American Mineralogist, Volume 73, pages 1172-1 178, 1988

End-membervillyaellenite from Mapimi, Durango, Mexico: Descriptive , , and implications for the ordering of Mn and Ca in type villyaellenite ANrrrorw R. K,lnrpr' Mineralogy Section,Natural History Museum of Los AngelesCounty, 900 Exposition Blvd., Los Angeles,California 90007, U.S.A. CH,c.RLnsR. Ross II* Department of Earth and SpaceSciences, University of California at Los Angeles,Los Angeles,California 90024, U.S.A. AssrRAcr The secondoccurrence of villyaellenite, at the Ojuela mine, Mapimi, Durango, Mexico, has yielded the nearly pure end member of the species.The mineral occurs as a compact spray of orange-pinkprismatic crystalsup to 4 cm in length, associatedwith ogdensburgite, arseniosiderite,adamite, and chalcophanite. Crystals are elongate parallel to [001] with forms {100}, {l l0}, and {101}. There is a good {100} ,and the hardnessis about 4. Optically, it is biaxial (-) with refractiveindices a: 1.713(2),p : t.723(2),and,y : 1.729(2);2V^.u":70(5)'and2V-u:75"; X:b, Z A c:40". Absorptionis Z >> X > Y (X : very pale orange-pink, I: exceedinglypale orange-pink, Z : pale orange-pink). The specificgraviry is 3.69, and the density (calc.) is 3.72 g/cm3. Chemical analysis by electron microprobe yielded CaO 0.5, MnO 36.2, FeO 0.1, ZnO 2.9, and AsrO' 50.6, and moisture evolution analysisgave 9.9 HrO, total 100.2 wto/0.The empirical formula basedupon 20 oxygensis

(MnourZno rrCao.orFeo o,)r, oo(HrO)o oo(AsOrOH), nr(AsO)r.., (Z: 4), close to pure Mn,(HrO)o(AsOrOH)r(AsOo)r. Single-crystal X-ray precession data indicate space groups Cc or C2/c; the latter is confirmed by structure analysis.Cell parametersdetermined during crystal-structureanal- ysisarea:18.015(5),b:9.261(2),c:9.770(3)A,p:96.238(7),V:t620.3A,.The strongestlines in the X-ray powder-diffraction pattern (d in A, I, hkl) are 8.96(50X200), 8.21(100X1l0),3.243(80X5ll),3.209(70)(222),3.068(70X222,srr),2.978(50)(600), and r.678(60)(642). The refractive indices, specificgravity, chemistry, and cell parametersdiffer markedly from those of type villyaellenite from Sainte-Marie aux Mines, France, which is near the midpoint of the villyaellenite-sainfelditeseries. A third occuffenceof villyaellenite at Ster- ling Hill, New Jersey,consists of material nearly identical chemically to the villyaellenite from Mapimi. Villyaellenite is isostructural with sainfeldite and hureaulite. The crystal structure of the near end member from Mapimi has been refined by the method of least-squareson the basis of I183 reflections measuredon an automated single-crystaldiffractometer to R : 0.076 (R* : 0.089). The averageMn-O bond lengthsfor the three nonequivalent octahedra that make up its five-member edge-sharingchain fragments are Mn(lfO : 2.21, Mn(21 O : 2.14, and Mn(3)-O : 2.24 A. Distance least-squaresrefinements of the type villyael- lenite structure[Mn/(Mn + Ca) : 0.565]yield M(IFO : 2.27,M(2FO : 2.20,and M(31 O:2.36 A, suggestingthe orderingscheme M(l) : Mn * Ca, M(2) : Mn, and M(3) : Ca. The abnormally large a cell parameter in type villyaellenite relative to the Mn and Ca end membersof the seriesand its structural and parageneticrelationships to fluckite further suggestthat Mn and Ca are at least partially ordered in type villyaellenite.

INrnooucrroN this locality. Type sainfeldite is the pure-Ca end member Villyaellenite (Mn,Ca)r(HrO)o(AsOrOH)r(AsOo),,was of the series,whereas type villyaellenite has an average first describedby Sarp (1984) from Sainie-Mari'i aux cation_contentofMn:Ca:2.69:2.O7 [Mn/(Mn + Ca): Mines, France. tte noteOit to be the Mn-dominant ana- 0.5651. The second occurrence of villyaellenite, as the logue of sainfeldite, a mineral also first described from nearly pure end member, is describedhere from the Ojue- la mine, Mapimi, Durango, Mexico. * present The-crystal structuresof sainfeldite and hureaulite (the address:Bayerisches Forschungsinstitut liir Experi- - mentelleGeochemieundGeophysik,UniveisitetBayreuth,post-phosphate analogue of villyaellenite) have been deter- fach 10 12 51, 858Bayreuth, Federal Republic of Germany. mined by Ferraris and Abbona (1972) and Moore and 0003-004x/88/0910-1172$02.00 n12 KAMPF AND ROSS: END-MEMBER VILLYAELLENITE It73

Araki (1973), respectively.The Mn and Ca atoms in these minerals are located in three different octahedral sites. The octahedraare joined by sharing edgesto form a five- memberedchain fragment:M(2)-M(3)-M( l)-M(3)-M(2). Corner sharing between chain fragments and with arse- nate or phosphatetetrahedra yields an open three-dimen- sional framework. The five-member chain fragments in the sainfeldite- hureaulite structure type closelyresemble segments of the octahedral edge-sharing chain found in fluckite, Fig. 1. Crystaldrawing of villyaelleni te from Mapimi. CaMn(HrO)r(AsOrOH),, a mineral that is intimately as- sociated with villyaellenite at Sainte-Marie aux Mines. spray for most of their lengths. Most of the crystals are BecauseCa and Mn have been determined to be com- slightly curved. Some etching is in evidence, especially pletely ordered in fluckite (Catti et al., 1980), it was sus- toward the end of the spray.Where the spray extendsinto pected that they might also be ordered in type villyael- the small remaining open spaceof the vug, the euhedral lenite. form of some of the crystals has been preserved.These Unfortunately, the structure of type villyaellenite could crystals are simple prisms bounded by the forms {100} not be solved becauseof the small size and poor quality and { I l0} and terminatedby the pinacoid {l-01}. figure of its crystals. The determination of the structure of the I is a crystal drawing of villyaellenite from Mapimi. near end member from Mapimi was undertaken to ex- Villyaellenite from Mapimi has a good {100} cleavage amine the sizesof its octahedralsites and, in so doing, to and a hardnessofabout 4. It is orange-pink in color and investigatethe likelihood of Mn and Ca ordering in type exhibits a nearly white and a vitreous luster. It is villyaellenite. The results of this structure determination optically biaxial (-) with refractive indices (measuredin then also provided data for a distance least-squaresre- white light) a -- 1.7l3(2), : 1.723(2), and : 1.729 (2); finement of the structure of type villyaellenite. P 7 2V*.":70(5)" and 2V*n: 75".Moderate pleochroism Occunnrh{cr was observedwith absorption Z >> X > Y (X: very pale Villyaellenite has been identified on a single specimen orange-pink, I: exceedinglypale orange-pink,7: pale collectedin 1981from a sectionof the Ojuela mine known orange-pink).The optical orientation is X: b, Z A c: as the San Judas Department. The villyaellenite speci- 40.. men, provided for study by Mr. John Whitmire, also con- The specificgravity of villyaellenite from Mapimi, de- tains the secondoccurrence ofogdensburgite, already re- termined on a Berman balance,is 3.69. This value com- ported by Kampf and Dunn (1987). The villyaellenite pares favorable to the calculateddensity of 3.72 g/cm3. occurs as orange-pink, prismatic crystals up to 4 cm in length that form a compact spray mostly filling a vug. CHBursrnv The walls of the vug and some surfacesof this spray are Villyaellenite from Mapimi was chemically analyzed coated with alternating layers of cryptocrystalline arse- with an electron microprobe using the following stan- niosiderite and bladed ogdensburgite.An aureoleof mas- dards: garnet (Mn and Fe), wollastonite (Ca), ZnO (Zn), sive chalcophanite surrounds the vug and extends from and synthetic olivenite (As). In Table 1, the analysis of 1 to 3 cm into host rock, limonitic gossan. the a Subhe- Mapimi villyaellenite is compared to microprobe analy- dral to euhedral crystals and irregular stringers ofcolor- ses by Sarp (1984) for material from Sainte-Marie aux less to pale green adamite are imbedded in the limonite, Mines and by P. J. Dunn (pers. comm.) for villyaellenite chalcophanite,and layered arseniosiderite-ogdensburgite from the Sterling Hill mine. assemblage.This specimenis preservedin the collection Water was directly determined using a 903-H DuPont Natural History of the Museum of Los Angeles County moisture-evolution analyzer yielding a total of 9.9 wto/o, (LACMNH 25414). in closeagreement with the differencebetween 1000/oand Villyaellenite has also been found at the Sterling Hill the analytical sum from the microprobe analysis of ele- mine, Ogdensburg,New (P. Dunn, pers. Jersey J. comm.), ments with atomic number >9. as transparent orange-pink crystals in flattened, circular, The empirical formula for villyaellenite from Mapimi radial (7-mm diameter) aggregates.It is found on - (basedon 20 oxygenatoms) is (Mn ulnorrCa"orFeoo,)rroo- bearing willemite-franklinite ore and apparently repre- (HrO)ooo(AsOrOH),nr(AsOo)ror,which is very near the sentsa seam phases occurrence.Other unusual found in pure-Mn end member, Mn, (HrO)u (AsOrOH), (AsOo)r. the samepart of the Sterling Hill deposit include ogdens- burgite, sterlinghillite, wallkilldellite, and a Mn-bearing X-nrv CRYSTALLOGRAPHY adamite(Dunn, l98la, l98lb; Dunn and Peacor,1983). A single crystal of villyaellenite from Mapimi, studied Prrvsrc.ql, AND oPrIcAL PRopERTTES by precessionX-ray single-crystaltechniques, confirmed The villyaellenite crystals from Mapimi are elongate the monoclinic symmetry and indicated the spacegroups on [001] and are tightly intergrown in a slightly divergent Cc or C2/c. The latter was confirmed bv the crvstal struc- tt74 KAMPF AND ROSS: END-MEMBER VILLYAELLENITE

TneLe1. Microprobeanalyses of villyaellenite TeeLe3. Positionaland isotropicthermal (A'z) parameters for villyaellenitefrom Mapimi SterlingHill Ste.-Marie Mapimi" (Dunn,pers. auxMines vlb U\1U (Thisstudy) comm.) (Sarp,1984) Mn(l) o.s 0.3943(4) 0 75 171(14) FeO 0 1(0.1-0.1) 0.3 Mn(2) 0.31s0(1) 0.0874(21 0 1878(2) 205(12) CaO 0.5(0.4-0.6) 1.0 13.58 Mn(3) 0.3257(1) 0.4713(21 0.137s(2) 211(14) ZnO 29 (2.4-3.8) 2.8 As(l) 0.4168(1) 0.31s7(2) 0.4109(1) 201(9) MnO 36.2(34.9-37.1) 36.0 22.40 As(2) 0.6611(1) 0 2363(2) 0.623e(1) 192(9) AsrO. 50.6(48.9-51.9) CU,J 52.99 o(1) 0.4237(61 0.4360(11) 0.2892(10) 232(61) H.o 9.9." e.6t 11.42+ o(2) 0.3389(7) 0.2240(12) 0.3631(11) 271(651 Total 100.2 100.0 100.39 o(3) 0.4150(6) 0.3915(13) 0.5657(11) 297(67) . o(4) 0.6648(6) 0.2574(11) 0.4528(9) 1e6(se) Average of five points over several crystals; ranges in parentheses. *. o(s) 0.7069(6) 0.0874(11) 0.6788(10) 239(63) Water determinedby moisture-evolutionanalysis. o(6) 0 7040(6) 0.3774(121 0.7046(11) 265{65) t Water by difference. o(7) 0.5707(6) 0.2276(121 0.6557(11) 266(64) + Water by rcA. OH 0.4897(7) 0.1981(13) 0.4226(11) 322(71) ow(1) 0.2416(6) 0.4219(12\ 0.4702(11) 272(67) ow(2) 0.5835(7) 0.0053(12) 0.8397(11) 343(74) ture analysis. The cell parametersrefined during crystal Note; Standard deviationsare in parentheses.U* : [1/(61)]>>AFF:. structureanalysis are a: 18.015(5),b:9.261(2), c: 9.770(3)A, B: eA.ZZt(7)",and V: t620.3A'. The X-ray powder diffraction data for d values greater than 1.6 A adequatequality. This euhedral fragment measured0. l7 given are for the Mapimi material in Table 2. x 0.19 x 0.24mm. Sarp( I 9 84) reported the following cell parametersmea- The fragment was mounted on a Huber automated dif- suredfrom precessionphotographs: a: 18.55,b:9.52, fractometer employing monochromatized MoKa radia- c: 10.01A, and A:97".In the courseof the present tion. A total of ll83 independent reflections was mea- parameters study these were refined from the powder dif- sured to a 20 limit of 110' using the 0/20 scan method. fraction data reported by Sarp by the method of least- The data were collectedfor Lorentz and polarization fac- squares.The I 9 observedd valueswith intensitiesgreater tors and for absorption. than l0 were utilized in the refinement, yielding the cell The space group C2/c was chosen by analogy to sain- : : : parametersa 18.515(37),b 9.484(9),c 10.000(7) feldite, and the atomic positions reported for sainfeldite p:96.72(7)'. A,and were utilized to initiate the structure refinement.All com- putations were done with the UCLA crystallographic Cnysral STRUCTURE computing package.All of the observed structure factors Data collection and structure refinement (F) were considered nonzero since all exceeded3"(F"). The villyaellenite crystals from Mapimi exhibit pro- The structure factors were assignedweights based upon nounced mosaic spread.Most fragments examined proved their standard deviations. Scattering factors for neutral to be unsatisfactoryfor intensity-data collection. The ter- atoms and anomalous dispersion coefficientswere taken mination of one narrow prism was determined to be of from International Tables for X-ray Crystallography

TneLe2. X-ray powder-diffractiondata for villyaellenitefrom Mapimi

d (A). ilh- hkr d (A)-- d(A). d(A).. 8.954 25 200 8.96 2.675 27 331,602 2.674 8.226 53 110 8.21 2.653 023 6.438 14 1t1 6.41 2.631 8 512 6.128 11 111 6.13 2.605 IJ 331 2.610 4.856 002 4.84 2.485 9 621 2.487 4.632 25 311,020 4.63 2.428 14 004 2.431 4.477 34 202,400 4.45 2.377 621 2.382 4.300 20 311 4.30 2.335 33 711,530 2.337 4.113 220 4.11 2.298 8 712 2.302 3.857 10 221 3.851 2.238 24 800,404,i33 2.239 3.721 17 3.724 2.069 713 2.063 3.345 23 o22,312 3.345 2.043 11 404,333 2.054 3.262 45 511 3.243 2.016 8 424 2.019 3.219 100 222,420 3.214 1.970 6 730 1.990 3.128 30 421,402 3.131 1.774 8 334 1.778 3.064 71 222,511 3.063 1.747 a 534 1.752 2.973 44 113,600 2.978 1.677 20 931,352 1.678 2.841 1'l 313 1.640 I 643 1.641 2.785 o 422 1.624 22 135,206,733 1.630 2742 o 330 1.609 10 i53,5s1 1.614 - Calculatedfrom structure data. The calculatedd value has been averagedfor multiple lines. '. Interplanarspacings measured from film, 114.6-mmGandolfi camera, Ni-filtered CuKc radiation KAMPF AND ROSS: END-MEMBER VILLYAELLENITE rt7 5

TABLE4. Anisotropicthermal parameters (A, x 104)for villyaellenitefrom Mapimi

uzz u8 Uo u8 Mn(1) 21O(20) 240(1s) 62 0 11(13) 0 Mn(2) 220(12) 220(12) 170(1 1) 14(8) 5(8) e(7) Mn(3) 218(14) 238(13) 1750 3) -6(s) 15(9) -21(s) As(1) 218(9) 236(9) 146(8) 7(7) 5(6) 0(6) As(2) 1s8(e) 226(s) 149(8) -1(6) 1(6) 4(6) o(1) 224(60l, 270(57) 1e8(ss) 38(49) 1(441 29(45) o(2) 297(671 3s4(64) 156(s3) 3(52) 3(47) -22(46) o(3) 187(64) 418(67) 263(59) 6(s3) -85(48) 4s(51) o(4) 244(62\ 275(s8) 73(47) -31(47) 28(42) -2(40) o(5) 272(671 239(s9) 1s3(52) -42(48) -26(46) 104(43) o(6) 207(641 317(61) 266(59) s3(s0) 2(47l. -134(49) o(7) 97(54) 396(66) 298(62) 26(50) -3(46) - 85(51) OH 257(68) 400(68) 294(64) 87(55) -43(51) -69(s3) ow(1) 245(64) 320(65) 236(58) 5(52) -39(47) 32(47) ow(2) 3s3(78) 343(67) 2s6(64) 11 (57) 54(5s) 55(52) Note.'Theanisotroipc thermal parametersare of the torm expf-2r2(Uulf a*2+ U22k2t2+ Usl2d2 + 2u12hka'U + 2ughla"d + 2U4k b-d)]. Standard deviationsare in oarentheses.

(1974). The octahedralsites were assumedto be occupied DrscussroN solely by Mn, but the occupanciesof these sites were Descriptive mineralogy refined,yielding Mn(l) : 0.97(l), Mn(2) : 1.19(l),and Mn(3) : 1.05(l). All other siteswere assignedunit oc- Villyaellenite from Mapimi is significantly superior in cupancres. crystal size and quality to that from Sainte-Marie aux The structurerefined to R : 0.076and R* : 0.089.The Mines. It is also much closerto the ideal Mn+nd member relatively high final R values were probably due, in part, composition. It differs markedly from type villyaellenite to the large mosaic spread. The positional and isotropic in its physical and optical properties and in its cell di- thermal parametersare listed in Table 3, and anisotropic mensions. Mapimi villyaellenite is remarkably similar temperature factors are given in Table 4. The observed chemically to villyaellenite from Sterling Hill. and calculated structure factors are listed in Table 5.' The variations in refractiveindices, cell parameters,and specificgravity relative to Mn/(Mn + Ca) for sainfeldite, DLS refinement of the type villyaellenite structure type villyaellenite, and Mapimi villyaellenite are shown A distance-least-squares(ors) refinement of the type in Figure 2.Ideal linear variations betweenthe end mem- villyaellenite structure was perfbrmed using the oprors bers are shown for comparisbn. The a cell constant and program of Dollase (unpub.). The refined cell parameters the specificgravity of type villyaellenite difer markedly for type villyaellenite reported above were employed, and from the values predicted by these linear variations. starting bond distanceswere obtained by averagingthose The Gladstone-Dale compatibility index (Mandarino, of sainfeldite and Mapimi villyaellenite. A total of 67 l98l) calculatedfor Mapimi villyaelleniteis -0.016. This bond lengths were prescribed and weighted as follows: value falls in the range indicating superior compatibility As-O : 1.0,M-O : 0.14,O-O (<3.2 A; : O.OZ,O-O of refractive index, specific gravity, and chemical com- (>3.2 A) : 0.04, M-M : 0.04. A total of 43 positional position. The compatibility index for sainfelditeis -0.013, parameterswere refined. The refinement convergedin three also in the superior range,whereas that for type villyael- cycles. lenite, -0.060, falls in the rangeofonly fair compatibility. A secondDLS refinement was performed assumingthe It is not clear whether the poorer compatibility seen in ordering schemeimplied by the above refinement.In this type villyaellenite is due to experimental error or to some refinement the initial bond distances were obtained as inherent anomalousGladstone-Dale behavior. However, follows: averagevalues (as determined above) were used it may be noted that, if the cell dimensions of type vil- for the M(1) site, the M(2) site was modeled after the lyaellenite did not deviate from the linear relationship Mn(2) site in Mapimi villyaellenite, and the M(3) site was describedabove, its calculated specific gravity would be modeledafter the Ca(3) site in sainfeldite.This refinement 3.33. This would yield a compatibility index of -0.018, also convergedin three cycles.The positional parameters in the range of superior compatibility. for both ors refinements are provided in Table 6. The octahedral metal-oxygen bond distances from the struc- ture refinement, and the prs refinementsare provided in Octahedral bond distancesin Mapirni villyaellenite Table 7. The averageMn4 distancesin Mapimi villyaellenite are Mn(lFO :2.21, Mn(2!O :2.14, and Mn(3FO : I To receive a copy of Table 5, order Document AM-88-388 2.24 L. The total octahedral-sitecontent provided by the from the BusinessOfrce, Mineralogical Societyof America,1625 I Street,N.W., Suite414, Washington, D.C. 20006,U.S.A. Please empirical formula is Mno urZnorrCa" orFeo o, [with one Mn( I ) remit $5.00 in advance for the microfiche. site and two eachofthe Mn(2) and Mn(3) sitesrepresented n76 KAMPF AND ROSS:END-MEMBER VILLYAELLENITE

TnaLe6. Distance-least-squarespositional parameters for typevillyaellenite

DLSnO. I os no.2 ylb xla ylb M(1) 1t) 0.3929 314 1t, 0.3913 314 M(2) 0.3158 0.0829 0.1846 0.3240 0.0755 0.1855 M(3) 0.3255 o.4707 0.1387 0.3233 0.4764 0.1367 As(1) 0 4156 0.3192 0.4118 o.4210 0.3172 0.4104 As(2) 0 6602 0.2370 0.6237 0.6629 o.2392 0.6248 o(1) 0 4245 0.4361 0.2824 0.4252 0.4398 0.2942 o(2) 0.3399 0.2286 0.3613 0.3514 0.2131 0.3530 o(3) 0.4146 0.3924 0.5636 0.4146 0.38s6 0.5627 o(4) 0.6634 o.2579 0.4565 0.6669 0.2551 0.4573 o(5) 07062 0.0903 0.6755 0.7029 0.0856 0.6737 o(6) 0.7030 0.3743 0.7045 0.7122 0.3710 0.7022 o(7) 0.5727 o.2284 0.6565 u,c/c I 0.2344 0.6565 OH 0.4859 0.2009 0.4238 0.4974 o.2142 0.4238 ow(1) 0.2414 0.4195 0.4691 o.2367 o.4125 0.4570 ow(2) 0 5805 0.01s4 0.8400 0.5802 0.0638 0.8072 in the formula unitl. The bond distancesand the refined lyaellenite, being significantly larger than the POo tetra- site occupanciessuggest that Zn is preferentially contained hedra in hureaulite, perhaps necessitateadjustments in in the Mn(2) site and Ca in the Mn(3) site, yielding Mn(l) the octahedral framework, yielding the Mn-O distances : Mn, Mn(2) : MnoroZno,u,Mn(3) : MnonuCaooo.(Note observed. that these site contents were not used in the structure oo refinement.) Although the crystal used for the structure o analysis was not chemically analyzed, microprobe anal- s I .:y.; o y> g o ysesshowed the variability of Zn and,Ca to be very lim- xg i> o ited. The point, found to be richest both in Zn and Ca, provided the total octahedral site content 'I lNlnoorZnoorCao,oFeoo,,yielding Mn(l) : Mn, Mn(2) : MnornZnorr,Mn(3) : MnorrCaoor.The amount of sub- stitution indicated in these sites is, therefore, relatively v(43) 1 small in comparison to the variation in averageMn4 distances. 1 The averageMn-O distancesin hureaulite (Moore and Araki, 1973),Mn(lfO : 2.188,Mn(2FO : 2.159,and Mn(3)-O : 2.209 A, show a similar, though significantly a(A) smaller, variation. The AsOo tetrahedra in Mapimi vil-

TABLE7. Octahedralmetal-oxygen distances (A) for villyaellen- b(A) a(A) ite to.2 Bond Mapimi" DLsno. 1 DLSno. 2 c(A) 10.1 b(A) M(1)-o(1r- 2.15 2.22 2.20 10.0 M(1)-o(3).' 2.23 2.30 2.31 99 M(1)-O(7)-. 2.26 2.33 231 c(A) Average 2.21 2.28 227 pf) M(2)-o(2) 2.14 2.25 2.14 pf) M(2)-o(4) 2.-t5 2.24 2.24 M(2)-o(s) 2.14 2.23 2.17 M(2)-o(6) 2.05 2.16 2.16 p n M(2)-OW(1) 2.33 2.35 2.31 .68 M(2)-OW(2) 2.07 2.18 2.21 1.66 Average 2.14 2.23 2.20 3.8 M(3)-o(1) 2.20 227 2.34 o.o M(3)-o(3) 2.22 2.30 2.33 3.4 S.G. M(3)-O(4) 2.18 2.25 2.32 3.2 3.2 M(3)-o(s) 2.29 2.35 2.37 s.G. M(3)-o(6) 219 2.26 2.32 1.O M(3)-ow(1) 233 2.40 2.50 Mn / Mn+Ca Average 224 2.31 2.36 Fig.2. Variation in cell constants,refractive indices, and spe- . Estimatedstandard deviations are +0.01 A. .- cific gravity for the sainfeldite-villyaelleniteseries. Note that the Two such bondsfor M(1)-O octahedron lines indicate ideal linear variations betweenthe end members. KAMPF AND ROSS: END-MEMBER VILLYAELLENITE

DlS-refined bond distancesin type villyaellenite .!- The first distance-least-squares(ors) refinement ofthe "l type villyaellenite structureusing bond distancesaveraged from sainfelditeand Mapimi villyaellenite yielded the fol- lowingaverage distances: M(IFO :2.28, M(2FO :2.23, and M(3lO :231 A. Thesevalues suggest that M(l) : Mn * Ca, M(2) : Mn, and M(3) : Ca, although the averageM(2FO distancesare somewhatlarger than those observedfor Mn4 in hureaulite and fluckite and M(31 O is somewhat smaller than those observed for Ca-O in sainfeldite and fluckite. The second ols refinement, as- suming this ordering scheme,yielded M(l)-O : 2.27, M(2FO : 2.20, and M(3)-O : 2.36 A. TheseM(2rc and M(3lO distancesshow excellentagreement with dis- tancesobserved in similar compoundsfor Mn-O and Ca- O, respectively.

Correlation with the fluckite structure In Figure 3 the octahedral edge-sharingchain portions of the villyaellenite (atomic positions from the secondors refinement) and fluckite structures are compared. The five- ab member octahedral chain fragment in villyaellenite and Fig.3. Comparisonofthe octahedraledge-sharing chain por- the infinite chain in fluckite are both zig-zag-type chains tionsofthe (a)villyaellenite and (b) fluckite structures. The atom- in which nonopposingedges are shared.In both structures ic positionsfor the villyaellenitechain are from the secondprs the oxygensof the shared edgesalso belong to arsenate refinement.The fluckite chain is modifiedfrom Catti et al. (1980). tetrahedra.The remaining two octahedralcorners are each sharedwith one other octahedron.These corners are either water or arsenateoxygens. sites should yield a [0I] dimension for fluckite inter- The series Ca(H,O)(AsO.OH)-Mn(H,O)(AsO,OH) in- mediatebetween those found in haidingerite and krautite. cludesthe specieshaidingerite, the Ca end member (Cas- In factthe [ 10 1] celldimensions are 10.542, | 1.706, I 1.089 sienet al., 1966),fluckite, with Ca:Mn : l:1, and krautite, A for traidingerite, fluckite, and krautite, respectively. This the Mn end member (Catti and Franchini-Angela, 1979). cell dimension for fluckite is greater than it is for either Although their symmetries differ (Pbna, Pl, and P2,, re- end member, suggestingthat the edge-sharingoctahedral spectively),these minerals are structurally similar to one chain in fluckite may be abnormally lengthenedas a result another. Fluckite and krautite are particularly similar, of the ordering of Mn and Ca. having topologically identical pyroxeneJike octahedral The octahedral chain fragment in sainfeldite and vil- chains. The differencesobserved in their symmetries and lyaellenite is parallel to the a axis. The a cell dimension structures are not due to ordering per se, but can be at- in type villyaellenite is considerablygreater than that pre- tributed to shifts between the chains induced by the dif- dicted by Vegard'slaw. By analogyto fluckite, the length- ferencein size of Ca and Mn. ening ofthe octahedralchain fragmentsin responseto the Two comparisonsare of interest in consideringthe pos- ordering of Mn and Ca is seenas the possible cause. sibility ofordering in villyaellenite. First, the octahedral sites in krautite and Mapimi villyaellenite, the Mn end Parageneticevidence membersof the two series,can be compared.Krautite has Heretofore there have been only two limited compo- four octahedralsites with the averagedistances Mn(llO sition rangesreported in the seriesCar(HrO)o(AsO3OH)r- :2.20, Mn(l'lO :2.20, Mn(2lO :2.77, Mn(2'fO: (AsOo)r-Mn,(HrO)o (AsO,OH), (AsOo)r.These are repre- 2. l8 A. Mapimi villyaellenite exhibits a much larger vari- sentedby the types for sainfeldite and villyaellenite, both ation in averageMn-O distancesover its three distinct from Sainte-Marie aux Mines. The nearly pure Mn end sites.Even without any distortion of the Mapimi villyael- member has now been found at both Mapimi, Mexico, lenite structure,some degree of ordering is to be expected. and Sterling Hill, New Jersey.At Mapimi, it is in direct Second,the cell dimensionsparallel to the chains in the associationwith arseniosideriteand ogdensburgite,min- structuresof the minerals of thesetwo seriescan be com- erals that contain essentialCa; at Sterling Hill, it occurs pared. In haidingerite-fluckite-krautite, the octahedral on calcite-bearingwillemite-franklinite ore. If Mn and Ca chain is parallel to [01] (The a and c axesin haidingerite were disordered in intermediate members of this series, must be interchanged).Although Vegard's law cannot be then a complete solid-solution seriesshould exist, and it applied strictly becauseof the diferences in the structures is likely that the villyaellenite from Mapimi and Sterling of theseminerals, disorder of Ca and Mn over the chain Hill would contain more substantial amounts of Ca. I 178 KAMPF AND ROSS: END-MEMBER VILLYAELLENITE

Further parageneticevidence for ordering in type vil- Museum of Natural History (SmithsonianInstitution) for making available lyaellenite is in associationwith fluckite reported by Sarp his data on villyaellenite from the Sterling Hill mine, GeorgeR. Rossman (1984)and observedby us on a samplefrom Sainte-Ma- of the California Institute of Technologyfor making available the equip- ment usedin the water determination, CharlesStrouse of the UCLA De- rine aux Mines obtained from Sarp via PeteJ. Dunn. The partment of Chemistry for assistancein the collection and refiflement of minute bladesofvillyaellenite on this sampleproject out- the structuredata, and Wayne Dollase of the UCLA Department of Earth ward from the surface of a crystal of fluckite, and the and Space Sciencesfor the use of his program orrors and for helpful boundary between the fluckite and villyaellenite crystals discussions.The manuscript benefitedfrom reviews by Paul B. Moore of the University of Chicago,Pete J. Dunn, and John M. Hughes of Miami is indistinct. This textural relationship suggeststhat the (Ohio) University. One of us (C.C.R ) would like to acknowledgesupport crystallization of villyaellenite closely succeededthat of of NSF grant 4-443875-22146-3(W. G. Emst, P.I.). fluckite, probably from the samesolution and under only slightly differing conditions. SinceCa and Mn are ordered RnronnNcns crrno in the fluckite octahedralchain, it is reasonableto suspect that they may also be ordered in the villyaellenite chain. Cassien,M., Herpin, P, and Permingeat,F. (1966) Structure cristalline CoNcr-usroNs de la haidingerite.Bulletin de la Soci6t6franqaise de Min6ralogie et de Cristallographie,89, 18-22. The ordering of Mn and Ca in type villyaellenite, al- Catti, M., and Franchini-Angela,M. (1979)Krautite, Mn(H,O)(AsO,OH): though probable, remains unproven. Without evidence Crystal structure,hydrogen bonding and relationswith haidingepiteand from a direct method such as structure analysis, the ex- .American Mineralogist, 64, 1248-1254. istence of such ordering can only be inferred. Neverthe- Catti, M., Chiari, G, and Ferraris, G. (1980) Fluckite, CaMn(HAsO), related pseudo-polytypismto krautite, MnHAsO4 less,the evidence 2HrO, a structure by at hand suggeststhat Mn and Ca are at H,O. Bulletin de Min6ralogie, 103, 129-134. least partially ordered in this mineral. Dunn, P.J (198la) Ogdensburgite,a new calcium-zinc-ferric iron arsenate The octahedralsites in near end-member villyaellenite mineral from SterlingHill, New Jersey.Mineralogical Record, 12,369- vary substantially in size, as evidenced by their average 370. - metal-oxygenbond distances.The or,srefinements oftype (l 98 lb) Sterlinghillite,a new hydratedmanganese arsenate mineral from Ogdensburg,New Jersey.American Mineralogist, 66, 182-184. villyaellenite generatebond distancesappropriate for or- Dunn, P.J., and Peacor,D.R. (1983) Kittatinnyite and willkilldellite, sil- dering according to the schemeM(l) : Mn + Ca, M(2) icate/arsenateanalogues containing calcium and manganese,from : Mn, and M(3) : Ca, correspondingto the ideal formula Franklin and Sterling Hill, New Jersey. American Mineralogist, 68, CarMnr(HrO)o(Mn,Ca)(AsOrOH)r(AsOo)r.The correla- r029-t032 Ferraris, G., and Abbona, F. (1972) The crystai structure of tion between type villyaellenite and fluckite in terms of Ca,(HAsO)dAsO), 4H,O (sainfeldite).Bulletin de la Soci6t6frangaise paragenesisand structure further supports the likelihood de Min6ralogie et de Cristallographie,95, 33-41. of ordering in type villyaellenite. Finally, the lack of other International Tablesfor X-ray Crystallography.(1974) J.A. Ibers and W.C. intermediate members of the sainfeldite-villyaellenitese- Hamilton, Eds., vol. 4. Kynoch Press,Birmingham, England. (1987) ries, despite the occurrence of the nearly pure-Mn end Ikmpl A.R., and Dunn, P.J. Ogdensburgitefrom Mapimi and new data for the species.American Mineralogist, 72,409412. member in relatively Ca-rich associations,suggests that Mandarino, JA. (1981) The Gladstone-Dalerelationship: Part IY. The a complete solid solution may not exist and that ordering compatibility conceptand its application. Canadian Mineralogist, 19, may set type villyaellenite apart from the two end mem- 441450. bers. Moore, P.B.,and Amki, T (1973)Hureaulite, Mn,(H,O).[PO,(OH)],PO"L: American Mineralogist, 58, 302-307. If direct proposed Its atomic arrangement. evidenceof the ordering in type vil- Sarp,H. (1984)Villyaellenite, H,(Mn,Ca)r(AsOJa 4HrO un nouveaumrn- lyaellenite is forthcoming, the near end-member villyael- 6ral de Sainte-Marie aux Mines (France). SchweizerischeMineralo- lenite from Mapimi and Sterling Hill may qualify as a gischeund PetrographischeMitteilungen, 64, 323-328. distinct species. AcxNowr-rncMENTS We wish to thank John Whitmire for providing the only specimenof Merquscmsr R.EcErl'EDJnm;env 11, 1988 villyaellenite known from Mapimi, Pete J. Dunn of the U.S. National M,arvuscmp'rAccEprED Mlv 24, 1988