illustrating mineral chemistry: graphical displays of multi … 18_chem2.pdf · 2008-10-23 ·...
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Initial concepts: two freely substituting components
End‐members: two idealized starting components (cations) that could ideally fill the same site in a mineral structure
This leaves the possibility that a mixture of these components could occur
For example, the alkali‐feldsparsNaAlSi3O8 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ KAlSi3O8
Exchange of Na+1 and K+1 can occur regularly
Plotting the ratio of the cations can be done independently or the relationship can be plotted based on the relative wt% of each species
3 ways of expressing mineral composition
Talc (sheet silicate usually formed from metamorphosed ultramafics)
In terms of weight percent oxides (wt%)
Chemical formula MgO SiO2 H2O MgO SiO2 H2OMg3[Si4O10](OH)2 31.88 63.37 4.75 3 4 1
In terms of wt% elements and apfu’
Mg Si O H Mg Si O HMg Si O H Mg Si O H19.23 29.62 50.62 0.53 3 4 12 2
l1
Feldspar GroupFeldspars are the most common silicates in the Earth’s CrustFeldspars are the most common silicates in the Earth s Crust.
C Al Si O Th i “ ld” t fCaAl2Si2O8Anorthite (An)
There is an “old” system for feldspar nomenclature, based on composition.
Or37‐100 = SanidineOr10‐37 = Anorthoclaseb l lAb90‐70 = Oligoclase
Ab70‐50 = AndesineAb50‐30 = Labradorite
NaAlSi3O8 KAlSi3O8Alkali Fsp
Ab50‐10 = Bytownite
NaAlSi3O8Albite (Ab)
3 8Orthoclase (Or)
Alkali Fsp
The Ternary diagram
Three distinct end‐members: each corner of the diagram is 100% of one component and 0% of the other two
Usually ternary plots are divided into a triangular gridded set with each line representing a 10% composition change
Returning to the feldspars:CaAl2Si2O8‐‐‐‐‐‐‐‐‐‐‐‐NaAlSi3O8 ‐‐‐‐‐‐‐‐‐‐‐‐ KAlSi3O8
In this case, the ternary diagram represents two distinct but inherently linked solid‐solution series
C Al Si OMiscibility gap: range of
CaAl2Si2O8Anorthite (An)
composition(s) where there are no stable assemblages in nature. In the example of the feldspars, there is no Or –An solid‐solution series
NaAlSi3O8 KAlSi3O8Alkali FspNaAlSi3O8Albite (Ab)
3 8Orthoclase (Or)
Alkali Fsp
COMMON SUBSTITUTIONS
Si4+ Al3+
Al3+Mg2+
Mg2+ Fe2+
One for one cation substitution are a very common manner by
which chemical formulas can be Na1+ K1+
Na1+ Ca2+
altered; however, notice the charge balance is often not
maintained.
K1+ Ca2+
Na1+Si4+Ca2+Al3+ In these cases, additional cation substiutions and/or a change in
Na1+Al3+2Mg2+
3Mg2+ □ + 2Al3+
substiutions and/or a change in the number of anions present in the formula is necessary to re-
achieve an equilibrium electricalOH1‐ Cl1‐ + □
OH1‐ F1‐ + □1 2
achieve an equilibrium electrical state.
OH1‐ 02‐ + □
F1‐ Cl1‐ □
Structural sites in minerals
Based on the size and charge of cations, we already know that there are particular types of coordination polyhedra based on the size of the
i ( ) h b d d h ifi l i ication(s) that must be accomodated, there are specific locations in a mineral where particular species will and will not fit.
Just as with creating or individual coordination polyhedra, there are specific sites in a mineral assemblage that are the appropriate size and capacity for a specific set of elements
These locations are given notations so that we can write generalized mineral formulas for a related mineral family irrespective of the specific mineral composition
Returning again to the feldspars:M1‐2T4O8
Structural sites in minerals
Most simply the terms we will use for the major sites are:M (1‐4): “metal” cation site
h d l iT: tetrahedral siteA, B, C: interstitial sites between coordinated locations
Returning again to the feldspars:
M1‐2T4O8
Plagioclase: CaxNa1‐xAl1+xSi3‐xO8
Generalize formulas and sites for the major minerals
Garnet:A2B2Si4O12
dA > B andA = Ca2+, Mg2+, Fe2+, Mn2+
B = Al3+, Fe3+, Cr3+
Generalize formulas and sites for the major minerals
Olivine:M2M1SiO4
2 1 dM2 > M1 andM2 = Ca2+, Mg2+, Fe2+/3+, Mn2+
M1 = Mg2+, Fe2+/3+, Mn2+
fayalite (Fe2SiO4) vs. monticellite (CaFeSiO4)
An example from the Olivines
Olivine is a solid solution series between an Mg and Fe rich set of end‐members: Forsterite and Fayalite
The general formula for Olivine is: (Mg, Fe)2[SiO]4g ( g, )2[ ]4
Chemical analysis (from Floran & Papike, 1973)SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaOSiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO30.09 0 0 0 69.42 0.28 0.91 0.08
Which Olivine is this an analysis of?Which Olivine is this an analysis of?
How can we calculate the formula in apfu’?
How many oxygen atoms will we need to base the calculation on?
An example from the Olivines
Olivine is a solid solution series between an Mg and Fe rich set of end‐members: Forsterite and Fayalitey
The general formula for Olivine is: (Mg, Fe)2[SiO]4
Based on the general formula, we can now assign a new formula to this specific species of Olivine:
(Mg0.0453Fe1.937Mn0.008Ca0.003)[Si1.004O1]4Sum of M1 and M2 cations: 0.0453+1.937+0.008+0.003 = 1.9933 (~2)
Octahedral coordination polyhedra of the mineral fayalitep y y
Generalize formulas and sites for the major minerals
Pyroxene:M2M1Si2O6
2 1 dM2 > M1 andM2 = Ca2+, Na1+, Fe2+, Mg2+
M1 = Mg2+, Fe2+/3+, Mn2+, Al3+, Cr3+, Ti4+
enstatite (Mg2Si2O6) vs. diopside (CaMgSi2O6)
Generalize formulas and sites for the major minerals
Amphibole:A0‐1M42M3M22M12Si8O22(OH)2
C O (O )A0‐1B2C5T8O22(OH)2A > M4 > M3 ~> M2 = M1 or A > B > C > T andA = K1+, Na1+
B = Na1+, Ca2+, Mg2+, Fe2+, Mn2+
C = Mg2+, Fe2+/3+, Mn2+, Al3+, Cr3+
T = Si4+, Al3+, Ti4+
OH = (OH)1‐, F1‐, Cl1‐, O2‐
Anthrophyllite (Mg7Si8O22(OH)2) vs. Tremolite (Ca2Mg5Si8O22(OH)2) vs. Richterite (Na(CaNa)Mg5Si8O22(OH)2)