Review of the Minerals

Key Points: Native Elements

• About 20 native elements are known exclusive of gases
• These occur as metals, semimetals and nonmetals
• The metals display cubic or hexagonal symmetries
• Ionic radii is the key to understanding solid solution in the metals
• Polymorphism is important in understanding the nonmetals: introduce the various types of polymorphic transformations (displacive, reconstructive, order/disorder)

Key Points: Sulfides

• Majority of metal ore minerals are sulfides
• Hard to generalize about structure although most sulfides have structures related to or derived from a few basic ones
• Polymorphism is fairly common.
• Reasonable to consider sulfides as nearly close packed structures.
• Largely covalently bonded because generally transition metals.
• Generally soft, pliable minerals.
• Little if any solid solution with "ionic" group Ia and IIa elements.
• Commonly difficult to 'charge balance' sulfide formulas especially when they involve semimetals such as Se and As.

Key Points: Halides

• large -1 anions and large +1 or +2 cations
• high symmetry structures
• the halite structure was the first to be determined by X-ray diffraction techniques: FCC array of Na (or Cl) with the Cl (or Na) forming 6 bonds in the form of a regular octahedron around each
• slightly larger +1 cations have the CsCl-structure which has 8-fold coordination: both the Cs and the Cl are at the centers of a cube of neighboring atoms
• +2 cations commonly have the fluorite structure: FCC array of Ca with F in tetrahedral coordination - each Ca has 8 fluorine bonds at the corners of a cube
• nearly pure ionic bonding
• you should know halite, sylvite, fluorite

Key Points: Carbonates

• carbonates and the rest of the '..ates' are characterized by strongly bonded anionic groups or complexes in their structure
• radius ratio relationships correctly predict that carbon should have 3 nearest oxygen neighbors and this occur as a planar triangle
• the double bonds in CO₂ are stronger than the single bonds in the CO₃^-2 complex so in the presence of hydrogen ion, carbonates breakdown and 'fizz' in acid
• 3 structurally distinct groups: calcite, aragonite, dolomite
• two important hydrous copper carbonates: azurite, malachite

Key Points: Sulfates, Phosphates, ...

• nitrates are extremely soluble and thus extremely rare in nature
• borates are source for boron which is important in glass manufacturing
• BO₃ triangles can be polymerized in exactly the same ways that silica tetrahedra can - sharing 1, 2, 3 corners.
• the covalently bonded SO₄^-2 tetrahedral anionic group is the fundamental structural unit in the sulfates
• various Ca, Fe, Mn tungstates are THE source of tungsten
• the covalently bonded PO₄^-3 tetrahedral anionic group is the fundamental structural unit in the phosphates

Key Points: Oxides

• hard, dense and refractory
• occur as accessory minerals in igneous and metamorphic rocks
• occur as resistant detrital grains in sediments
• approximate closest packed oxygen structures
• strongly ionic bonding
• no cations with charge/coordination number >= 1 and therefore no strong anionic groups like we will see in the carbonates and silicates.
• chief ores of iron, chromium, manganese, tin, titanium, and uranium

Orthosilicates

• No tetrahedral O shared with other tetrahedra
• Based on SiO₄ ^4-
• Also called nesosilicates
• A diverse group of minerals, examples covered here:
  • olivine
  • Garnet
  • aluminosilicates
    • kyanite
    • andalusite
    • sillimanite
  • topaz
  • staurolite
  • zircon
  • titanite or sphene
  • chloritoid
  • humite group
• structures determined by size(s) and charge(s) of cations in the structure
• often rather dense minerals - mantle phases!
• equant grains with generally poor cleavage
• low amounts of Al substitution for Si

Sorosilicates

Contain the Si₂ O₇ structural unit! (bow tie)

• many examples, most are rare!
• Epidote group - common mineral, low grade metamorphic rocks; hydrothermal alteration product
• Vesuvianite
• Hemimorphite
• Lawsonite

Cyclosilicates

• Rings of different sizes (3, 4, 6) all of which have Si:O ratios of 1:3.
• Simplest ring: 3-membered Si₃ O₉ - found in rare mineral benitoite (Ba Ti Si₃ O₉)

• 4-membered Si₄ O₁₂ - no remotely common examples

• 6-membered Si₆ O₁₈ - found in the more common ring silicates

Pyroxenes and Pyroxenoids

• Silica tetrahedra may form simple chains by sharing 2 of the 4 oxygens. These simple single chain silicates have -> Si:O = 1:3

• Opposing chains (whose apical oxygens point at one another) create octrahedral sites of two types:

  [1] Octahedral sites form a chain between the apical oxygens of the opposed tetrahedral chains.

  • These are rather regular sized 6-coordinated sites.
  • Accommodate: Mg, Fe (2+ and 3+), Al, Mn, etc.
  • These sites are known as M1 sites (which you will see are analogous to the M1, M2, and M3 sites in amphiboles).

  [2] Also, back to back chains provide cation sites to complete a second strip of sites.

  • These are large and distorted nominally 6-fold sites but 8-fold coordination is common.
  • These sites can accommodate: Mg, Fe, Al, [Mn], Li, Ca, Na, etc.
  • This second set of sites are designated the M2 sites (analogous to the M4 sites in amphiboles).

• As in layer silicates and amphiboles, formation of octahedral sites through opposing apical oxygens requires that the apical O do not superimpose exactly, but are offset.

  • RESULT: stagger in the I beam. Monoclinic and Orthorhombic varieties
• Exsolution is a common feature during cooling from igneous temperatures
• Pyroxenoid structures result from different chain length repeats

Amphiboles

• Double chain 'strands' with the apical oxygen atoms pointing toward each other -> module known as an I-beam. These are stacked back to back and tiled in a characteristic pattern
• Amphiboles are hydrous minerals
• Using two opposing chains and 2 OH to make I-beam -> [ ]Si₈O₂₂(OH)₂
• Complicated exsolution and solid-solution relationships

Pyroxene - Amphibole Comparison:

• Two chains joined side by side form a double chain silicate (the amphiboles) which, because half the tetrahedra share 2 oxygens and half share 3 oxygens, have a Si:O ratio is 4:11.
• Pyroxenes commonly alter to amphiboles under retrograde hydrous conditions.
• Pyroxenes are anhydrous (no structural OH or H₂O) whereas amphiboles have structural OH, Cl, F.
• Most members of each group are monoclinic but orthorhombic examples occur.
• The c-cell dimensions of both pyroxenes and amphiboles are roughly the same at 5.2 A (2 tetrahedra). The a-dimensions are also analogous but the double wide chain leads to a commensurate doubling of the b-cell dimension in the amphiboles.
• Pyroxenes tend to form short blocky crystals while amphiboles tend to elongate crystals.
• Pyroxenes form at higher temperatures than do amphiboles.

Sheet Silicates

• Built on a plane of bridging O, leaving an unsatisfied apical charge for each tetrahedra.

• Result is a 'sheet' of tetrahedra which is the basis for group name and physical properties.

• Structure usually sandwiches of tetrahedral and octahedral layers.
• Composition of sheets: Si₂O₅ (count T and O in net)
• 1:1 Layer silicates: Kaolinite and Serpentine
• 2:1 Layer silicates: Pyrophyllite and Talc
• 2:1 Layer silicates with Interlayer cations: Muscovite and Biotite
• 2:1 Layer silicates with Interlayer octahedral sheet: Chlorite

Framework Silicates: Silica

• Rougly 2/3 of the crust of the Earth is composed of Framework silicates
• We examined:
  • SiO₂ group
  • Feldspars
  • Feldspathoids
  • Scapolites
  • Zeolites

   

• Start with Si and O
  

  Si:
  • 4 coordination:
  • 14th element on periodic table: [Ne] 2s² 3p²
    -> Si⁴⁺ with tetrahedral sp3 hybrid orbitals
  • Si bonds with O: 50% ionic, 50% covalent.

  Simplest compounds based on Si and O : formula = ?

  Structural arrangement ? Use Pauling bond strengths in tetrahedra - unsatisfied charge on O ?

  Bridging oxygens

  Polymerization network: polymorphs

Feldspars and other Framework Silicates