangement of the Na + and Cl - ions making up the structure.

13 HRTEM (high resolution transmission electron microscopy) image of cordierite (Mg 2 Al 4 Si 5 O 18 ) illustrating the periodicity and symmetry of atoms making up the crystal. White areas show the positions of atoms and black areas are empty channels! ~2 nm

14 How do minerals form?!

15 Crystal Chemistry (Chap 3 of Nesse s book)! ü Nature of chemical elements?! ü What mechanisms cause chemical elements to bond?! ü What are the sizes of atoms and elements?! ü Abundance of chemical elements available on Earth?!

16 Atoms! Shells and subshells s subshell: 2 electrons p subshell: 6 electrons d subshell: 10 electrons 1st shell: s, 2nd shell: s + p, 3rd shell: s+p+d, See complete table in Nesse Table 3.3

17 Chemical Bonding! ü Try to reach Noble Gas status! ü Fill orbitals to match unpaired electrons!

18 Chemical Bonding! Pauling (1931): On the Nature of the Chemical Bond! ü 1. The electron-pair bond forms through the interaction of an unpaired electron on each of two atoms! ü 2. The spins of the electrons have to be opposed! ü 3. Once paired, the two electrons cannot take part in additional bonds.! ü 4. The electron-exchange terms for the bond involves only one wave function from each atom! ü 5. The available electrons in the lowest energy level form the strongest bonds! ü 6. Of two orbitals in an atom, the one that can overlap the most with an orbital from another atom will form the strongest bond, and this bond will tend to lie in the direction of the concentrated orbital.!

19 Table of electronegativities which is a relative measure (maximum value is 4) of the ability of an element to attract electrons into its valence shell. Fluorine has the highest electronegativity and Cesium the smallest.

20 S: 6 and -2 Cl: -1 Ti-V: 2, 3, 4 Cr: 3, 6 Mn: 2, 3, 4, 5 Fe: 0, 2, 3 Oxidation state! Fig. 3.3 Nesse s book ,

21 Chemical bonds Chemical bond: force of attraction between two or more atoms/ions Types of bonds in crystals: Ionic bond: electrostatic attraction between two oppositely charged ions. This type of bond involves electron transfer and is non-directional. The ionic bond predominates between elements whose electronegativity differs by 2 or more Covalent bond: forms between elements with high but nearly equal electronegativity. This type of bonding involves sharing of valence electrons In some minerals, other (less important) bond types include: Hydrogen bonds; when H forms a covalent bond its single electron is largely confined to the overlap zone leaving the proton unshielded and able to attract negative ions. Metallic bonds: the outer shell of atoms in metals contain loosely held valence electron which can be shared among neighbor metal atoms forming a weak bond Van der Waals bonds: weak bond observed between electrically neutral molecules which may develop a slight positive charge at one end and a slight negative charge at the other end forming a weak dipole. The attraction between these molecules forms a very weak Van der Waal bond.