Greetings colleagues!
I know it's a lot massages from me on the forum last weeks.
I've read the topic about nagative poroptions of end memebers in spinel that is published here a time ago, but I observe the large values of negative propotions and it's confusing.
What is the right way to interpretate this data?
QuoteTitle: osyb9
T = 1205,00 (C) P = 9,000 (kbars) log(10) f O2 = -7,58 delta HM = -4,92 NNO = -0,07 QFM = 0,00 COH = 1,83 IW = 4,11
Constraint Flags: fO2 path = QFM
Liquid mass = 27,95 (gm) density = 3,09 (gm/cc) viscosity = 1,49 (log 10 poise) (analysis in wt %)
G = -388332,13 (J) H = -280978,92 (J) S = 72,63 (J/K) V = 9,05 (cc) Cp = 38,42 (J/K)
SiO2 TiO2 Al2O3 Fe2O3 Cr2O3 FeO MnO MgO NiO CoO CaO Na2O K2O P2O5 H2O CO2 SO3 Cl2O-1 F2O-1
41,29 5,16 13,04 2,97 0,00 23,16 1,04 3,10 0,00 0,00 7,64 2,61 0,00 0,00 0,00 0,00 0,00 0,00 0,00
clinopyroxene mass = 30,84 (gm) density = 3,39 (gm/cc) (analysis in mole %)
cpx Na0,06Ca0,63Fe''0,41Mg0,61Fe'''0,12Ti0,05Al0,41Si1,72O6
G = -463007,38 (J) H = -356018,61 (J) S = 72,38 (J/K) V = 9,09 (cc) Cp = 35,52 (J/K)
diopside clinoenstatit hedenbergite alumino-buffo buffonite essenite jadeite
-6,76 31,38 41,26 16,54 -7,09 18,89 5,77
clinopyroxene mass = 14,56 (gm) density = 3,42 (gm/cc) (analysis in mole %)
cpx Na0,03Ca0,32Fe''0,62Mg0,79Fe'''0,09Ti0,02Al0,35Si1,77O6
G = -212375,73 (J) H = -161371,57 (J) S = 34,51 (J/K) V = 4,25 (cc) Cp = 16,95 (J/K)
diopside clinoenstatit hedenbergite alumino-buffo buffonite essenite jadeite
-53,03 64,95 62,06 13,29 -9,29 18,66 3,35
feldspar mass = 26,90 (gm) density = 2,65 (gm/cc) (analysis in mole %)
K0,00Na0,47Ca0,53Al1,53Si2,47O8
G = -459947,06 (J) H = -361093,18 (J) S = 66,88 (J/K) V = 10,14 (cc) Cp = 33,22 (J/K)
albite anorthite sanidine
46,73 53,27 0,00
Total solids mass = 72,31 (gm) density = 3,08 (gm/cc)
G = -1135330,17 (J) H = -878483,36 (J) S = 173,76 (J/K) V = 23,49 (cc) Cp = 85,69 (J/K)
Viscosity of the System cannot be computed.
System mass = 100,26 (gm) density = 3,08 (gm/cc)
G = -1523662,30 (J) H = -1159462,28 (J) S = 246,39 (J/K) V = 32,54 (cc) Cp = 124,11 (J/K)
Oxygen delta moles = 0,00145398 delta grams = 0,0465258
G = -495,59 (J) H = 57,85 (J) S = 0,37 (J/K) V = 178,69 (cc) Cp = 0,05 (J/K)
Dear Anna,
Negative values of end-members is normal and not a matter of concern. You should only be concerned if the quantity of some actual element were negative. Let's look at why you get negative end-members so you can understand my answer. The pyroxene components are:
diopside CaMgSi2O6
clinoenstatite Mg2Si2O6
hedenbergite Ca(Fe2+)Si2O6
alumino-buffonite Ca(Ti0.5Mg0.5)(AlSi)O6
buffonite Ca(Ti0.5Mg0.5)(Si(Fe3+))O6
essenite CaFe3+SiAlO6
jadeite NaAlSi2O6
So, for starters, consider how to make the perfectly legal and legitimate pyroxene ferrosilite, (Fe2+)2Si2O6. It isn't one of the end-members, but you can make it by a linear combination of them: ferrosilite = clinoenstatite + 2 hedenbergite ââ,¬â€œ 2 diopside. So that means, in normalized percent terms, ferrosilite is
diopside clinoenstatit hedenbergite alumino-buffo buffonite essenite jadeite
-200 100 200 0 0 0 0
In general, any low-Ca, high-Fe2+ pyroxene is going to have negative diopside (geometrically, this happens if it plots below the diagonal from enstatite to hedenbergite in the pyroxene quadrilateral and so is outside the enstatite-hedenbergite-diopside triangle).
For the non-quadrilateral components, you will note that Al-Bf has Al and Ti. Bf has Fe3+ and Ti. Ess has Al and Fe3+. By linear combinations of these three, you can make whatever amount of Al, Ti, and Fe3+ you need. But the abundances of the end-members may well be negative. Consider a pyroxene that contains tetrahedral Al but is free of either Ti or Fe3+. To make that, you have to have positive Al-Bf and Ess and negative Bf (to cancel out the Ti and Fe3+ that come along with the Al in Al-Bf and Ess).
Hope that helps!
-- Paul