In our study on some granitic rocks we observed some samples containing high perthitic orthoclase in thin sections but they have very low concentrations of K2O in their whole rock analyses but instead contain considerable amounts of Na2O. It is not interpretable for us that why modal content of feldspars are not compatible with the results of chemical analyses. We repeated the chemical analyses many times with many methods and many accurate instruments but the results is similar. Would you please guide us?
It would be interesting to see an image of your perthitic orthoclase to get some idea of the ratio volumes of plagioclase exsolution lamellae: host orthoclase. In your modal analysis are you counting perthitic orthoclase as orthoclase or estimating proportions of lamellae and host?
Perthite is an intergrowth with K feldspar forming the host for Na feldspar (K>Na). Antiperthite is the intergrowth displaying the Na feldspar hosting the K feldspar (Na>K). Check your intergrown minerals by means of the petrographic microscope or the EMPA. Whole-rock XRF and only routine thin section examination alone may be accountable for the mis-interpretation.
HGD
Could it not have something to do with the sample size used for whole rock analysis compared to that represented by a thin section? Presumably you crushed at least a fist-size lump of rock for whole-rock analysis, possibly more if the granite is really coarse grained or porphyritic? Now, in comparison a thin section represents a tiny sample of a rock....how many thin sections were analysed to get your modal mineralogy?
Feldspar is not only the most frequent mineral in the Earth Crust but also one of the most complex s.s.s. The interrelationship of the various domains covers the size range from macroscopic lamellae down to the nanoscale. A precise mineralogical examination is paramount and conducted ahead of any chemical analysis. Even the mafic silicates associated with feldspar , amphibole, can be the source of Na. In the source rock analysis a great deal of Na may have been derived from Na-dominated amphiboles as shown by the most common amphibole groups: Mg-Fe-Mn-Li amphibole, Ca amphibole, Na-Ca amphibole, alkali amphibole, Na-Ca Mg-Fe-Mn-Li amphibole.
HGD
Dear Ali,
It is pity that this photo is not the best.
However, you have there an amphibole surrounded by K-feldspar? what kind of granitic rock is it?
Did you try to determine chemical composition of this K-fs by EPMA? Sometime one can be suprised by differences in results between values with focused beam and "de-focused" beam in diameter of 10-15 microns.
Question is - how you determined modal composition = did you use Eltinor for counting all minerals?
Other question is what kind of normative minerals content calculation was used - there are big differences between CIPW norm, Mesonorm or Peranorm (Usdanky, 1986)... I have good experience comparing modal composition (counted by Eltinor) with Mesonorm composition of my granitic rocks.
Best wishes,
Milan
***********
It seems most of perthites are antiperthite in composition. If you have opportunity please do EMPA analyses and take BSE images it will answers your question.
Dear Ali,
to quote somebody I don't remember right now, rocks don't lie, but they might be misunderstood. The most probable source of this discrepancy is the misidentification of K-feldspar. I don't think an antiperthite is likely to be mistaken for a perthite as suggested. What's happened then, also noticing that the quality of the microphotograph is not very helpful? One pretty remote possibility is that the phase identified as cross-hatched microcline is in fact Schachbrettalbit, but a more probable answer might lie in the optics of Na-rich plagioclase. The birefringence decreases with uptake of Ca, recording a minimum at ca. 45% mole An, the relief stay negative up to 30% An, the optical sign is (-) between 15-30% An, resembling K-feldspar. One additional complication is the unmixing of peristerite compositions, leading to blurred extinction, patchy microstructures and even anomalous interference colours. The only way out of this dilemma is as suggested by Prof. Dill and Amjad, a more careful optical and compositional (SEM) investigation using BSE pictures and compositional maps, resolution allowing.
- Badges
- Science topic
- Similar topics
- Philosophy
- Esthetics
- Interpretation