American Mineralogist, Volume 67, pages 90-96, I9E2 A contribution to the crystal chemistry of ellestaditeand the silicate sulfate apatitest RolnNo C. RousB Departmentof GeologicalSciences University of Michigan Ann Arbor, Michigan 48109 euo PEtp J. DUNN Departmentof Mineral Sciences Smiths o nian Inst itution Washington,D. C. 20560 Abstract A seriesof calcium silicate sulfate apatitesfrom Crestmore,California, which contain the coupled substitutionSilvsvl for 2Pv, has been investigatedusing electron microprobe, powder diffraction, and single-crystal diffraction methods. Chemical analysis of eighteen specimensof differentphosphorus contents proves that the Si:S ratio is essentiallyI : I and yieldsthe idealizedgeneral formula Caro(SiOn):-*(SO4)3-^@O4)2.(OH,F,CD2,where x : 0 to 3. The membersof this seriesfor which x : 0 and 3l2 have beenlabelled "ellestadite" and "wilkeite", respectively, by previous workers. "Ellestadite" is actually a solid solutioninvolving the end-membersCale(SiO a,){SOq)tZz, where Z: OH (hydroxylellesta- dite), F (fluorellestadite),or Cl (chlorellestadite).The term ellestaditeis redefinedto make it a group name for all compositions having >(Si,S) > P. Wilkeite is not a valid mineral species,since it is only one of many solid solutions involving the six end-members fluorapatite,hydroxyapatite, chlorapatite, fluorellestadite, hydroxylellestadite, and chlor- ellestadite. Although natural hydroxylellestadite is monoclinic, precession photographs of type "ellestadite" and "wilkeite" show hexagonalsymmetry and no evidenceof Si-S ordering as suggestedby the Si: S ratio of I : I . The silicate sulfate apatites from Crestmore show a strong linear relationshipbetween their P and F contents,such that these two variables simultaneouslygo to zero. Linear relationshipsalso exist betweentheir unit cell parame- ters and their P, F, and (Si+S) contents.These correlations imply a convergenceof the Crestmore apatite series towards a hypothetical member-of composition Caro(SiOa)r (SO4)3(OH,CI)zand cell constantsa:9.543 and c = 6.9174. Introduction ellestadite,conventionally formulated as Ca5(SiOa, Crystal structures of the apatite type, general SO4,PO4)3(OH,F,Cl)and Ca5(SiO4,SO4)3(OH,F,CI), formula Ato(XO)622, &r€ rot?ble for their ability to respectively.Since these phases are of someindustri- (Pliego-Cuervo accommodatea greatvariety of atomsin their A, X, al and Glasser, 1978; Trivino Yaz- and Z sites by means of simple or coupled ionic quez, 1979)as well as crystal chemical and mineral- substitutions (Dihn and Klement, 19421'Schwarz, ogicalinterest and since very little modern analytical 1967a,b, c; Cockbain, 1968;Ito, 1968;Kreidler and data exists for either the natural minerals or their Hummel, 1970;McConnell, 1973).An example of syntheticanalogues, we have undertakena re-exami- the latter mechanismis the substitutionSirvsvr - nation of ellestadite and wilkeite using type and 2Pv, which occurs in the minerals wilkeite and cotype materialsfrom Crestmore,California. In this report we present new analytical chemical, powder difraction, and single-crystaldiffraction data, which tContribution No. 370,The MineralogicalLaboratory, Depart- bear upon the crystal chemistry of the silicate sulfate mentof GeologicalSciences, The University of Michigan. apatitesand the interpretationsmade of it by previous 0003-0M)vE2l0I 02-0090$02.00 ROUSEAND DUNN: ELLESTADITE investigators.Nomenclatural changes were approved The term "hydroxylellestadite" was introduced prior to publication by the Commission on New by Harada et al. (1971)to denote an impure form of Mineralsand Mineral Names,I.M.A. Cals(SiOa)3(SO4)3(OH)2,which occurs at Doshin- kubo in the Chichibu mining district of Japan.The Historical review new mineral was found to be crystallographically The replacementof a substantialportion (-50%) and chemically similar to ellestadite, except for the of the POaby (SiO4,SO4)in a new mineral of the fact that OH > (F + Cl), and the spacegroup was apatite group was reported by Eakle and Rogers assignedas P63lmon the basisof precessionphoto- (1914) in their description of wilkeite from Crest- graphs.More recently,however, Sudarsanan (1980) more. This was the first example of a coupled has reportednatural hydroxylellestaditeto be mono- atomic substitution in an apatite structure to be clinic-pseudohexagonalwith spacegroup PZ/m and,a : recognizedas such. Additional data on wilkeite 9.476(2),b : 9.508(2),c : 6.919(1)4,and 1 = were given by Rogers (1929), McConnell 093j. 119.53(2).The monocliniccell is not of the sametype 1938),and Taylor (1953),who showedthe supposed as that found in monoclinic phosphate apatites, mineralcrestmoreite to be a submicroscopicinter- sincethe latter is a supercell having b : 2a &nd 1 : growth of wilkeite and the calcium silicate hydrate, 120.0'(Young, 1975).Whether all hydroxylellesta- tobermorite.More recently, wilkeite has been re- dites are monoclinic remains an open question. ported from Timna', Israel (Wtirzberger, 1970), Sudarsanan'sresults were obtainedfrom one crys- Takiue,Japan (Harada et al.,l97l), and the Adiron- tal and it is possiblethat,like hydroxyapatite,some dack Mountains of New York (J. W. Valley, per- crystals are monoclinic and others hexagonal. sonal communication).Apatites containing a few Trivino Yazquez (1979)reported ellestadite as a percentSiO2 and SO3occur at other localities,e.g., constitutent of the incrustations formed in cement the Rhineland (Brauns, 1916) and the Ural and heat-exchangecyclones. He further stated that the Aldan regionsof the Soviet Union (Vasileva,1958), thermaldecomposition of ellestaditein the presence but these are better termed silicatian sulfatian apa- of potassiumyields a "K ellestadite" of probable tites rather than wilkeite. In fact, as we shall later formula Ca1sKSi3S2O22F.The existence of this show, the name wilkeite is itself superfluousand compoundremains unproven as of this writing. shouldbe discarded. Ellestaditewas also first describedfrom Crest- Experimental more by McConnell (1937),who distinguishedit Chemicalanalysis of eighteensilicate sulfate apa- from wilkeite chiefly on the basis of chemical tites including type ellestaditeand cotype wilkeite composition. Whereas type wilkeite had 20.85% are presentedin Table 1. All analyseswere per- P2O5,McConnell found a specimenwith only 3.06% formed on an ARL-seuqelectron microprobe using P2O5and called it ellestadite. No difference in an operating voltage of 15 kV, a beam current of paragenesisor spacegroup symmetry was evident 0.15 pA, and the following standards:hornblende for ellestadite and the mineral is visually indistin- for Si; fluorapatitefor P, Ca, and F; scapolitefor Cl; guishablefrom wilkeite. The crystal chemistry of and celestinefor S. The data were conected using ellestadite,wilkeite, and intermediatecompositions Bence-Albeefactors. The low summationsin Table was further explored by McConnell (1938).Dihn I are due mostly to the fact that CO2 and H2O were and Klement (1942) synthesizedthe pure com- not determined. Published values for these two pounds Caro(SiO+)r(SO4)3F2and Caro(SiO+): componentsare 2.l0VoCO2 and a trace of H2O in (SO4)3(OH)2,which we shall call fluorellestadite wilkeite (Eakle and Rogers, l9l4) and 0.61% COz and hydroxylellestadite,respectively, along with a and 0.63VoH2O in ellestadite(McConnell, 1937). numberof other exotic apatiteisotypes containing Although McConnell reported minor quantitiesof PO4,SiO4, and SOain variousproportions, some of MgO, MnO, Al2O3 and Fe2O3(<0.5Vo each with which deviated from the ideal A19(XO)622 stoichi- the Fe and Al attributed to admixed vesuvianite) in ometry. Pliego-Cuervoand Glasser(1978) synthe- ellestadite,our specimensshow tracesof Mn as the sized a third ellestadite-like phase, Cals(SiOa)3 only other detectable element of atomic number (SO4)3C12,which we shall call chlorellestadite.Fur- greater than nine. ther developmentsalong these lines appearin pa- Single-crystal fragments of type ellestadite pers by Schwarz (1967a, b, c), Ito (1968), and (NtrrNn #103072) and cotype wilkeite (NuNH Kreidler and Hummel (1970). #95685)were examinedby the precessionmethod. 92 ROUSE AND DUNN: ELLESTADITE Table l. Electron microprobe analysesof silicate sulfateapatites NBS silicon (a : 5.43088A)internal standard.The from Crestmore, California d-values represent a considerable improvement over those previously published for ellestadite and NTII{H# si02 Cao 50: PzOS F Cl o=F,Cl Total wilkeite. In addition, unit cell parametersfor five of Bl72l9 3.8 55.5 4.2 33.1 2.6 0.5 1.2 98.5 the chemically analyzed specimens(but not for the R5256 4.1 55.5 5.r 33.0 2.8 0.5 1.3 99.7 9s68s 9.4 55.2 ll.7 r8.2 1.6 0.8 0.8 96.r samecrystals which had beenanalyzed) were calcu- 94033-3 9.6 55.9 12.1 l8.l t.3 0.8 0.7 97.7 94463 I 0. I 55.8 13.9 15.4 I .3 0.7 0.7 97-5 lated from the powder data using a silicon internal 94033-2 10.8 55.5 14.0 I 5.6 1.2 0.7 0.7 97.1 standard. These were refined by the method of 93417 10.8 54.7 ',14.5I 4.3 I 5.0 I .3 0.7 0.5 96.2 The 94415 11.4'n 55.2 14.2 I .l 0.8 0.5 96.6 least-squaresassuming hexagonal symmetry. c40s0 .I 55.I I 4.9 13.8 1.2 0.7 0.7 96.1 R1874? t1.5 s5.0 14.7 13.4 l.l 0.8 0.6 95.9 results appear in Table 3. 94033-1 12.9 55.2 17.3 9.9 0.8 0.8 0.5 96.4 c4105 t 2.8 54,7 16.5 9.8 0.9 0.8 0.5 95.1 87281 t4.l 55.8 17.1 8.8 0.7 1.0 0.6 97-5 Resultsand discussion R9217-l 15.5 55.7 20.4 4.7 0.4 0.4 0.3 96.8 R92',13 16.0 55.4 21.0 4.3 0.4 0.5 0.3 97.3 Chemicalcomposition R9217-3 16.2 55.3 19.9 3.8 0.3 0.5 0.2 95.8 R92l5 16.2 56.3 20.9 3.8 0,4 0.5 0.3 97.8 formula assigned to ellestadite in I 03072 '17.4iI 5.8 55.7 21.2 3.2 0.3 1.7 0.5 98.4 The chemical Theoryf 54.12 23.18 6.84 t.55 I 00.0 Theoryt+ 17,98 55.90 23.94 3.78 1.60 I 00.0 most referencebooks is Cas(SiO+,SO4)3(OH,F,CI)or Theoryt+f 18.05 56.12 24 04 I 00.0* somevariant thereof.