American Mineralogist, Volume 76, pages 661-662, l99I

Structural variation in the -ankeritesolid-solution senes: An X-ray, Miissbauer, and TEM study-Reply

Rrcn.q.RDJ. Rrnonn Department of Earth and SpaceSciences, State University of New York at Stony Brook, Stony Brook, New York I 1794, U.S.A. W.lvNn A. Dor,r-.lsB Departmentof Earthand SpaceSciences, University of California,Los Angeles,Los Angeles,California 90024, U.S.A.

IJpon reconsideringthe effectsof octahedraldistortion with just over 700/oFe2* (sample AMNH 8059) deviate on the stabilities offerroan carbonates,Rosenberg (1991) from 90" by 1.80". Thus, it appearsthat the oc- agrees that the cause of the apparent instability of tahedra are more distorted, but by only 0.25". CaFe(CO,), and limited substitution of Fe2* in dolomite A slightly greater octahedral distortion in siderite is does not seem to lie with excessivedistortion of the oc- consistent with the observation of a larger quadrupole tahedral sites in ankerite. Rather, he now suggeststhat splitting in the Mdssbauerspectra of siderite (Nagy et al., the lack of significant octahedral distortion in ankerite 1975) compared to that of ankerite (Reederand Dollase, and the presenceof slightly greaterFe2+ site distortion in 1989; DeGrave and Vochten, 1985). Similarly, the lower siderite serve to stabilize the latter, thereby favoring the rate of changeof quadrupole splitting (QS) as a function assemblagecalcite + siderite solid solutions over ankerite of temperature observed in siderite (Nagy et al., 1975) with CaFe(COr), content greater than roughly 70 molo/0. compared to ankerite (DeGrave and Vochten, 1985) is We agree with Rosenbergthat the presenceof different very likely due to slightly greaterdistortion in the former, Fe'?+site distortions in these allows for the pos- and hence,greater splitting ofthe first excited stateabove sibility of differing crystal field stabilization. However, we the ground state. also believe the magnitudes of the distortion differences However, the differencesobserved between siderite and that have been documented are sufficiently small that at- ankerite are very small, especiallywhen compared with tributing differencesof stability to crystal field effectsmay examples of Fe'z+-bearingsilicates. The crystal field en- be unwarranted. Nevertheless,we find Rosenberg'ssug- ergetic consequencesof such small differencesin distor- gestion very interesting and believe that it points to the tion are also likely to be small. need for greater study of Fe-bearingcarbonates. We also Trigonal elongation of the Fe2* octahedra in siderite applaud Rosenbergfor indicating that observed octahe- and ankerite results in splitting of the tr, energy level, dral distortions in rhombohedral carbonatesare not fully producing an orbital doublet ground stateand singlet first consistent with the previously proposed stability series excited state, for which the splitting is denoted by D.The for transition metal dolomites (cf. Rosenbergand Foit, additional crystal field stabilization energy (CFSE) for 1979). high-spin Fe2+resulting from this trigonal elongation is Since Rosenberg'sarguments are based on differences 7:6, so that the total CFSE is 2/rLo I %0. Values of Ao of the magnitudesof Fe2+site distortion betweensiderite have not beenreported for siderite or ankerite, and there- and ankerite, we examine this first. The sitesoccupied by fore, evaluation of total CFSE is not possible. However, Fe2*inbothsideriteandankeritehavesymmetry 3, and DeGrave and Vochten (1985) have estimatedd : 370 static octahedral distortions are constrained to be either cm ' for an ankerite based on the temperature depen- trigonal elongationsor compressions.Magnitudes of dis- denceof QS. A comparable analysisfor siderite using the tortion are commonly expressedeither by quadratic elon- data given by Nagy et al. (1975) yields approximately 600 gation (Robinson et al., 197l) or, somewhat more con- cm ', which is consistentwith its slightly greatersite dis- veniently,bydeviationfrom90'oftheoctahedralangles. tortion. We emphasize,as did DeGrave and Vochten, In both siderite and ankerite, the observed static distor- that these values should be considered little more than tion is a trigonal elongation of small magritude. We have rough estimates,since the simple crystal field models used previously noted that cation sites of all calcite- and do- cannot adequatelydescribe the linear temperaturedepen- lomite-structure carbonatesexhibit small trigonal elon- dence of QS observed for both siderite and ankerite. gations, both larger and smaller in magnitude than the However, theseestimates do suggestthat the crystal freld ferroan carbonatesdiscussed here (cf. Reeder, 1983). energeticdifferences are likely to be small-on the order The structure refinement of siderite by Etrenbergeret of only a few hundred callmol greater stabilization in al. (1981) showsthat the O-Fe'?+-Oangles (the O atoms siderite. are situated at different levels along c) of the trigonally Further evidence that the additional crystal field sta- elongated octahedra deviate from 90' by 2.05". Refine- bilization of siderite is minimal as a consequenceof its ments of the ankerite structure(Reeder and Dollase, 1989) distortion can be found in the linear relationship between show that the O-(Fe'z+,Mg)-Oangles of the octahedrafilled the free energiesof formation of aqueous metal species 0003-o04x/91/0304-{66 I $02.00 661 662 REEDER AND DOLLASE: DOLOMITE-ANKERITE SERIES:REPLY and their respective (calcite structure) carbonate shown refinements of magnesite,calcite, ,siderite, smithsonite, by Sverjensky(1984). Ifthe distortion ofthe Fe2+site in and dolomite, with discussionof sorne aspectsof the stereochemistry siderite contributed of calcite type carbonates.Zeitschrift fiiLrKristallographie, 156, 233- significantly to enhancingits stabili- 243. ty, one would expect its free energyvalpe to deviate from Nagy, D.L., Deszi, I., and Gonser, U. (1975) Mdssbauerstudies of FeCO, the straight line connecting the other carbonates(Sver- (siderite).Neues Jahrbuch fiir Mineralogie Monatshefte, l0l-114 jensky, 1984,Fig. l), for which there are no crystalfield Reeder, R.J. (1983) Crystal chemistry of the rhombohedral carbonates. effects. In Mineralogical Society of American Reviews in , I l, l- 47. A small CFSE may (or may not) be significant,as noted Reeder,R.J., and Dollase, W.A. (1989) Structural variation in the dolo- by Rosenberg,but it should not be viewed as the only mite-ankerite solid-solution series:An X-ray, Mdssbauer,and TEM factor affecting the apparent instability of CaFe(COr), and study. American Mineralogist, 74, 1159-1167. the limited Fe2+ substitution in ankerite. For example, Robinson, K., Gibbs, G.V., and Ribbe, P.H. (1971)Quadratic elongation: quantitative polyhedra. the energeticconsequences A measureof distortion in coordination Sci- of mixing in calcite and sid- ence.172.567-570. erite solid solutions [which are seemingly more stable than Rosenberg,P.E. (1991) Structural variation in the dolomite-ankeritesolid CaFe(COr)r-rich ankeritesl could easily be on the order solution series:An X-ray, Miissbauer, and TEM study-Discussion. of hundreds of callmol. In view of the lack of data for American Mineralogist, 76, 659-660. Rosenberg,P.E., Foit, F.F., (1979)The stability metal the various energeticcontributions, we believe that it is and Jr. oftransition probably dolomites in carbonatesystems: A discussion.Geochimica et Cosmo- unwarranted to attribute any causeto small dif- chimicaActa. 43. 951-955. ferencesin octahedral distortions, unless other observa- Sve{ensky, D.A. (1984) Prediction ofGibbs free energiesofcalcite-type tions can be found to support it. carbonates and the equilibrium distribution of trace elements between carbonatesand aqueoussolutions. Geochimica et CosmochimicaActa, RnrpnnNcns cITED 48,1127-1134. DeGrave, E., and Yochten, R. (1985) An 5?FeMdssbauer effect study of ankerite. Physicsand Chemistry of Minerals, I 2, 108-1 I 3. M,lNuscRrFr REcETvEDMe,ncH 12, 1990 Effenberger,H., Mereiter, K., and Zemann, J. (1981) Mexuscnrrr AccEmEDJnNumv 10, 1991