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