Compositional Variations in Apatite and Petrogenetic Significance: Examples from Peraluminous Granites and Related Pegmatites and Hydrothermal Veins from the Central Iberian Zone (Spain and Portugal)
, Alfonso Pesquera 1, Alexandre Lima 2,3, Idoia Garate-Olave 1, Enrique Merino-Martínez 4, Joana Cardoso-Fernandes 2,3 and Jon Errandonea-Martin 1Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
- Line 19: replace "peraluninousity" by "peraluminosity".
Done
- Line 52, after reference [15]: parenthesis is missing.
Added
- Line 102 Cerny and Ercit, line 177 Cerny & Ercit : please uniformize. Also, please add the correct diacritical in the name of Peter Cerny: the correct spelling is "Černý".
Done
- Lines 332 to 366. In this section, the authors describe the variations in wt. % for the different However, these variations are huge: for example, CaO from 37 to 55 wt. %, MnO from 1 to 10 wt. %, P2O5 from 33 to 44 wt. %. Such compositions do not reflect the real composition of the apatites: in Figure 2, you can clearly see that these extreme values are far away from the mean values. For example, in Figure 2, MnO is from 0 to 6 wt. %, P2O5 is from 37 to 43 wt. %, and CaO is from 45 to 55 wt. % (if you observe only the values in the boxes). I think that the extreme values, which may correspond to analytical errors, should be omitted from this discussion: only the reliable values should be used (for example, the values of the boxes in Figure 2, or the values appearing in the tables, which are even narrower).
We agree with the reviewer that variations of the content in most of the major elements in the studied apatite are huge, as we actually indicate in the text. However, we do not agree with the rest of this comment. As it is evidenced in Figs. 3, 4 and 5, the variations given for the major elements are continuous and the chemical trends show a high correlation for all the elements. No compositional gaps are observed for the major elements. Therefore, to omit part of those values would imply to give just a partial information on the chemistry of the studied apatite. We consider that it would be undoubtedly problematic to decide arbitrarily since which composition we should omit some of the data. It is needed to say that, for example, in the well-known textbook “An Introduction to the Rock-Forming Minerals” (by Deer, Howie & Zussman) the dataset for apatite includes 6 representative analyses that show P2O5 values in a range that goes from 35.01 to 42.40 wt.%, as broad as the one we present. On the other hand, if we just leave the data of the “boxes” from Fig. 2, we could hardly see any chemical variation in the apatite composition. It is needed to indicate that all the data that could correspond to analytical errors had previously been deleted.
- Indeed, an apatite-group mineral with 10 % MnO is certainly not an apatite any more: it may be a new Mn-dominant species....
As indicated in the text (Discussion chapter, point 5.1.1), the highest MnO content reported for apatite is nearly the double (≈19% wt.% MnO, (Pieczka, A. (2007) Beusite and an unusual Mn-rich apatite from the Szklary granitic pegmatite, Lower Silesia, Southwestern Poland. Can. Mineral. 45, 901-914).
All the compositions of the apatite crystals included in this study fall in the area given for apatite related to “Pegmatites” and “Felsic-intermediate ore-related systems”, for the Ca-Fe-Mn space, in the chapter entitled “Apatite in Igneous Systems” (Piccoli & Candela (2002), in “Phosphates”, Reviews in Mineralogy and Geochemistry, vol 48, Mineralogical Society of America).
- Table 2. I don't understand the choice of the basis for the calculation of the apatite formulae. The authors decided to choose a basis of 26 O atoms pfu, but the formula of apatite-group minerals is Ca5(PO4)3(F,OH,Cl), which contains 12 O and one other anion (F,OH,Cl). If you multiply this content by 2, it gives 24 O pfu, not 26. The total of positive charges, which equals the total of negative charges, is 25, multiplied by two it becomes 50. This corresponds to 25 oxygen, not 26. Consequently, I think that the correct basis, to be used here, is 25 oxygen, not 26.
As also noticed by the other reviewer, there was a mistake in this regard. The structural formulae have been calculated on the basis of 26 anions, not on the basis of 26 oxygens. This mistake has been already corrected.
- Lines 518 to 549. In this section the authors explain that the fO2 variations may be a parameter governing the incorporation of Mn end fe in the apatite structure, but that the most probable reason for the Mn and Fe variations is the availability of these elements in the melt. Of course, Mn2+ shows an ionic radius and a charge very close to those of Ca2+, and for that reason, Mn2+ is the most easy substituent in apatite. Fe2+ also shows a similar ionic radius, but it is smaller. Mn3+ and Fe3+ are both too small to be significantly incorporated in the apatite structure, and for that reason, a very high fO2 would not be favorable for Fe and Mn incorporation in apatite. However, it depends of the fO2 occurring in the pegmatitic and granitic rocks investigated here. For the oxidation of Mn2+ to Mn3+, a very high oxygen fugacity is necessary, which is observed during low-temperature hydrothermal or meteoric conditions. So, if the fO2 is close to the Ni/NiO buffer, Fe and Mn should mainly be divalent, and the fO2 should not play an important role in the control of these substituants. The main parameters being their availability in the melt, and the crystal-chemical constraints. In conclusion, I think that these paragraphs can be shortened and modified a bit....
We have reduced the length of this part of the discussion, pointing out the main parameters controlling the Mn-Fe contents in apatite, as indicated by the reviewer.
Reviewer 2 Report
The manuscript provides a useful compilation of data on apatite from different granites, showing petrogenetic significance of its chemical composition.
My major concern is that the authors choose to calculate structural (or crystal-chemical) formulae of apatite on the basis of 26 O, taking into account that F, Cl and OH can be present as well as O2- anions. The IMA recommends three ways of calculation, from which the calculation based upon 13 anions is preferred:
‘Calculation on the basis of 13 total anions. If the analytical value for H2O is available, this will not result in exactly 12 O and 1 (F + Cl + OH). Typically, the total amounts of O and (F + Cl + OH) should not dramatically shift from the above values; however, in some cases, depending on the overall positive charges associated with cations, it could also be that some O2 substitutes for monovalent anions at the X crystallographic site. If the analytical value for H2O is lacking, it is reasonable to assume a calculated wt% H2O so as to give 13 total anions (O + F + Cl + OH), with O = 12 and (F + Cl + OH) = 1, unless (F + Cl) >= 1 (in those cases H2Ocalc can be omitted).’ [Nomenclature of the apatite supergroup minerals: Eur. J. Min., 22 (2010),163-179.]
Or did the authors mean that they calculated crystal-chemical formulae on the basis of 26 anions (it seems correct)? Please make clear which way of calculation was used in order to avoid misunderstanding.
Misprints:
Line 122
Make ‘1’ a subscript
Line 180
Insert space between ‘50’ and ‘m’
Line 214
Insert space between ‘7’ and ‘m’
Lines 299 and 301
Are these molar or weight percents? Please make clear.
Author Response
The manuscript provides a useful compilation of data on apatite from different granites, showing petrogenetic significance of its chemical composition.
My major concern is that the authors choose to calculate structural (or crystal-chemical) formulae of apatite on the basis of 26 O, taking into account that F, Cl and OH can be present as well as O2- anions. The IMA recommends three ways of calculation, from which the calculation based upon 13 anions is preferred:
‘Calculation on the basis of 13 total anions. If the analytical value for H2O is available, this will not result in exactly 12 O and 1 (F + Cl + OH). Typically, the total amounts of O and (F + Cl + OH) should not dramatically shift from the above values; however, in some cases, depending on the overall positive charges associated with cations, it could also be that some O2 substitutes for monovalent anions at the X crystallographic site. If the analytical value for H2O is lacking, it is reasonable to assume a calculated wt% H2O so as to give 13 total anions (O + F + Cl + OH), with O = 12 and (F + Cl + OH) = 1, unless (F + Cl) >= 1 (in those cases H2Ocalc can be omitted).’ [Nomenclature of the apatite supergroup minerals: Eur. J. Min., 22 (2010),163-179.]
Or did the authors mean that they calculated crystal-chemical formulae on the basis of 26 anions (it seems correct)? Please make clear which way of calculation was used in order to avoid misunderstanding.
We really appreciate this comment of reviewer 2. Crystal-chemical formulae were not calculated on the basis of 26 O, but on the basis of 26 anions. We have corrected this mistake in the new version.
Misprints:
Line 122
Done
Make ‘1’ a subscript
Line 180
Done
Insert space between ‘50’ and ‘m’
Line 214
Done
Insert space between ‘7’ and ‘m’
Lines 299 and 301
Done
Are these molar or weight percents? Please make clear.
Done
