3/4) Silicate mineral groups

what are the classes of silicates

orthosilicates / nesosilicates
disilicates / sorosilicates
ring silicates / cyclosilicates
chain silicates / inosilicates
sheet silicates / phyllosilicates
framework silicates / tectosilicates

details on ortho/nesosilicates, examples, formula?

least degree of polymerization
no Oxy shared between adjacent tetrahedra
isolated structural entities that bond with cations such as Mg, Fe, Al
SiO₄ ^-4

Ex: olivine, zricon, topaz

details on disilicates/sorosilicates, example, formula?

share a single Oxy between two silicon tetrahedra
Ex: epidote

Si₂O₇ ^-6

details on ring silicates/cyclosilicates, example, formula?

share 2 Oxy each, form 6-sided rings
rings can stack to form columns, adjacent columns are linked by other cations
additional ions may occupy the center of the column
Ex: tourmaline, beryl, cordierite

Si₆O₁₈ ^-12

details on single chain silicates, example, formula?

two Oxy per tetrahedral are shared with neighbouring tetrahedra, cleavage at 90°
Ex: pyroxenes (MgSiO3)

Si₂O₆ ^-4

details on double chain silicates, example, formula?

alternate sharing 2 Oxy and 3 Oxy, may appear like rings, amphiboles use doubled
Ex: amphibole

Si₄O₁₁ ^-6

details on sheet silicates

share 3 Oxy to form continous sheets, 1 Oxy is facing upwards not shared
Ex: biotite, muscovite, chlorite

Si₄O₁₀ ^-4

divalent vs trivalent, example

divalent: Mg2+, Fe2+, need 3 to balance the -6 charge (trioctahedral sheet because of 3 ions)
Ex: kaolinite, pyrophyllite

trivalent: Fe3+, Al3+, need 2 to balance the -6 charge (dioctahedral sheet from 2 ions), will have vacancies
Ex: serpentine, talc

what is the interlayer cation in muscovite and biotite?

Potassium, to balance the charge after replacing a Si with Al (need one more positive)

explain the O and T layers

O: octahedral = OH, cation, OH
T: tetrahedral = O^2-, Si or Al, O^2-

OT, TOT, OT-OT, TOT-TOT, TOT+cations, TOT+O+TOT

in the octahedral layer, whether it’s __ or __ has a big impact on the properties of the mineral

Al or Mg/Fe

details on framework silicates

all four Oxy on each tetrahedra are shared with adjacent tetrahedra to form a 3D framework
more than 2/3 of the earths crust is composed of framework silicates
4 Oxy are surrounding the silicon, they are all shared, so all 4 represent a half, so it’s 4 halfs = 2
most highly polymerized silicate group, crystal structure is fairly open, can easily accomodate big cations like Ca2+, Na+, K+
tend to not be stable at high pres, restricted to earth’s crust

SiO₂ neutral

details on feldspar group

feldspars have aluminum so we need more positive charge to combat the lesser positive charge that Al supplies, can use K, Na, or Ca
K: K-feldspar: every 4th tetrahedra has an aluminum in it, tetrahedra’s surround the potassium. potassium fills in the empty space
Na: albite
Ca: anorthite, need one MORE Al, one LESS Si

details on substitution

complete solid solution is possible from Ca and Na, same size but different charge. No solid solution between Ca and K because they are too different

explain order-disorder polymorphism: kspar

difference is the location of cations in structure
sanidine: fast cooling rate, volcanic rock, completely disordered (Al can sub for any Si)
orthoclase: intermediate cooling rate, intermediate number of sites with Al
microcline: slow cooling rate, completely ordered, Al restricted to one site

explain feldspathoid group

less Si relative to amount of Na and K
occur in Si-poor igneous rocks (which are uncommon), so feldspathoids are also uncommon
feldspathoids and quartz will never be in the same rock
Ex: nepheline, leucite

list the mineral groups and corresponding anion/anion group

native elements: N/A
oxides: O^2-
hydroxides: OH-
halides: Cl-, Br-, F-
sulfides: S^2-
sulfates: SO₄^2-
carbonates: CO₃^2-
phosphates: PO₄^3-
silicates: SiO₄^4-