American Mineralogist, Volume 72, pages 319-328, 1987

Petrologicand crystal-chemicalimplications of cation order-disorderin kutnahoriteICaMn(COr)rl* D. R. Pucon, E. J. EssBNn Department of Geological Sciences,The University of Michigan, Ann Arbor, Michigan 48109, U.S.A. A. M. G.qlNns Division of Earth Sciences,National ScienceFoundation, Washington, D.C.20550, U.S.A.

Ansrucr The crystal structuresof kutnahorite from Bald Knob (BK), North Carolina, and Sterling Hill (SH), New Jersey,have been refined. The BK kutnahorite (CaorMn'oMg'Feo)is dis- ordered in Ca-Mn distribution, but the SH kutnahorite is substantially ordered: (Ca".roMno1u)(Cao rrMno rrXCOrL. Long-rangecation order in kutnahorite is not detectable using conventional powder X-ray diffraction techniques,but it may be measuredby single- crystal techniques.The largeionic radius of Mn relative to Mg in leadsto coupled distortion of the Ca and Mn octahedrathat may result in a low ordering potential. Thus, the BK kutnahorite lacks significant cation order despite slow cooling from amphibolite- facies regional metamorphic conditions. Long-range cation order in SH kutnahorite is compatible with a low-temperature solvus between and kutnahorite as well as between kutnahorite and . Two-phase intergrowths of (CarrMn,rMg) and calcian kutnahorite (Ca.rMnrrMgr) from SH are interpreted as due to primary coprecipitation of calcite and ordered kutnahorite from solution in the two-phase region at temperaturesbelow the solvus crest. Data on metamorphic Ca-Mn carbonates indicate complete solid solution between calcite and rhodochrosite at 600qCwith solvi betweenkutnahorite-calcite and kutnahorite-rhodochrositeforming at lower temperatures.

INrnooucrrou and calcite. The ternary system CaCOr-MnCOr-MgCO. Kutnahorite was originally described by Frondel and was evaluated at 500 to 800"C by Goldsmith and Graf Bauer (1955) as having ideal composition CaMn(COr), (1960), who found wide solvi separatingmanganoan cal- and ordering of Ca and Mn, resulting in a dolomitelike cite, manganoan dolomite, and manganoan magnesite. structure. The ordering was determined by the apparent They were unable to detect ordering reflections in syn- presenceof hll, l: 2N + I reflections (basedon hexag- thetic kutnahorite and concludedthat the similar scatter- onal axes)in the powder-diffraction pattern that are com- ing powers of Ca and Mn result in very weak and perhaps patible with spacegroup (R3) of the ordered compound undetectableordering reflections even if Ca and Mn are but incompatible with spacegroup (R3c) of the disor- highly ordered.Goldsmith (1983) further pointed out that dered calcite-typestructure. Schindler and Ghose (1970), short-rangeorder may occur in carbonatesbut be unde- Wildeman(1970), Lumsden and Lloyd (1984),and Lloyd tectableby X-ray diffraction. Capobiancoand Navrotsky et al. (1985) showed that Mn2+ substitutesin both the A (1987) combined experimental and thermodynamic data and B sites of dolomite, but with a strong preferencefor on synthetic CaCOr-MnCO. solid solutions to calculate the B(Mg) site. This implies strong Ca-Mn order in man- a solvus for disordered carbonatescresting at ca. 330'C ganoan dolomite, but the results are for very low Mn and X.^.o, of 0.44. The calculated solvus is metastable concentrationsand may not apply to CaMn(COr)r. Bol- relative to a solvus or solvi involving kutnahorite with zenius et al. (1985) have briefly describeda structure re- short- or long-rangeorder (Capobianco and Nawotsky, finement of a magnesian kutnahorite of an unreported 1987).Although the coherentand spinodal solvi were not composition but did not report on the inferred cation given, their data suggestthat disordered carbonatesnear ordering. the kutnahorite composition are unstableat Earth surface The binary systemCaCOr-MnCO. was experimentally temperatures. investigatedby Goldsmith and Graf (1957) and de Cap- Despite the absenceofobserved ordering reflectionsin itani and Peters(198 l), who found a solvus betweenrho- the synthetic materials, the experimental solvus inferred dochrositeand kutnahorite but none betweenkutnahorite between kutnahorite and rhodochrosite (Goldsmith and Graf,1957 de Capitaniand Peters,l98l) suggestscation * Contributionno. 422 from the Mineralogicaltaboratory, ordering in kutnahorite by analogy with the system Departmentof GeologicalSciences, the University of Michigan. CaCOr-MgCOr.This also would suggesta solvus between 0003404x/87/030443r 9$02.00 319 320 PEACORET AL.: CATION ORDER.DISORDERIN KUTNAHORITE

TABLE1. Microprobeanalyses of manganoancarbonates could be detectedin single-crystalX-ray photographs. This specimen contains only small amounts of other cations weight percent such as Mg. Becausesuch additional components would 8K.14 H-109405 sH-525K SH-525C have significant effects on order-disorder relations and FeO 0.2 0.8 0.8 0.2 becauserelatively pure kutnahorites are unusual, we chose MnO 32.6 31.8 zI -o v.b refine its structure in order to directly determine the Mgo 0.4 u.c 1.1 0.5 to CaO zJ-o 25.3 37.8 56.5 state of order-disorder. In addition, Pete J. Dunn kindly ZnO n.d. 1.4 0.8 0.1 made available a specimen of Sterling Hill (SH), New 40.6 40.3 31.1 32.5 Sum 99.4 100.1 99.2 99.4 Jersey,kutnahorite that has a composition nearly iden- Atomfraction tical to that from BK. Becausesome SH Ca-Mn carbon- Fe 0.003 0.012 0.008 0.002 ates are two-phase intergrowths (Frondel and Bauer, I 9 5 5; Mn 0.496 o.477 0.351 0.117 Goldsmith, 1959, 1983),the presenceof a solvusis im- Mg 0.011 0.013 0.025 0.010 plied, and this in tum suggestsordering in these kutna- Ca 0.490 0.480 0.607 0.888 Zn n.d. 0.018 0.009 0.003 horites; we therefore chose to refine the structure of a specimenfrom that locality. Nofe: CO, calculatedfrom stoichiometry; n.d. : not determined; sam- ple 525 from SterlingHill for kutnahorite(K) and calcite(C). Saupr-r cHARACTERIZATToN The Bald Knob, North Carolina, sample (BK-14) is kutnahorite and calcite, but this solvus has not yet been from a fine-grained metamorphic rock that equilibrated demonstratedexperimentally. The compositions of both under middle amphibolite-facies conditions at tempera- synthetic and natural metamorphic phasesthat fall close turesof 550-600"C (Winter et a1.,I 98 l). This sampleand to the CaCO.-CaMn(COr),binary join are consistentwith others at Bald Knob display an obvious gneissiclayering complete solid solution although some two-phase mate- that reflects differencesin proportions and grain sizesof rials and compositional gapssuggest low-temperatwe sol- Mn carbonatesand silicates,such that the carbonatesmay vi (Frondel and Bauer, 1955; Goldsmirh, 1959, 1983; vary significantly in composition between layers. How- Winter et al., I 98 I ; de Capitani and Peters, I 98 I ; Essene, ever, analysesof six different grains from sample BK-14 1983;Fig. l). The continuum of compositionscould be gave results that are identical within analytical error. Al- due to (1) crystallization of metastable,cation-disordered though the analytical totals ofthe carbonate analysesin carbonateswithin a two-phase field as in the calcite-do- Winter et al. (1981) are erratic, reanalysisof kutnahorite lomite system(Goldsmith, 1983);(2) locationof a solvus BK-14 with the University of Michigan automated ce- at a temperaturebelow that of the formation of the min- vrrca Camebaxmicroprobe yields a formula closeto that erals studied; or (3) the lack ofresolution offine-grained, originally reported but with better totals (Table l). A two-phaseintergrowths. Extrapolation of the experiments cleavagefragment was separatedfrom the original spec- in the temary system CaCO,-MgCOr-MnCO, (Gold- imen, and its composition was assumedto correspondto smith and Gral 1960) indicates that the solvus will in- the average(Ca. onMno,oMgo o, Feo ooCOr) for the specimen tersect the CaCO3-MnCO.join at T < 400'C. This sys- (Table l). tem demonstratesthe importance of even minor Mg in The Sterling Hill sample (SH) was obtained from Har- generating a solvus assemblagethat may be driven by vard University sample H-109405. The kutnahorite oc- ordering of cations in the dolomite-type structure. curs as a coarselycrystalline vein cutting (Fron- Lastly, the "dolomite problem" has long been an enig- del and Bauer, 1955). This sample was chosenbecause a ma to crystal chemists. It is not at all clear why Ca and systematic study of compositions of Sterling Hill and Mg are ordered in dolomite, resulting in a phase that Franklin, New Jersey, kutnahorites (P. J. Dunn, pers. determines the form of the CaCOr-MgCO, system and comm.) had shown that it is nearly stoichiometric with having far-reaching consequencesin natural carbonate only minor amounts of Mg and thus is very close to the systems.Reeder (1983) has reviewed the factors that af- Bald Knob sample in composition. Refinement of the SH fect the relative stabilities of possibleordered compounds composition during X-ray structure analysisis consistent with the dolomite structure.Interpretation is hindered by with the composition(CaorrMnoor)COr. The singlecrys- a lack ofdetailed knowledgeofsteric factors in the struc- tal that was studied was ground away before microprobe tures ofseveral carbonates.As kutnahorite is one ofonly analysiscould be obtained; analysesof the crushedgrains four known dolomite-structure , it is essentialto from which the single-crystalmount was selectedyielded define its detailed structure so that meaningful compari- (CaoorMno orZno o, Mg o,Feoo, )COr. However,it is evident sons of steric parameters can be made and to provide that the SH sampleis somewhatvariable around the ideal insight into factors that lead to ordered dolomite-type kutnahorite composition. In the discussionthat follows, structures. it is assumed that the composition derived from the As part of a study of Mn minerals from Bald Knob X-ray study is correct. (BK), North Carolina, Winter et al. (1981) characterized Powder-diffractometer patterns were obtained from a phase having a composition very near to that of stoi- both BK and SH kutnahorites. We attempted to index chiometric kutnahorite, for which no ordering reflections these patterns initially based on the indexing of Frondel PEACOR ET AL.: CATION ORDER-DISORDER IN KUTNAHORITE 321

Trale 2. Powder diffraction data for kutnahorite TneLe3. Crystaldata for Bald Knob and SterlingHill kutnaho- (SampleBK-14, Bald Knob, North Carolina) rite

ilh SH

U ?74 J./5 012 R3 R3 100 2.939 2.936 104 Lattice parameters 3 2.438 2.437 110 a (A) 4.8732(81 4.894(1) 4 2.225 2.224 113 16.349(6) 16.50(2) 0 2.O44 107 v(A') 336.2(1) 342.3(3) 10 2.O42 2.043 202 Ditfractometer Weissenberggeometry o-scan 2 1.875 1.875 o24 Radiation MoKa, monochromatedwith graphite I 1.839 1.839 018 crystal 0 1.817 009 Detector Scintillationcounter 11 1.816 1.816 116 Absorption correction 1 1.587 1.588 211 p (cm ') 50 50 o 1.565 1.566 122 crystal shape rhombohedron rhombohedron 't.486 3 1.486 214 averagevertex to vertex 0.26 0.20 1 1.468 1.468 028 distanc€ (mm) 2 1.456 1.456 110 sin d/Xlimit 0.65 0.65 1 1.435 1.434 125 Reflectionsmeasured h>0;h+k>0 h>0;h+k>0 2 1.407 1.407 300 Number of reflections 322 319 1 1.291 1.292 02,10 Number of unobserved 78 54 1.256 1.257 128 reflections Finalresiduals Notei The ordering reflections (107) and (009) are in- F (all data) 3.9% 4.O% ferred to have an intensity oJ 0 based on single-crystal R (observeddata) 3.1% 3.1"h observations Largest A/sigma 0.12 0.09

and Bauer (1955) in order to obtain data for least-squares disordered distribution of Ca and Mn and that the SH refinementof lattice parameters.Ambiguities becameap- kutnahorite is at least partially ordered. In order to test parent in the indexing of the ordering reflections, how- the BK sample for the possibility of short-range order ever, and we therefore reindexed the pattern (Table 2). that might occur in anti-phasedomains of a size so small This showedthat all observedreflections for both the BK as to be undetectableusing X-ray difraction, a sample and SH samples,as well as those of Frondel and Bauer was studied using transmission-electronmicroscopy. An (1955),are explainedby nonorderingreflections alone. ion-thinned sample was observedusing the University of Michigan JEoLJEM roocx electron microscope. The rela- Cnvsr,l,r,-srRucruRE REFINEMENTS tively featurelessimages so obtained are consistent with Rhombohedral cleavagefragments of kutnahorite from a randomly disordered structure. both localities were mounted along c on a Supper-Pace Refinement was carried out with the program RFTNE2 diffractometer system. MoK" radiation, monochromated (Finger and Prince, 1975) using neutral scatteringfactors by a flat crystal of graphite, was used. Intensities of re- of Doyle and Turner (1968) with equal Ca and Mn oc- flectionswith sin d/tr < 0.65 were measuredand corrected cupanciesinitially assumedfor the A and B sites of both for Lorentz, polarization, and absorption effects,resulting kutnahorites. The occupancieswere not constrained by in 322 and 319 reflections for the BK and SH kutnaho- the analytical data, and the only restriction was that the rites, respectively. Absorption corrections were carried total site occupanciesare 1.0. The weighting schemeof out using a modified version of a program written by Cruickshank (1965) was used. All refinements were car- Burnham (1962a). Lattice parameters were determined ried out assumingspace group R3 for the ordered struc- by least-squaresrefinement of powder-diffractometer data, ture. No correction for extinction was utilized because obtained using CuI! with Si as an internal standard and examination of the structure factors implied that there utilizing the program rcrsq (Burnham, 1962b). The re- was no significant extinction factor. Refinement con- fined values area: 4.8732(8)and c : 16.349(6)A for verged after refinement of anisotropic temperature fac- BK and a: 4.894(l) and c: 16.50(2)A for SH (Table tors and occupancyfactors for both cation sites.Including Jt. unobserved reflections, the final R factors are 3.90/ofor Before proceedingwith measurementsof intensity val- BK and 4.0o/ofor SH kutnahorite. Final structure factors ues, single-crystalX-ray scanswere carefully made at co- are listed in Table 4,r structure parameters in Table 5, ordinates correspondingto ordering reflections.Scans for data from thermal ellipsoids in Table 6, and selecteddis- the Bald Knob specimen showed no observableintensi- tancesand anglesin Table 7. Distancesand angleswere ties for any ordering reflections except a barely observ- calculatedusing the program onrre (Busing et al., 1964), able peak for (107). This confirmed the single-crystalfilm observations for which no ordering reflections were ob- ' Table 4, order Document AM-87-332 served.By contrast, reflections To obtain a copy of severalordering were ob- from the BusinessOfrce, Mineralogical Society of America, 1625 served to have weak-to-medium intensities for the SH I Street,N.W., Suite 414, Washington,D.C. 20006,U.S.A. Please crystal. Thesedata imply that the BK samplehas a highly remit $5.00 in advance for the microfiche. 322 PEACOR ET AL.: CATION ORDER-DISORDER IN KUTNAHORITE

TneLr 5. Structure parameters for Bald Knob (upper lines)and Sterling Hill (lower lines) kutnahorite

A 0 0 0 0 0 0 B 0 0 Y2 0 0 Y2 0 0 0.2495(2) 0 0 0.2461(2) o 0.2634(3) 0.9995(4) 0.2496(2) 0.2s68(3) 0.98s0(4) o.2472(1)

a R P33 P23 A 0.0115(3) 0.0115 0.0010(1 ) 0.0057 0 0 0.0120(2) 0.0120 0.0011(1) 0.0060 0 0 B 0.0012(3) 0.0012 0.0010(1 ) 0.0056 0 0.0108(2) 0.0108 0.0011(1) 0.0054 0 0 0.0122(9) 0.0122 0.0011(2) 0.0061 0 0 0.0124(1 0) 0.0124 0.0013(1) 0.0062 0 0 U 0.0131(s) 0.0272(61 0.0019(1) 0.0125(4) - 0.0001(1) - 0.0003(1) 0.0122(6) o.0244(71 0.0017(1 ) 0.0111(6) - 0.0004(1) - 0.0007(1)

standard errors in lattice parameters,and the full vari- positions Ca"on,o.,Mno ,, and Caos3(4)Mno o, for the A and B ance-covariancematrix of the structure refinement. sites, respectively.The high occupancyvalue ofCa rela- tive to Mn in the B site may in part be attributed to DrscussroN ordering of the small amount of Mg in the site in that the Site occupancies form factor of Mg will be accommodatedby that of Ca. The results for the BK and SH kutnahorites are very Refined occupancies for the SH kutnahorite are different, as anticipated from the absenceand presenceof Caoro,o,Mno ,u and Caorr,r,Mrto, for the A and B sites, re- ordering reflections in single-crystalphotographs of the spectively, which correspond to the composition respectivesamples. The BK kutnahorite is almost com- Ca"rrMnoorCOr, neglecting the small contributions of Mg pletely disordered in Ca-Mn distribution; the SH kutna- and Fe. By comparison, the composition implied by the horite is largely ordered. lattice parametersis CaorrMnoor,assuming an ideal com- The disorder of BK kutnahorite is clearly indicated by position in the CaCO.-MnCO, binary and Vegard's law. the near-equivalenceof structure parametersfor both the This also is in fair agreementwith the composition as A and B cation sites (Tables 5-7). The averageM-O dis- determined by electron-microprobeanalysis. tances,for example,are 2.277(2) and 2.267(2)A, respec- The Ca-O and Mn-O distancesdetermined by Eflen- tively. Final refined occupanciescorrespond to site com- bergeret al. ( 19 8 I ) for calcite and rhodochrosite are2.362 and2.l90 A, respectively.Assuming the site occupancies given above, linear interpolations between these dis- TABLE6. Thermal€llipsoiddata for BaldKnob (upper lines) and SterlingHill (lower lines) kutnahorite Tner-e7. Selectedinteratomic distances (A) and an- Amplitude Angle to Angle to glesfor BaldKnob and Sterling Hill kutnahorite (A) a'f) Angle to b. (') c- f) Bald Knob SterlingHill 1 0.102(1 ) 90 0.104(1 ) 90 A polyhedron 3 0.118(2) 90 90 0 Ca-O 2.277(2) 2.351(2) 0.'t22(2) 90 90 0 o-o' 3.1s2(3) 3.244(31 3.404(3) 1 o-o' 3.288(3) 0.100(1 ) 90 87.59.(9) 87.76'(8) 0.09s(1 90 O-Ca-O' ) O-Ca-O" 92.41.(9) 92.74"(8) 3 0.118(2) 90 90 0 QE 1.001 7(5) 1.0022(3) 0.124(21 90 90 0 polyhedron 1 0.105(4) 90 B Mn-O 2.267(2) 2.217(2) 0.106(3) 90 o-o' 3.146(3) 3.074(3) 3 0.124(8) on 90 0 3.264(3) 3.196(3) on o-o' 0.137(9) 90 0 O-Mn-O' 87.90.(e) 87.76(7) 1 0.094(2) 176(1 ) e4(21 el(2) O-Mn-O" e2.10.(e) 92.24(8) 0.093(3) 174(2) 96(2) 8s(2) QE 1.001 3(1 2) 1.001 5(1 0) 2 0.1s4(2) 93(2) 36(11) 126(121 Carbon polyhedron 0.143(2) e5(2) 36(7) 126(7) c-o 1.285(1 ) 1.2s6(1) 3 0.162(2) s3(2) 54(11) 36(12) o-o 2.225(21 2.244(2) 0.160(2) e2(2) 54(7) 36(7) o-c-o 120.0e(1) 119.9812) PEACORET AL.: CATION ORDER-DISORDERIN KUTNAHORITE 323 tances result in predictions of M-O distances of 2.334 tion while accommodating the large difference in iomc and2.236 A, which are different than the observedvalues radiusof Ca and Mg. Reeder(1983) has emphasizedthis of 2.351 and 2.217 A. If such a linear interpolation is coupling of the steric effectsof the A and B sites; both valid, this implies either that the degreeof order is higher the absoluteand relative ionic radii are significant in sta- than indicated by the site occupancies,or perhaps that bilization of a dolomite-type structure. some stressfactor prevents their adjustment to ideal val- Attempts to explain the lack of a dolomite-type ues. structure for CaFe(COr), compared to CaMg(COr)r, CaMn(COr)r, CaZn(CO')r, and CdMg(COr), and to ex- Stereochemistryvs. ordering plain their relative stabilities on the basis offactors such Insofar as the SH and BK kutnahorites are the sub- as ionic radii or octahedral site-preferenceenergies have stantially ordered and disordered equivalents of nearly not been successful.The coupling of dimensions of the A stoichiometric kutnahorite, they provide a unique op- and B sites through a common oxygen atom may be the portunity to compare structure parametersin dolomite- critical factor (combined with site-preferenceenergies for type structures.The most striking comparison of parain- certain transition-element ions), but we can frnd no sim- eters is in the thermal ellipsoid data, which are nearly ple function for correlating such parameters. identical for both structures (Table 6). This implies that Lacking a more appropriate function, the parameter the values for BK kutnahorite do not include a contri- QE appearsto be the best available one for evaluating bution due to positional disorder. Even though different the relative stability of a dolomite-type structure.For BK individual oxygen sites must be associatedwith different kutnahorite, the QE values (Table 7) for the A and B sites speciesin A and B sites,the positions of all oxygenatoms are only slightly different than the averagevalue (1.0014) must be virtually identical. This implies a lack of short- for rhodochrositeand calcite,as expectedfor a disordered range order below the level ofdetectability by X-ray dif- structure (Reeder, 1983). The QE value for the A site of fraction for BK kutnahorite, as also concluded from reu the ordered SH kutnahorite is much greaterthan the av- observations.Reeder (1983) tabulated rms amplitudesof eragesof the calcite-type structures (Table 7). Thus, the thermal ellipsoids for available refinements of dolomite CO, group rotation results in interatomic distancesap- (Althofl 1977; Reederand Wenk, 1983; Reeder,1983) proximately consistent with ionic radii, but at the price and (Beran and Zemann, 1977). These values of significant A-site octahedral distortion. Although the are all smaller than those for BK and SH kutnahorites, distortions in kutnahorite are not significant enough to including those for vibration parallel to c. Similarly, causeinstability of the structure, they may be important Reederlisted values for severalcalcite-type structures for in constraining the order-disorder transition to low tem- refinementscarried out by Althoff (1977) and Effenberger peratures. The B-O distances in SH kutnahorite are et al. (1981). They are also smaller than those for kut- slightly larger and A-O distancessmaller than those pre- nahorite, although those for calcite are almost as large. dicted from radii, and site occupanciesmay possibly be We have no explanation for the disparity. We re-empha- due to the coupling ofdistortions in the A and B sites. size,however, that the samefactors must have equivalent Reeder (1983) and Reeder and Wenk (1983) have effectsin both the ordered and disordered structures. pointed out that the C atom is displaced from the plane The octahedrain calcite- and dolomite-type structures of three coordinating oxygen atoms in dolomite-type are trigonally distorted. Several authors (Althofl 1977; structures.The displacementis in responseto differences Beran and Znmann, 1977; Rosenbergand Foit, 1979; Ef- between the ordered cations in layers above and below fenbergeret al., l98l; Reeder,1983) have discussedthe the carbonate groups. In BK kutnahorite, this displace- significanceofthe distortion using the quadratic elonga- ment is nearly zero (0.002 A), but in SH kutnahorite, it tion (QE) parameterof Robinson et al. (1971). It is a is 0.016 A. It therefore provides a significant measureof function of rotation of the CO, groups about c, resulting the degreeof cation order as in the caseof dolomite. in rotation in an opposite senseofthe vertices ofboth A The original powder pattern of kutnahorite (Frondel and B octahedra. The rotation results in an increase in and Bauer, 1955) was incorrectly indexed as to ordering the A-O distanceand a decreasein the B-O distance. reflections. The scattering factors of Ca and Mn are so Reeder (1983) has plotted QE versus mean A-O and similar that such reflectionsare much weaker than in do- B-O distancesfor both calcite- and dolomite-type struc- lomite where they are a function of the large diference tures. In generalthere is an increasein QE with distance in scatteringpower of Ca and Mg (Goldsmith, 1983). If (and therefore with increasein ionic radius) for calcite- these reflections could be identified in powder patterns, type structures,where no CO, rotation is possibleand all the degreeof ordering would be much more easily deter- octahedraare equivalent. However, rotation in dolomite mined than through single-crystalpatterns. We therefore causesa nonequivalencein both QE and mean bond dis- obtained a diffractometer scanusing a portion of the Ster- tance for the A and B sites.QE for the B site (1.0008- ling Hill sample, maximizing conditions for observation 1.0009)is only slightly lessthan for magnesite(1.0009- ofweak reflections, but no ordering reflections could be 1.0010),but that for the A site (1.0016)is significantly detected. This implies that powder-diffraction data should less than that for calcite (1.0020). Rotation of the car- not be used to infer ordering in nearly pure Ca-Mn car- bonate groups thus results in smaller octahedral distor- bonates unless special care is used, as is the case with 324 PEACOR ET AL.: CATION ORDER-DISORDER IN KUTNAHORITE

onstratesthat the kinetics ofthe ordering processare slug- gish even for the geologictimes attendant with slow cool- ing of a regionalmetamorphic terrane (Winter et al., l98l).

Compositionsof natural Mn carbonatesin the GoldsmithI G.of (1957) systemCaCO.-MnCO, l possible pairs / de CopitoniI Peters (1981) Observationof solid solutionsand soh'us -.rr.rl i of natural Mn carbonatesprovides insight into their phase '-"i equilibria. Data on natural carbonatesclose to the Ca- CO.-MnCO, join give some constraints on solvi along this join. Using estimated equilibration temperatures,de Capitaniand Peters(1981) projected the compositionsof I AI , natural carbonatesonto a CaCOr-MnCO. binary in com- parison with experimentson the kutnahorite-rhodochro- 50 Co CO3 Mo| "/" lMnCO3 site (KR) solvus. Their figure has been expanded to in- (1981), Reinecke(1983), Fig.1. Manganoancarbonateswith less than 10molToMgCO, clude data from Winter et al. andFeCO3 projected onto the CaCOr-MnCO, binary join modi- and this paper (Fig. l). The compositions of the natural fied from de Capitaniand Peters(1981). SN : Serrodo Navio, carbonatesare generallyconsistent with the experiments MP : MurettoPass, B : Buritirama,A : Alagna,S : Scerscen. of Goldsmith and Graf (1957) and de Capitani and Peters Additionaldata are BK (BaldKnob, North Carolina;Winter et (1981) on the KR solvus (Fig. 1). However, its location al., 1981),AI (AndrosIsland, Greece; Reinecke, 1983), and SH is constrained only as a maximum becauseof the unre- (SterlingHill, N.J.;this work).Solid lines are the inferredsolvi versed nature of their experiments. Successfulexperi- of Goldsmithand Graf (1957)and of de Capitaniand Peters ments involving homogeneoussolid solutions with equi- (1981), possible and dashedlines are solvi consistentwith ex- librium distributions of cations and initially located within perimentaland natural data. the I(R solvus are neededto reverseits location (Essene, 1983). Attempts have yielded no detectablereaction to Rietveld-type refinements. Single-crystalpatterns, how- form two-phasematerials (Goldsmith and Graf, 1957;de ever, clearly show ordering reflections. Capitani and Peters,1981). Experiments would have to involve kutnahorites with the equilibrium ordering. For Nomenclature a complete understandingof this systemby analogy with In describing kutnahorite we have faced a seemingly CaCOr-MgCO, (Burton and Kikuchi, 1984),the temper- trivial, yet complicatingaspect of nomenclature.The name ature of order-disorder transition of kutnahorite in rela- kutnahorite was clearly meant to be applied to the or- tion to the equilibrium solvi and tricritical points needs dered phase with ideal composition CaMn(CO.), (Fron- to be determined for different CalMn ratios. del and Bauer, 1955). The probability that the type ma- Few two-phase pairs of carbonatesnear the join Ca- terial was ordered is high, even though ordering reflections CO,-MnCO, have been reported. Some analytical data were erroneouslyidentified. The name kutnahorite should are available on coexisting Mn carbonatesthat are close not be applicable to disordered phases, sensu stricto. to the CaCOr-MnCO, join: kutnahorite-rhodochrosite However, we recommend its use for all phases near (KR) from blueschists at Andros Island, Greece (Rei- CaMn(CO.), in composition, with adjectives such as necke, 1983),and calcite-kutnahorite (CK) from Sterling "disordered" or "calcian" specifying aspectsof structure Hill. From the appearanceof the sample 525 (Fig. 2l\), and chemistry as they becomedetermined. No confusion we tentatively infer that it formed by epitaxial growth seemsto result from such usage. rather than by exsolution. Sample 526 shows a more complex intergrowth of earlier euhedral calcite and later Irnplications of ordered vs. disorderedkutnahorite replacive (?) calcite in a calcian rhodochrosite host (Fig. The occurrenceofdisordered kutnahorite at Bald Knob 2B). The textures of the two-phase materials do not re- and ordered kutnahorite at Sterling Hill presumably re- semble those commonly observed for high-temperature stricts the order-disorder transition to a temperaturebe- calcitescontaining exsolveddolomite. However, an exso- tweenthose of the formation ofthese two carbonates.The lution or replacement origin for the Sterling Hill inter- formation of ordered kutnahorite in late, low-tempera- growths cannot at present be ruled out. In any case,this ture veins at Sterling Hill is inferred to indicate growth assemblageprovides direct evidence(Table l) for the ex- of an ordered phase within its stability field. Unfortu- istence of a low-temperature solvus between calcite and nately the equilibration temperature of the SH carbonate kutnahorite, but its shape and location remains ill-con- is uncertain. Our prejudice, based on estimates of late- strained. stage hydrothermal paragenesesat Sterling Hill would place the formation of the SH carbonate veins and the Compositionsof natural Mn carbonatesin the system order-disorder transition somewherein the range of 200 CaCOr-MnCOr-MgCO, to 400'C with the former at lower temperaturesthan the Before acceptingthat the compositions of natural car- latter. The survival of disordered BK kutnahorite dem- bonates in Figure I properly constrain the binary solvi, PEACOR ET AL.: CATION ORDER-DISORDER IN KUTNAHORITE 325

CoCO.

NilgCO3 Mn C03

Fig. 3. Manganoancarbonates with lessthan 5 molo/oFeCO, projected onto the CaCOr-MnCOr-MgCO, ternary in compari- son with the experimental solvi of Goldsmith and Graf (1960) at 600'C.References: Doelter (1912,1914,1917), Krieger (1930), Wayland (1942), Ham and Oakes (1944), Frondel and Bauer (1955), Goldsmith and Graf (1955), Lee (1955), Stankevich (1955),Graf and Lamar (1956),Muta (1957),Karado (1959), Van Tasseland Scheere(1960), Deer et al. (1962), Pavlishin and Slivko (1962),Sundius (1963), Saliya (1964),Trdlicka (196D, Gabrielsonand Sundius(1965), Pisa (1966), Sundius et al. (1966), Trdlicka and Sevcu(1968), Calvert and Price(1970), Wenk and Maurizio (1970),Della-Salla et al. (1973),Koritningetal. (1974), Shibuyaand Harada (1976),Kashima and Motomura (1977), Peterset al. (1977,1978,1980), Sanford (1978), Cortesogno et al. (1979), Suess(1979), Dabitzias(1980), Melone and Vurro (1980),Sivaprakash (1980), Bello et al. (1981),Fukuoka{1981), Greenet al. (1981),Tanida and Kitamura (1981, 1982),Winter et al. (1981), Zak and Povondra (1981), Pedersenand Price (1982), Iwafuchi et al. (1983),Nahon et al. (1983), Reinecke (1983). Point lying near center of three-phasetriangle is from Fig. 2. Backscattered-electronimages of manganoancalcite Nahon et al. (1983)and deservesfurther studv. (dark)intergrown in calciankutnahorite (light) from SterlingHill, N.J. (A) (SH-525)shows irregular "arrowheads" of manganoan calcitein kutnahorite(length ofphoto 335pm). Finedark lines Except for carbonates along the magnesite-calcite binary are cracksand cleavages.(B) (SH-526)shows hexagonal inter- join, most one-phase manganoan carbonates lie outside gowths andlate fillings (?) of manganoancalcite in coarse the 600'C isotherm consistent with lbrmation at or below kutnahorite(length of photo 720 pm). Spotswith dark circles that temperature (Fig. 3). However, if disordered kutna- arefrom surfacedust. horites grow metastably at low temperatures, it is also possible that phases with metastable intermediate com- it is important to consider the possible effects of addi- positions may form as authigenic or diagenetic minerals tional componentson the solvi. Compositionsof most in sediments (Suess,I 979; Pedersenand Price, I 98 2; Ber- natural manganoancarbonates are more completely rep- an et al., 1983; Boyle, 1983). No low-Fe carbonates are resented by the ternary system CaCOr-MnCOr-MgCO. known to occur as solid solutions between magnesite and becausethey commonly have 5 to l0 molo/osubstitution rhodochrosite (Essene, 1983, Fig. 3). A solvus could be of MgCO. (Fig. 3). The solid solution of FeCO. is gen- generated between magnesite and rhodochrosite by ex- erally much lessimportant; analyseswith > 5 molo/oFeCO, pansion of the experimental two-phase region between have been excluded from the plot. Despite the wide ter- calcian magnesite and calcian rhodochrosite at 600'C (Fig. nary solvi of Goldsmith and Graf (1960),few equilibrium 3) down to the MgCO.-MnCO. binary at low tempera- two- or three-phasemanganoan carbonate assemblages tures (Fig. 4A). Alternatively, it is possible that appro- have been reported. The Andros Island kutnahorite co- priate protoliths have not yet been found that would yield existing with rhodochrosite has significant MgCOr, these solid solutions (Essene, 1983). Permissive evidence whereasthe Sterling Hill kutnahorite coexistingwith cal- for a low-temperature magnesite-rhodochrosite solvus is cite, though low in MgCOr, varies erratically in CaCOr. provided by the experiments of Erenburg (1961), who 326 PEACORET AL.: CATION ORDER-DISORDERIN KUTNAHORITE

CoCO. bonates(Fig. aA) or formation of a three-phasefield (Fig. 4B). Shibuya and Harada (1976) and Zak and Povondra (1981) have described patchy-to-oscillatory variations betweenmanganoan ankerite and magnesiankutnahorite that they ascribe to disequilibrium processes.These car- bonates may bear upon possible solvi in the quaternary systemCaCOr-MnCOr-MgCOr-FeCOr, but no reversed experimentsare available for comparison in this system.

AcxNowr-nocMENTs We again thank W. B. Simmons, Jr., for introducing us to the Bald Knob manganesedeposit. C. Capobiancoand A. Navrotsky kindly pro- vided us with a preprint of their manuscript in advance of publication. C. B. Henderson,L. A. Marcotty, and W. C. Bigelow of the University of Michigan Microbeam Laboratory are acknowledged for technical as- ;' \,,"'.:j;) sistance.We are gateful to R. J. Reeder, W. B. Simmons, Jr., and an {iii?'I i \ \ anonymous reviewer for their insightful comments on our manuscript. We also acknowledgeNSF Grants EAR-8212764, 85R-83 and EAR- Mn CO. 8408168 in partial support ofthis research. CoCO3 RnpnnBNcBs Althoff, P.L. (197?) Structural refinementsofdolomite and a magnesian calcite and implications for dolomite formation in the marine environ- ment. American Mineralogist, 62, 77 2-7 83. Bello, R.M.S., Coutinho, J.M.V., Valarelli, J.W., Schultz, R.A., and Ya- mamoto, J.K. (1981) Dados odicionais sobre a mineralogia dos pro- tominerios de manganesde Serrado Navio-Amapa. AnnaesAcademia de Ciencias,Rio de Janeiro, 53,123-134. Beran, A., and Zemann, J. (1977) Refinement and comparison of the crystal structures of a dolomite and of an Fe-rich ankerite. Tschermaks Mineralogischeund PetrographischeMitteilungen, 24, 279-286. Beran, A., Faupl, P., and Hamilton, W. (1983) Die Manganschieferder Strubbergschichten(N6rdliche Kalkalpen, Osterreich)-eine diagene- tisch gepragte Mangankarbonatyererzung. Tschermaks Mineralogische und PetrographischeMitteilungen, 31, 175-192. Bolzenius,8.H., Farkas,L., and Will, G. (1985) Strukturuntersuchungan natiirlichem Kutnahorit (abs.).Fortschritte der Mineralogie, 63, 29. Boyle, E.A. (1983) Manganesecarbonate overgroMhs on foraminiferal Acta, 47, 1815-1819. Mg CO3 Mn COE tests.Geochimica et Cosmochimica Burnham, C.W. (1962a) rclse, crystallographic lattice-constant least- Fig. 4. Ternarydiagrams showing the compositionsof two- squaresrefinement program. CarnegieInstitution of Washington Year phase manganoancarbonates from Andros Island, Greece Book 61, 132-135. -(1962b) (Reinecke,1983), and from SterlingHill, N.J. (thiswork). Pos- Absorption corrections for prismatic crystals and evalu- American CrystallographicAssociation 1962 An' siblesolvi at 300 + l00qCare consistent with thesedata and the ation of end effect. nual Meeting Program and Abstracts, 19. experimentalsolvi of Goldsmithand Graf (1960)at higherZ. Burton, B., and Kikuchi, R. (1984) Thermodynamic analysisofthe sys- DiagramA showsno miscibilitygap between rhodochrosite and tem CaCO,-MgCO, in the tetrahedron approximation of the cluster magnesite;diagram B extendsthe ternarygap down to intersect variation method. American Mineralogist, 69, 165-175. this binaryjoin. Busing,W.R., Martin, K.D, and Lerry, H.A. (1964) A Fortran crystallo- graphic function and error program. U.S. National Technical Infor- mation ServiceORNL-TM-306. Calvert, S.8., and Price, N.B. (1970) Composition of manganesenodules precipitated two carbonate phaseswhen starting with a and manganesecarbonates from Loch Fyne, Scotland. Contributions to and Petrology, 29,215-233. high Mn/Mg ratio in aqueousexperiments on this binary. Capobianco, C., and Navrotsky, A. (1987) Solid solution thermody- The rare magnesianrhodochrosites (Fig. 3) need to be namics in CaCOr-MnCOr. American Mineralogist, 72, 312-3 18. examined carefully for coexistencewith or exsolution of Cortesogno,L., Lucchetti, G, and Penco, A M. (1979) Le mineralizza- manganoanmagnesite to test these two alternatives. We zioni a manganesenei Diaspri delle ofiolite Liguri: Mineralogia e ge- 151- have speculatedin Figures 4,{ and 48 as to possible re- nesi. Rendiconti SocietaItaliani di Mineralogia e Petrologia, 35, 197 lations of the 300'C isotherms in the ternary systemusing Cruickshank,D.W.J (1965) Errors in least-squaremethods. In J. S. Rol- the two-phasepairs from Andros Island and Sterling Hill lett, Ed. Computing methods in crystallography, p. 114. Pergamon Press, in combination with single-phasecarbonates and the ex- Oxford periments at higher temperatures.If equilibrium prevails, Dabitzias, S.G. (1980) Petrology and genesisof the Vavdos cryptocrys- Peninsula, Northem Greece. magnesiankutnahorite-rhodochrosite pair (Tsu- talline magnesite deposits, Chalhdiki a third EconomicGeology, 75, 1138-l l5l. sue, 1967) suggestseither strong compositional effectson de Capitani, C., and Peters,Tj. (1981) The solvus in the systemMnCOr the Ko for Mg-Mn distribution between these two car- CaCO3.Contributions to Mineralogy and Petrology,76,394400. PEACOR ET AL.: CATION ORDER-DISORDER IN KUTNAHORITE 327

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