American Mineralogist, Volume 78, pages 190-194, 1993

Ribbeite, a secondexample of edge-sharingsilicate tetrahedra in the leucophoenicitegroup

Rosnnr L. Fnnpo Department of Geology, Trinity University, San Antonio, Texas 78212, U.S.A. Ror-l.No C. Rousn, DoNllo R. Pracon Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan 48109, U.S'A.

Ansrn-lcr Ribbeite,Mn,(OH),(SiOo)r, is orthorhombicPnmawith a : 10.732(l)A, t : ts.Oz61 A, c : 4.81l(l) A, Z : 4, and V:809.2 43. Its structurehas beendetermined by direct methods and refined to a residual of 0.037 (unweighted) for all reflections. Ribbeite is structurally related to leucophoenicitein that it contains serrated,edge-sharing chains of Mn octahedra and two distinct types of Si tetrahedral sites: (l) a single, isolated, fully occupied Si tetrahedron, and (2) a pair ofedge-sharing,half-occupied Si tetrahedra,which are related by an inversion center. Ribbeite is the unit-cell-twinned dimorph of alleghany- ite; the disordered edge-sharingtetrahedra in ribbeite are a direct consequenceof mixed unit-cell glide operations in alleghanyite:statistically, half of the glides are alternate, half are en echelor. The valence requirements of O atoms in edge-sharingtetrahedra are met by two H bonds and one anomalouslyshort Si-O distanceof l'522(3) A.

INrnonucrroN ccp Mg(3)'?,i.e., the unit cell is based on two twin bands that are each three atoms wide. Other members of the The new mineral ribbeite, Mnr(OH)r(SiOo)r, was de- humite group are based on intergrowths of twin bands scribed from the Kombat mine in Namibia by Peacoret that are two and three atoms wide. The twin-layer repeat al.(1987).Theynotedthatitisorthorhombic,spacegroup notation is given in Table l White and Hyde (1983a) Pbnm or Pbn2, (nonstandardsettings), with lattice pa- noted that the older method of denoting humites, rametersa:4.799(l), b: 10.742(6),c:15.70(l) A nMgrSiOo'Mg(OH,F)r,corresponds to twinned ccp (obtained by least-squaresrefinement of the powder dif- Mg(3,2) with r : x * l- fraction data), Z : 4, and with the calculated chemical The relationship between leucophoenicitesand hum- formula (MnoroMgouoFeoorCaoor)onusi,e.HrroO,o(from ites was enigmatic(Dunn, 1985) until White and Hyde electron microprobe analysis).In addition, Peacor et al. (1983a) noted that leucophoenicites(1,21 are regular in- (1987) observed that ribbeite and alleghanyite are poly- tergrowths of a twin band one atom wide with bands two morphically related, with ribbeite belonging to the leu- atoms wide; the bridging structure common to (3,2) cophoenicitegroup and alleghanyiteto the humite group humites and (1,2') leucophoenicitesis (2) olivine. White (Table l). and Hyde (1983a) also observed that subjecting the ol- The structures of the humite-group minerals are well ivine structure to repeatedcrystallographic shear produc- known and have been reviewed by Ribbe (1982). They es successivemembers of the leucophoenicitegroup. Kato were originally describedas periodic layers ofolivine and et al. (1989) noted that the twin operation is a glide plane, brucite-sellaite(Taylor and West, 1928),but Ribbe et al. and Yau and Peacor (1986) generalizedthe relationship, (1968) showed that this layer concept for humite struc- stating that "a similar glide in every member of the hum- tures is inappropriate. Thompson (1978) st'ggesteda de- ite family (3,2") results in a structure of the leucophoeni- scription with alternating modules of olivine and nor- cite family (1,2'*'7." Table I summarizes the relation- bergite. White and Hyde (1982a, 1982b) described shipsbetweenleucophoenicitesandhumites. humites in terms of anion-stuffed cation arrays that cor- The structures of leucophoenicite (Moore, 1970) and respond to those in either well-known alloy types or la- jerrygibbsite (Kato et al., 1989) have been discussedin mellar intergrowths of such alloys. The arrays can be de- terms of unit-cell twinning by Yau and Peacor (1986). scribed as twinned cubic-closest-packed(ccp) bands of We now report the crystal structure of ribbeite, which is octahedrallycoordinated cations. Using this system,White indeed the unit-cell-twinned dimorph of alleghanyite,as and Hyde (1983a) described forsterite as twinned ccp predicted by Peacor et al. (1987). Ribbeite is similar to Mg(2)r, where the twin bands of Mg cations are two at- leucophoenicitein that it contains two distinct types of oms wide and the superscriptdenotes the number of twin tetrahedral sites: (l) a single, isolated, fully occupied Si individuals (bands) in a unit cell. Norbergite is twinned tetrahedron, and (2) a pair ofedge-sharing,half-occupied 0003-004x/93l0r 02-0 r 90$02.00 I 90 FREED ET AL.: RIBBEITE 191

TABLE1, Summaryof structuredata for the humiteand leucophoenicitegroups

nMZ SiO4M, . Twin Mineral ldealformula (OH,F), Spacegroup. formula'. Reference Humitegroup Norbergite Mg3(SiO4XOH), n: 1 Pbnm (3F Gibbsand Ribbe(1969) MANt Mn3(SiO"XOH), n: 1 Pbnm (3F Francisand Ribbe(1978) Chondrodite Mgs(SiO4),(OH), n:2 nllb (3,2) Gibbset al. (1970) Alleghanyite Mn5(SiO4),(OH), n:2 nllb (3,2) Rentzeperis(1970) Humite Mgi(Si04)3(OH), n:3 Pbnm (3,21" Ribbeand Gibbs(1971) Manganhumite Mn?(SiO4)3(OH), n:3 Pbnm (3,2)" Francisand Ribbe(1978) Clinohumite Mg,(SiO")4(OH), n: 4 n.lb (3,2") Robinsonet al. (1973) Sonolite Mn,(SiOr)4(OH), n: 4 n.lb (3,2") Kato et al. (1989) Olivinegroup Forsterite Mg,(SiO4) Pbnm (2)" Francisand Ribbe(1980) Tephroite Mn,(Si04) Pbnm (2)', Francisand Ribbe(1980) Leucophoenicitegroup Unknown Mn3(SiO4XOH), n:1 (1,2)? Ribbeite Mn5(SiO4),(OH), n:2 Pbnm (1,z',)', this study Leucophoenicite Mn?(SiO4)3(OH), n:3 n'lb (1,2') Moore(1970) Jerrygibbsite Mn,(SiO4)4(OH), Pbnm (1,2n)', Kato et al. (1989) t Convenlionalbut nonstandardsettings. -'After Whiteand Hyde(1982a, 1982b, 1983a). t Synthetic Mn analogueof norbergite.

Si tetrahedra,related by an inversion center.A transmis- Si, and most of the O atoms; synthesesof electron density sion electron microscopy (TEM) study of leucophoenicite and differenceelectron density revealed the positions of (White and Hyde, 1983b) supported the structure pro- the remaining atoms other than H. Least-squaresstruc- posedby Moore (1970). ture refinement was performed using the 802 reflections having 1.o. > 3o(1"o"),where o is the standard deviation S:rnucrunp soLUTroN AND REFTNEMENT from the counting statistics.The function minimized was Intensity measurementswere made using an anhedral )w(lFl.o" - lFl*J', wherew : 4lFlio"/o'1(lFl:b").With crystalof approximatedimensions 0.12 x 0. 16 x 0.31 isotropic displacementfactors, the refinement converged mm; this crystal was selectedfrom the type material of to a value of 0.059 for the unweightedresidual. However, ribbeite (Peacoret al., 1987; Smithsonian Institution cat- strong parameterinteraction betweenpairs of nonequiva- alog no. NMNH 163208).Intensities of I l3l reflections lent atoms and the inability to refine all O atoms with with sin d < 0.818 and constitutingone asymmetricunit anisotropic displacementfactors suggestedthat the acen- were measruedwith monochromatized MoKa radiation tric structure was incorect. Therefore, the centrosym- by an Enraf-Nonius CAD4 kappa-axis diffractometer, metric spacegroup Pnma was selected,and after appro- controlled by a MicroVAX 3100 computer. All calcula- priate reindexingof reflections,direct methods were again tions involved in the solution and refinement of the struc- employed. An .E map revealed the locations of Mn, Si, ture were performed using the Enraf-Nonius crystallo- and all O atoms. kast-squares refinement of this struc- graphic software system MolEN. The intensities of six ture with isotropic displacement factors convergedto an standard reflections were monitored during data mea- unweighted residual of 0.069. A refinement of the total surement, and these showed a total average intensity occupancyfactor ofSi2 yieldeda value of0.485(5). This changeof 0.850/0.Cell parameterswere obtained by least- correspondsto 3.88(4)atoms, which is equal to four at- squaresanalysis using the setting anglesof 24 reflections oms within three estimated standard deviations: i.e.. the in the range 5o < 0 < l2'. In the Pnma standardsetting, Si2 site is essentially500/o occupied. ribbeite is orthorhombic, with a : 10.732(l) A, b : Introduction ofanisotropic displacementfactors for all 15.672(6)A, c : 4.811(l)A, z : 4, and V: 809.2A.. atoms reduced the residual to 0.032. At this point an Intensities were reducedto structure factor amplitudes by inspection of the list of observedand calculatedstructure correction for Lorentz-polarization effects and applica- factors revealeda pattern ofintense, low-angle reflections tion of an empirical absorption correction derived from having l.Fl.o, < lFl*,.A combined isotropic extinction ry'-scansof six reflections. conection (i.e., combining the effectsof primary and sec- Peacoret al. (1987) determined that ribbeite has space ondary extinction) of the form l F l .o,(corr.) : l -Fl .o"[ * group Pbnm or Pbn2, (Pnma qr Pna2, in the standard g( | 1l *,)l was therefore applied, and that further reduced settings).Intensity statistics were inconclusive for select- the residual to 0.027. A differencesynthesis revealed two ing either the centric or acentric structure. The noncen- small peaks of 0.5 e/A3 situated at distancesof 0.6 and trosymmetric spacegroup Pna2rwas thereforearbitrarily 0.8 A from the hydroxyl O atoms 05 and 06, respec- chosenas the basis for structure solution by direct meth- tively. These were assumedto be H atoms and were la- ods, using the Multan ll/82 package incorporated in beled Hl andH2, respectively.It proved possibleto ob- MolEN. The first E map revealed the locations of Mn, tain reasonablebond parametersfor Hl but not for H2, t92 FREEDET AL.: RIBBEITE

TABLE2. Positional (decimal fractions) and displacement (A'?) tains the final positional and displacement parameters, parameters in ribbeite Table 3r the observed and calculated structure factors, Table 4 selectedinteratomic distances and angles, and B4 Table 5 the empirical bond valencescalculated from the Mn1 8d 86850(5) 14s01(3) 49334(9) 1.110) observeddistances and the constantsgiven by Breseand Mnz 4c 59124(6) 1/t s1449(1 4) 1.11(1) Mng 8d 64402(5) 06127(3) -00408(10) 1.24(1) O'Keefe (1991). Si1 4c 7779(1', 1/a 0712(2) 0.55(2) si2 8d 4613(1) 0448(1) s835(3) 0.65(3) Srnucrunn DESCRTPTIoNAND DrscussloN 01 8d 4616(2) 0432(1) 2672(5) 1.2214) 02 4c 7778(31 1/q 7381(6) 0.73(s) The ribbeite structure (Fig. 1) consistsof sheetsof hex- 0.78(s) 03 4c 9179(2) V4 2125(61 levels of approximately 04 8d 7101(2) 1681(1) 2123(4\ 0.75(3) agonal close-packedO atoms at 05 8d 5286(2) 1373(1) 7311 (5) 1.s8(4) z : 0.25 and 0.75. Cation sitesare located at levels of 06 8d 3109(2) 0439(1) 7344(5) 1.37(4) approximately z : 0.00 and 0.50. Mn octahedra form H1 8d 555(8) 126(s) 569(16) 1.7(171 parallel H2 8d 345 034 600 2.0 serrated edge-sharingchains that trend roughly to D. Moore (1970) described the leucophoenicite struc- Note; Valuesin oarenthesesare estimatedstandard deviations. Si2, H1, and H2 sites are 50o/ovacant. Disolacementfactor for H1 was refined ture in terms of a cluster of five edge-sharingMnOu oc- isotropically:that for H2 was arbitrarilylixed. No estimated standard de- tahedra; this cluster is also found in ribbeite (Fig. l). viations are shown for H2 because its coordinates were fixed at those Viewed as an anion-stuffed cation array (White and Hyde, taken from the difierencesynthesis. 1982a, 1982b, 1983a), ribbeite has the twinned ccp Mn(l,2'z)'zarrangement. Ribbeite is also similar to leucophoenicite in that it and accordingly the coordinatesof H2 were fixed at those contains two distinct types oftetrahedral sites: (l) a sin- taken from the differencemap, whereasthe displacement gle, isolated, fully occupied Si tetrahedron and (2) a pair factor for H2 was arbitrarily fixed at 2.0 A'. The ditrer- ofedge-sharing,half-occupied Si tetrahedrarelated by an ence synthesis showed no evidence of atom splitting affecting the disordered atom Si2 or its coordinating an10ns. I A copy of Table 3 may be ordered as Document AM-93-515 The final values of the residual are 0.027 (unweighted) from the BusinessOfrce, MineralogicalSociety of America' 1130 and 0.050 (weighted)for the 802 observedreflections and SeventeenthStreet NW, Suite 330, Washington, DC 20036, 0.037 (unweighted)for all I l3l reflections.Table 2 con- U.S.A. Pleaseremit $5.00 in advance for the microfiche.

TABLE4. Selected interatomic distances (A) and angles (') in ribbeite MnO.octahedra Mn1-O1 2.262(2) Mn2-o2 2.273(3) Mn3-O1 2.360(2) 02 2.245(2) o3 2.157(3) o1 2.369(2) o3 2.194(2) o4 2.321(2)x 2 o4 2.0e5(2) o4 2.202(2) o5 2.158(2)x 2 o5 2.140(2\ o5 2.174(2) MEAN 2.231 o6 2.123(2) o6 2.146(21 o6 2.153\2) MEAN 2.204 MEAN 2.207 SiO.tetrahedra si1-02 1.603(3) o2-o4 2.717(31x 2 04-si1-03 103.7(1)x 2 o3 1.649(3) o2-o3 2.733(4) o4-si1-o2 114.7(1)x 2 o4 1.624(21x 2 03-04 2.573(3)x 2 o4-si1-o4 104.4(2) MEAN 1.625 04-04 2.566(4) o3-si1-o2 114.4(21 MEAN 2.646 MEAN 109.3 si2-o1 1.522(3) 01-o1' 2.744(sl o1-si2-o1' 113.2(1) 01' 1.761(3) 01-o5 2.770(3) 01-si2-o5 114.5(1) o5 1.769(3) 01-06 2.77O(3) o1-si2-o6 114.4(1) o6 1.770(3) o1'-o5 2.831(3) o1'-si2-o5 106.6(1) MEAN 1.706 01',-o6 2.798(3) o1',-si2-06 104.8(1) 05-06 2.757(31 05-si2-o6 102.3(1) MEAN 2.778 MEAN 109.3 H-O in edge-sharing tetrahedra H1-O5 0.85(8) o5-H1.. 01 126(7',) 01 2.19(8) o5-H1-O1', 85(5) 01' 2.77(8) o5-H1-O6 64(5) 06 3.02(8) o5-H1-O6', 135(6) 06' 3.36(8) H2-O6 0.76 o6-H2...01 161 01 2.04 o6-H2-O1', 107 o1', 2.48 06-H2-O5 91 05 2.63 06-H2-O5', 139 05' 3.40 ivote..No estimatedstandard deviationsare given for distancesor angles involvingH2 becausethe position of this atom was fixed at the coordinates from the differencesynthesis during the refinement. FREED ET AL.: RIBBEITE 193

TraLe5. Empiricalbond valences(vu) for ribbeite

Mn1 si2 ol 0.28 0.21(x2) (0.35.,0.66.)- 1.71 (0.70,1.32) I 02 0.29(x2) - 0.27 1.06 1.91 0.29l o3 0.34(x2) - 0.37 0.94 1.99 0.34I o4 0.33 O.24- o.44 1.00- 2.01 0.24(x2) I 1.00(x2) I o5 0.35 0.37- 0.39 034'- 1.45 0.37(x2) I 0.68I UO 0.38 0.37,0.41 0.34-- 1.50 0.68I >v. 1.97 1.86 203 4.00 3.38 Nofe.'Valences for O-Si2 bonds marked with asterisks are given as 50% of their calculatedvalues to account for the 50% occupancyof the Si2 site.

inversioncenter. As noted by Moore (1970),the absence edge of the tetrahedral pair. However, in this case the of Si in one of the edge-sharingtetrahedral pairs implies valencerequirements of Ol'(0.73) cannotbe satisfiedby that (OH)- has replaced O2-. Thus, when Si2(0.58) (Fig. the formation of H bonds becausebond distances(Table 2) is absent,Hl(0.57) and H2(0.60)are present,forming 4) aretoo long:2.77A for Hl-Ol', and2.48A for HZ- OH groups with 05(0.73) and 06(0.73), respectively. Ol'. Baur (1972) notes that "only H ' ' ' O distances However, the valencerequirements of Ol(0.27) are also shorter than 2.4 A should be consideredto be hydrogen unsatisfied in the absenceof Si2(0.58) and can only be bonds." Instead,the distanceSi2(0.42)-O1'(0.73) in the met by the formation of H bonds from Hl(0.57) and occupied Si2 tetrahedron contracts to a very short 1.522 H2(0.60).This is supportedby short H-Ol distances(Ta- A to satisfythe valenceneeds of Ol'(0.73). ble 4) of 2.19(8)A for Hl(0.57)-0l(0.27) and 2.04A for When Si2(0.58)(Fig. 2) is absent,Si2(0.42) must be H2(0.60)-O1(0.27).Additional support comesfrom the present. This fulfills the valence requirements of its as- empirical sums of bond valence (Table 5): the sum for sociated05' and 06' ligands, and the latter are therefore O I is conspicuouslylow at 1.7I vu, whereasthe sums for oxide rather than hydroxyl O atoms. Atoms Hl(0.43) and the other nonhydroxyl O atoms 02, 03, and 04 are all H2(0.40) must be absent,and the occupanciesof Hl and within 0.10 vu of their ideal values. H2 are therefore correlated with the occupancy of Si2, Valencerequirements of Ol'(0.73) (Fig. 2) are likewise i.e., at 500/oin eachcase. unsatisfiedin the absenceofSi2(0.58), since this Si atom Tetrahedral Si2-O distances(Table 4) are unusual,with is coordinatedto both O I and O l', which form the shared long distancesassociated with each basal O atom and a very short distance with the apical O atom. Similar dis- tanceswere observedby Moore (1970) for leucophoeni- cite and Kato et al. (1989) for jerrygibbsite. Two distanc- es,Si2-O5 and Si2-O6, must representan averageof Si2 with YzO'?-+ r/zOH-, depending on the presenceor ab-

02 26

Fig. 1. Polyhedral diagram ofthe ribbeite structure, project- ed down the c-axis. Numerical values are z coordinatesin hun- dredths. The serratededge-sharing chain of Mn octahedracen- tered at z = 1.0 is portrayed with an intermediate stipple and is superimposedon Mn octahedra(light stipple) centeredat level Fig. 2. Polyhedral diagram of a portion of the ribbeite struc- z = 0.5. The Si tetrahedra centeredat level z = I.O, both fully ture centered at level z x 0.5 projected down the c axis and occupied, isolated units and half occupied, edge-sharingpairs, showing the relationship of H atoms to Mn octahedra and Si are shownin darker stipple. tetrahedra.Numerical values are z coordinatesin hundredths. r94 FREED ET AL.: RIBBEITE senceofSi2, as discussedabove. Support for this comes dation. This study is contribution no. 486 from the Mineralogical Labo- from anisotropic displacement parameters: U, for 05 ratory, Department of Geological Sciences,University of Michigan, Ann Arbor, MI 48109 suggestsan anisotropic ellipsoid elongated along b; U,, for 06 suggestsan anisotropic ellipsoid elongatedalong RrrnnnNcns crrno a. The remaining two Si-O distancesare associatedwith (1972) hydrogen atom (Fig. Baur, W.H. Prediction of hydrogen bonds and Ol 2). As noted above, when Si2(0.58)is absent, positionsin crystallinesolids Acta Crystallographica,28B, 1456-1465. Ol forms H bonds with 05 and 06 and an oxide bond Brese,N.E., and O'Keeffe,M. (1991) Bond-valenceparameters for solids. with Si2(0.42),ar a disranceof 1.761(3)A. when Si2(0.58) Acta Crystallographica,478, | 92-197 . is present,H bonds are not formed, and the Si2-Ol dis- Dunn, P.J. (1985) Manganesehumites and leucophoenicitesfrom Frank- tance contracts to 1.522(! A to satisfy valence require- lin and Sterling Hill, New Jersey:Parageneses, compositions, and im- plications for solid solution limits American Mineralogist, 70, 379- ments. Thus the position of Ol is constrained by satis- 387. fying two bonding patterns,depending on the presenceor Francis, C.A, and Ribbe, P.H. (1978) Crystal structuresof the humite absenceofSi2. minerals. V. Magnesian manganhumite. American Mineralogist, 63, As noted earlier, the structuresof alleghanyiteand rib- 874-87't . -(1980) I. refine- glide-plane The forsterite-tephroiteseries. Crystal structure beite are indeed related by a unit-cell twin ments. American Mineralogist, 65, 1263-1269 operation,as proposedby Kato et al. (1989).In terms of Gibbs, G.V , and Ribbe, P.H. (1969) The crystal structuresof the humite the nine original designationsfor unit-cell twinning given minerals. L Norbergite. American Mineralogist, 54, 376-390. by Ito (1950),this is a G2,or alternate,glide operation. Gibbs, G.V., Ribbe, P.H., and Anderson, C.P. (1970) The crystal struc- Mineralogist, Using the polyhedral diagram of alleghanyite given by tures of the humite minerals. II. Chondrodite. American 55,I 182-l194. Rentzeperis(1970), the serratededge-sharing chain of Mn Ito, T. (1950) X-ray studieson polymorphism, p. l-6. Maruzen, Tokyo. octahedra in ribbeite can be generatedby a c/4 glide in Kato, T, Ito, Y., and Hashimoto, N. (1989) The crystal structures of alleghanyite repeated in the G'z mode. In the ribbeite sonoliteand jerrygibbsite.Neues Jahrbuch fiir Mineralogie Monatshef- structure (Fig. 1), this glide is parallel to a. The fully te,410-430. Moore, P.B. (l 970) Edge-sharingsilicate tetrahedra in the crystalstructure occupied, isolated Si tetrahedral site in ribbeite can also ofleucophoenicite.American Mineralogist, 55, I 146-1166 be generatedby this glide; however, only one ofthe dis- Peacor,D.R., Dunn, P.J.,Su, S.C.,and Innes,J. (1987)Ribbeite, a poly- ordered,edge-sharing pair ofSi tetrahedrain ribbeite, the morph of alleghanyiteand member of the leucophoenicitegroup from upward-pointing member in Figure l, is generated.Va- the Kombat mine, Namibia. American Mineralogrst,72,213-216. lence needsof O atoms disrupted by the glide operation Rentzeperis,P.J. (1970) The crystal structure of alleghanyite, Mn,[(OHL | (SiOo)'].Zeitschrift fiir Kristallographie, I 32, l- I 8. are met with H bonding. If the glide operation is G', or Ribbe, P H. (1982) The humite seriesand Mn-analogs.In Mineralogical en echelon (Ito, 1950), the reverse situation occurs: the Societyof America Reviews in Mineralogy, 5, 231-274. downward-pointing member of the disordered pair is Ribbe, P.H., and Gibbs, G.V. (1971) Crystal structures of the humite produced, with the valence needs of O atoms again ful- minerals III. Mg/Fe ordering in humite and its relation to other fer- romagnesiansilicates. American Mineralogist, 56, I 155- I I 73. filled by H bonding. Thus, it appearsthat the disordered Ribbe, P.H., Gibbs, G.V., and Jones,N.W. (1968) Cation and anion edge-sharingtetrahedra in ribbeite are a direct conse- substitution in the humite minerals. Mineralogical Magaztne,37 ,966- quence of a mixed-glide operation: statistically, half of 975. the glides are alternate, half are en echelon.We do not Robinson, K., Gibbs, G.V., and Ribbe, P H. (1973) The crystal structures Amer- know if the glide types are randomly mixed or if they of the humite minerals IV. Clinohumite and titanoclinohumite. ican Mineralogist, 58, 43-49. occur in a regularpattern; in either case,the simultaneous Taylor, W.H., and West, J. (1928)The crystalstructure of the chondrodite occurrenceof both types is designatedG3, or complex, in series.Proceedings ofthe Royal SocietyofLondon, SeriesA, 117, 517- the notation of Ito (1950). We suggestthat the leuco- s32. phoenicite structure may be related to that of mangan- Thompson, J.B., Jr. (1978) Biopyriboles and polysomatic series.Ameri- can Mineralogist, 63, 239-249. humite by complex-glideunit-cell twinning, as defined by White, T.J., and Hyde, B.G. (1982a) Electron microscope study of the Ito (1950).It is interesringro norethat Kato et al. (1989) humite minerals. L Mg-rich specimens.Physics and Chemistry of Min- did not observehalf-occupied, edge-sharingpairs oftet- erals,8, 55-63. rahedra in jerrygibbsite, a structure based on only an al- - ( 1982b)Electron microscopestudy of the humite minerals.II. Mn- 167-174. ternate glide. may be a unique member of rich specimens.Physics and Chemistry of Minerals, 8, Jerrygrbbsite -(1983a) A description of the leucophoenicitefamily of structures the leucophoenicitegroup; alternatively, the high R-fac- and its relation to the humite family Acta Crystallographica,39B, l0- tor (0.088)obtained by Kato et al. (1989) may indicate 17. that the structure is partially in error, and that a refine- -(1983b) An electron microscopestudy of leucophoenicite.Amer- ment basedon a structure with half-occupied edge-shar- ican Mineralogist,68, 1009-1021. Yau, Y.C., and Peacor,D.R. (1986) Jerrygibbsiteleucophoenicitemixed ing tetrahedra may convergeto a smaller residual. layeringand generalrelations between the humite and leucophoenicite families. American Mineralogist, 7 l, 98 5-98 8. AcxNowlnocMENT The Enraf-Nonius CAD4 kappa-axisdiffractometer used in this work Mnxuscnrm REcEIvEDM,qncH 4, 1992 was acquiredunder grant EAR-89 I 7350 from the National ScienceFoun- MaNuscnrpr AccEprEDAucusr 28. 1992