Mid.sem.ex. Flashcards
Type of covalent bond in which valence electrons are free to move from atom to atom in the crystal structure
Metallic bond (metals), relatively weak bonds, non-directional.
Electrons are fully exchanged between the two ions so that both obtain a noble gas electron configuration
Ionic bonds (NaCl), ions are packed closely together.
Sharing of electrons between two atoms by overlapping orbitals so that noble gas electron configurations is obtained
Covalent bonds, strongly directional and specific g.
Sigma bonds: strongest, end to end overlap.
Pi bonds: weaker, side to side overlap.
Does not involve valence electrons, but result from weak electrostatic attraction due to short-term polarization of molecules.
Van der Waals bonds, additional to valence electron bonds, weak.
Result from electrostatic attraction between a proton in one molecule and an electronegative atom in the other.
Hydrogen bonds.
What increases when more shells are filled with electrons, and decreases when sub shells are filled with electrons, and which way on the table?
Atomic radius, bigger on the left and smaller towards the right. Determines how atoms fit together in a crystal structure.
What decreases when more shells are filled with electrons, and increases when sub shells are filled with electrons? Which way on the periodic table?
Electronegativity, bigger on the right (F) and smaller towards the left. Determines what bonds are formed between atoms.
What is the oxidation state of an element determined by?
How many electrons must be gained or lost to reach the noble gas config.
What are valence electrons, and which types do we have?
Valence electrons are electrons that occupy sub shells in the outermost shells that are not entirely filled. Anions (- charge) and cations (+ charge).
What is the most important control on the crystal structure of a mineral?
The type of chemical bonds that holds the elements together.
How does metal atoms tend to pack together?
In a highly ordered atomic arrangement that minimizes void space: closest packing.
Hexagonal closest-packing will have a sequence of layers like_____
ABABAB, the third layer is placed in depressions directly above the spheres in the first layer.
Cubic closest packing will have a sequence of layers like_____
ABCABC, the third layer is placed in depressions above the gaps in the first layer.
How will structures with ionic bonds approximately be?
Close-packed, with anions as close-packed structures and cations in interstitial sites between atoms.
Which five rules can be used to predict the crystal structure for ionic compounds?
Pauling’s rules:
- The Coordination Principle
- The Electrostatic Valency Principle
- Sharing of Polyhedral Elements I
- Sharing of Polyhedral Elements II
- Principle of Parsimony
The Coordination Principle (Pauling’s Rule 1) tells us that____
Large cations only fit in large interstitial sites, and a large interstitial site requires a large number of anions.
The relative size of the cation and anion determines by how many anions a cation will be surrounded.
What is Coordination Numbers (C.N.)?
The numbers of anions that surround a cation.
Depends on the radius ratio = R_c/R_a
The C.N. determines the coordination polyhedron forming around the cation.
Cations with C.N. that will from a VI (octahedral) polyhedron are____
Mn2+, Fe2+, Mg2+, Fe3+, Ti4+, Al3+
Cations with C.N. that will form a IV (tetrahedral) polyhedron are____
Al3+, Si4+, P5+, S6+
Cations with C.N. that will form a III (triangular) polyhedron are____
C4+
Cations with a C.N. that will form a VI-VIII (octahedral to cubic) tetrahedron are____
Na+, Ca+
Cations with a C.N. of VIII-XII (cubic to close-packed) are___
K+
The Electrostatic Valency Principle tells us that___
The total strength of the valence bonds that reach an anion from all nearest neighbor cations is equal to the charge of an anion.
The strength of an evb = (charge on the cation, Z)/(Coordination Number fo the cation, C.N.).
Which three types of ionic bonds do we have?
Isodesmic, anisodesmic, and mesodesmic.
What is an isodesmic bond, and which minerals tend to have this type of bonding?
All the ionic bonds have the same strength that is less than half of the charge on the anion.
All ionic minerals with a single cation and a single anion (NaCl) tends to have a close-packed structure.
What is an anisodesmic bond, and which minerals tend to have anisodesmic bonding?
The bond strength is non-uniform and some bonds take more than half of the charge on the anion.
All ionic mineral with small and highly charged cations that form distinct anisodesmic anionic groups (CO3, PO4, SO4) tends to have decreased symmetry to accommodate groups.
What is a mesodesmic bond, and which minerals tend to have this type of bonding?
The bond strength is non-uniform and some bonds take exactly half of the charge of the anion.
Each oxygen has a - 1 charge leftover that is not shared, and can be used to bond to another cation or SiO4 tetrahedron: polymerization of silica tetrahedra.
Sharing of Polyhedra I tells us that____
Repulsion between cations is stronger when polyhedra share edges or faces, so it is most stable when sharing corners.
Silicon tetrahedra never share faces.
Sharing of Polyhedra II tells us that___
High-charges cations minimizes the number of anions that they share between polyhedra so that the cations can be kept apart at the maximum distance.
Principle of Parsimony tells us that___
Crystal structures have a limited set of distinctively different cation and anion sites.
Covalent bonds get structures that___
Are directional because a specific geometry is needed to make the orbitals overlap, so the structures are NOT close packed.
Isostructural minerals and isostructural groups are___
Isostructural minerals have atoms that are arranged in the same type of crystal structure.
Isostructural groups have isostructural minerals and are chemically related by a common anion or anionic group.
NaCl and PbS.
What is polymorphism and polymorphic groups?
Polymorphism is the ability of minerals with the same chemical composition to crystallize in different structures.
Polymorphic groups are sets of structurally different minerals with the same chemical composition.
Conversion from one polymorph to another involves a major reorganization of the crystal structure (incl. bond breaking) - what type of polymorphism is this?
Reconstructive polymorphism, ex:
Al2SiO5
Symmetry changes, quenchable, metastable phases.
Conversion from one polymorph to another involves distortion or bending of the crystal structure, what type of polymorphism is this?
Displacive polymorphism, ex: Alpha-quartz and beta-quartz.
Lower symmetry in low temperature phase, not quenchable.
Conversion form one polymorph to another involves the redistribution of cations over structural sites - what type of polymorphism is this?
Order - disorder polymorphism, ex: Si and Al in K-feldspar.
How can you describe a crystal structure?
Crystal structure = crystal lattice + motif
What represents the periodic of a crystal as a geometrical array of chosen points, each of which has identical surroundings?
The crystal lattice, a 2D lattice can be described by lattice vectors a and b, and the angle between these vectors.
What is the atomic arrangement or pattern that is associated with each lattice point?
The motif.
What is the smallest building block that can be repeated in all directions to reconstruct the crystal lattice and possess the symmetry of the mineral?
The unit cell, has lattice points at its corners, and the size and shape of the unit cell is determined by the length and angle of the lattice vectors.
The unit cell represents the ‘ordered atomic arrangement’.
How many 2D plane lattices do we have, and what are they named?
5,
4 with lattice points only at corners: square, hexagonal, rectangular, oblique.
One with lattice point at center too: centered rectangular.
What defines the translational symmetry of a structure (the periodic repetition of a unit cell)?
The lattice.
The point symmetry in 2D has three elements of symmetry, what is repeated and what are the elements?
The motif is repeated, and we have point symmetry: rotation, glide and reflection.
How many plane groups do we have, and what are they describing?
We have 17 plane groups that describe all 2D symmetry.
What describes the translational symmetry in 3D, can you name them?
The 14 space lattices (Bravais), we have cubic (3), tetragonal (2), orthorhombic (4), monoclinic (2), triclinic (1), trigonal (1), and hexagonal (1)
What are the unit cells in 3D called, and how many are there?
Name them from highest symmetry to lowest.
The Bravais lattices produces 7 shapes of unit cells, that are referred to as the 7 crystal systems.
Cubic (isometric), hexagonal, tetragonal, trigonal (also hex), orthorhombic, monoclinic, triclinic.
The 3D point symmetry is the same as in 2D, but with three additional elements, which ones?
Center of symmetry (inversion), roto-inversion, and screw axes.
Name the three phases of crystal growth.
Nucleation, growth and post-growth.
What are the two types of nucleation?
Describe them.
Homogeneous: free atoms and ions find each other and chemically bond to form an embryo and eventually a mineral nucleus.
Heterogeneous: An existing surface helps with the formation of a critical nucleus so that the formation of embryos is not needed.
What can we describe the energy changes involved in the formation of a nucleus with?
Gibbs free energy of formation, the most stable phase has the lowest Gibbs free energy of formation.
What is spontaneous nucleation driven by?
A decrease in energy levels from the solution to the solid.
If the volume of a nucleus is larger, the delta G_v will become___
More negative.
The larger the radius (surface area) the more___
Positive delta G_s
Fast cooling magma will have a high high degree of under cooling, and will have___
Small critical nucleus and many nuclei (small crystals).
Slow cooling magma will have a low degree of under cooling, and therefore will have____
Large critical nucleus and few nuclei (large crystals).
The longest dimensions of a crystal is typically associated with the _____ unit cell dimension.
shortest
Compositional variability in a mineral can be called____
Solid solution, chemical variations but within limits.
Substitutions of elements (cations) in a crystal structure.
A continuous or complete solid solution means that___
all of the intermediate compositions are possible.
Discontinuous or incomplete solid solution series means that___
Only a restricted range of compositions between the end -members are possible.
What are the two requirements for substitution?
Similar ionic radius: Pauling’s first rule, complete solid solution if the difference is < 15% (more at high T).
Charge neutrality: crystal structure must remain electrically neutral, usually no more than 1 charge diff.
“Substitution of cations with identical charges and identical ionic radii.” - is called?
Simple substitution, ex: Fe2+ and Mg2+ in olivine.
“Coupling one substitution that increases the charge with another one that decreases the charge.” - is called?
Coupled substitution, ex: Ca2+ and Al3+ exchanges for Na+ and Si4+ in plagioclase.
“Substitution of an ion with different charge and leaving another structural site unfilled.” - is called?
Omission substitution, ex: 2Fe3+ and vacancy exchange for 3Fe2+ in Pyrrhotite.
“Substitution of ions in structural sites that are normally not filled with any ion (vacancies or open ring structures).” - is called?
Interstitial substitution, ex: K+ into the center of Si4+ rings, and Al3+ for Si4+ for charge balance.
What does recrystallization do with the surface energy?
Recrystallization lowers the surface energy, and because recrystallization requires the diffusion of atoms and ions, this process occurs at high temperatures.
Real crystals have growth defects, name the three and explain them.
Point defects: defects around a single lattice point.
Line defects: misaligned atoms along a line (dislocation).
Planar defects: structural mismatch across a surface.
Range Moh’s hardness scale.
Talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, diamond.
“Some 3d orbitals get a higher and some a lower energy when a cation is placed in a coordination polyhedron with anions.”
What is this called?
Crystal Field Theory.
“Differently charged cations in adjacent sites exchange valence electrons by absorptions of light (charge transfer transitions)”
What is this called?
Molecular orbital transitions
“Un paired electrons that are dislocated in the crystal structure and can absorb light in exited states, with electron missing (after irradiated)”
What is this called?
Hole color center
“Unpaired electrons that are dislocated in the crystal structure and can absorb light in exited states, with anion missing”
What is this called?
Electron color center
How much a mineral stands out from its surroundings is called___
Relief
Changes in color when you rotate a mineral in polarized light is called___
Pleochroism, and results from different light paths in a crystal.
In crossed polarized light (XPL) it is possible to see___
the interference color of a mineral, also called birefringence.
Minerals that split light into two directions with different velocities (show birefringence), are called___
anisotropic minerals.
Minerals that appear black under XPL are called___
isotropic minerals (cubic and glass)
What does X-ray diffraction (XRD) use as sample, and what does it determine?
XRD is used on a powdered sample to determine the crystal structure of a mineral.
What does Electron Microprobe Analysis (EMPA) use as sample, and what does it determine?
EMPA uses thin sections to determine the chemical composition of a mineral, and can use this to derive the mineral formula.
Elementene til venstre i periodesystemet er _____ og har ____ elektronegativitet.
større og mindre
Elementer til høyre i periodesystemet er ____ og ____ elektronegativ.
mindre og mer
