American Mineralogist, Volume 68, pages 04, l9E3 An electronmicroscope study of leucophoenicite Triuorny J. Wnrrer AND BRUcEG. Hvoe Research School of Chemistry, Australian National University P.O. Box 4, Canberra,A.C.T. 2600,Australia Abstract Three leucophoenicitespecimens were examinedin the transmissionelectron micro- scopeand by X-ray powder diffractionand electron-probemicroanalysis. A comparisonof observedand computer-simulatedstructure images supports Moore's proposalsthat the leucophoenicitestructure possessesedge-shared pairs of 04 tetrahedra that are half- occupiedby Si, and that the crystalscontain planarfaults parallelto (001).The latter are interpretedas lamellaeof other membersof a proposedleucophoenicite series (analogous to the well-known humite series).Such faults frequentlycause microtwinning, often on a scaleof less than 1004. Humite seriesmembers were also very occasionallyintergrown with leucophoenicite,and one specimen(from Pajsberg,Sweden) was associatedwith chrysotile. In the electron beam leucophoenicitereadily decomposesto tephroite, Mn2SiOa. Introduction to a simplealloy structure(White and Hyde, 1983a).The Leucophoeniciteis dimorphicwith manganhumite,ide- cation array is twinned, cubic-close-packed(c.c.p.) Mn ally 3Mn2SiO+'Mn(OH)2, although the former always with Si in three-quartersof the Mn6 trigonalprisms at the containsseveral weight percent of CaO (in the present compositionplanes. Two protons occupy each of the specimens-6%). A morphological study by palache remainingprisms. Oxygenatoms are in SiMn3tetrahedra (1928) indicated that it is crystallographicallydistinct and Mn3 triangles (HMnr tetrahedra). from the humite minerals, and a preliminary power X-ray The cation array is a regular intergrowth of the Ni2In examination(Moore, 1967)suggested the existenceof alloy type (twinnedc.c.p. Ni . 2,2. with In in Ni6 severalleucophoenicite types with monoclinicor ortho- trigonalprisms) and the CrB alloy type (twinnedc.c.p. Cr rhombic symmetry. Finally, a singlecrystal X-ray struc- . I,l . with B in Cr6 trigonal prisms) (Hyde et al., ture analysisby Moore (1970)showed that leucophoeni- 1979). Thus, leucophoenicite is twinned c.c.p. cite is monoclinic P2/a, with the crystallochemical Mn . 1,2,2,2,1,2,2,2. .or (1,23)withSior2Hineach formula Mnz[SiO+]z[(SiO4)(OH)2].We have reorientedto of the Mn6 trigonal prisms. The twin bandsare one or two P2/b, a-axisunique in order to facilitate a direct compari- atoms wide. This approach yields simple relations be- son with the structures of the humite minerals (pbnm or tweenthe structuresof a proposedleucophoenicite family P2rlb) and olivine (Pbnm). and those of the humite family and olivine. Figure I However, following Moore's studiesmany incomplete- shows leucophoenicitein both descriptions.Here we ly identifiedspecimens remained; in particularthe ques- concentrateon the cation array. tion of orthorhombic polymorphs was not settled (see Electron microscopy also Cook, 1969). Three leucophoenicitespecimens were examined;de- Structural considerations tails are given in Table 1. For examinationin the trans- Moore (1970)described leucophoenicite as a hexago- mission electron microscope, fragments were ground nally-close-packed(h.c.p.) anion array with the larger under ethanol in an agate mortar and then mounted on cations (mainly Mn) in octahedral interstices and the holey-carbonfilms supportedon coppergrids. They were smaller(Si) in tetrahedralinterstices. We havepreviously studiedat 100kV in a rr,ol l00CX microscopefitted wirh proposedan alternativedescription in which anions are a high-resolution,side-entry stage or at 200kV in a rrol insertedinto the intersticesof a cation array, which is 200CX microscope with an ultra-high-resolution,top- rather more regular than the anion array and corresponds entry stage. Both instruments had standard hair-pin fila- ments.Correction of objective-lensastigmatism was car- rPresent ried out on the carbon support film by Address:School of Science,Criffith University, Bris- minimizing contrast.High-resolution bane,Queensland 4lll, Australia imageswere recordedat magni 1009 1010 WHITE AND HYDE: LEUCOPHOENICITE Table l: Leucophoenicitespecimens used in the study Plefixes for Musem Nutrels: BM British Musem of Natural HisEory sI Smithsonian Institute (Natural History) Su Swedish MuseN of Natural History Sample and Source constituents Found Observatlons Almost all crystals showed a considerable FrankIj.n Mine, sussex county, (1,23) feucophoenicite, ^ (221 N.J-. U.S.A. r. rilFr6 t-^hr^ire degree of fau1tin9. APproximately half (i.e- (BM 1946, 185) and sonolite (2r,3). were twinnedomicroscoPicafty twin bands < I00 A wide) . fragments Franklin Mine, sussex county, (1, 23) leucophoenicite Most crys!a1s were "Perfcct". fuo N.J., U.S.A. were heavily faulted, and one fragnent (sr c6800) exhibited twinning. perfect; Pajsberg, sweden (.?, 23) leucophoenicite, fi\e (1,23) leucophoenicite was nainly (sM 740214) + a little manqanhwite (22,3) two crystals wele twinned microscoPically. wj.th another + some orthochrysotsile The orthochrysotale was intergrown phase, possibly ta1c. fications of 380,000x, 550,000x or 810,000x. Linear Image interpretation imaging conditions (i.e., crystals less than 804 thick) were alwaysutilized. Imagesfrom the l00CX instrumentconsisted of (001) Cleavagenormal to [00] (the principal zone of interest) fringes. The 200CX yielded higher-resolution lattice im- was well defined,and large thin crystalswere abundant. agesand structure images which could be matched with Beam damage was severe and rapid, particularly for computer-simulated images. The program used for the heavilv-faultedcrvstals. image calculations was purchased from Arizona State o o \/\ 'j ir o, oo 'i a o a ,t a a !i.u o aa a toa si O Mn (o,5O)O,O oo Fig. L Moore's (1979)stnicture of leucophoeniciteprojected on (100).a. Structurein conventionalterms: h.c.p. anions with cations in octahedral and tetrahedral sites. Also emphasizedare sheetsof twinned c.c.p. cations, the repeat unit consisting of one band one atom wide (1) followed by three bandstwo atoms wide (2), which leadsto the twin formula (1, 23).b. Structure emphasizing the alloy-type structure of the cation array. (Note that this is more regular than the anion array above). Rows of edge-connected SiMn6 trigonal prisms are of the Ni2In type; the pairs of face-sharing trigonal prisms are of the CrB type. In the proposed leucophoeniciteseries various widths of the Ni2In type are inserted betweenCrB-like elements.(The olivine structure is anion-stufed Ni2In, with no CrBlike layers.) WHITE AND HYDE: LEUCOPHOENICITE l0lI and experimentalimages (Figs. 5 and 6) supports the structure proposedby Moore, at least over the thick- nessesstudied. Lamellar faults As expectedfrom Moore's earlierX-ray studiesfaults parallel to (001) were found, especially in one .of the samplesfrom Franklin, N.J. (BM 1946,185). (The other sampleswere less imperfect.) A low-resolutionlattice imageof an inhomogeneouscrystal is shownin Figure7. High-resolution lattice imagessupport the notion that the variations are very thin lamellae (one or two unit cells wide) of highermembers of a leucophoeniciteseries, (1, 2^)with x > 3 (Figs. 5 and 8). No lower members(x < 3) were observed. tG ttati3rkol 4l dirtriburkrn , of $i Fig. 2. orderingof theff:XTit 23)leucophoenicite can producemany polymorphs. The four simplest, designated A, B, C, andD, areshown projected down [100]. (Si-free Mnu trigonal prismsare outlined by dashes.)Note that the (1,23) cation array is maintained.Manganhumite (22,3)2 is alsoshown. University and adapted to run on the A.N.U. Univac computerby Dr. J. R. Sellarand Dr. R. A. Eggleton.The following instrumental parameterswere employed for the imagecalculations: incident beam convergence: 0.2 or 0.4 mrad, objective aperture radius 0.39 or 0.79A-r, coefficientof sphericalaberration, C" : L5 mm, depthof gaussianfocus = 150A,slice thickness = 44. Results The distribution of Si atoms Moore's (1970)determination of the leucophoenicite(1, ,tt*t'*pq,* 23)structure showed two crystallographicailydistinct Si trltl atoms.The first SiOatetrahedron (in our descriptionthe * ,#'* S # i{{{{{t alta edge-sharingMn6Si trigonal prisms) is isolatedand the Si lr*tr {{{{{{{ .-'S sitefully occupied.The secondSi is statisticallydistribut- le 4 , I *,**.{{ I ed between two tetrahedra of an edge-sharedpair , i.e ., in an [(SiOa)(OH)z]group (or alternativelya pair face- of I sharedMn6 trigonal prisms). I If the latter Si arrangement is ordered a number of polymorphsare possible:four simple ones emphasizing i{t:{ff{tt{fi the trigonal prism array are shown in Figure 2. Structure r ff{lffffft imageswere calculatedfor both [100] and [010] projec- Fig. 3. Calculatedthrough-focus series of imagesfor the tions of Moore's statisticalmodel and polymorphsA [100] and zoneusing the statisticalmodel of leucophoenicite,and alsothe B (Figs. 3 and 4). (The calculatedimages for polymorphs ordered polymorphs A and B. (Computationalparameters were: C and D are readily deducedby consideringthose of the thickness: 404, objective aperture radius = 0.39A-r, C, = other ordered polymorphs, particularly around the opti- 1.5A, beam divergence: 0.2 mrad, chromatic abberation: mum defocusof -6004.) A comparisonof the calculated 150A,and defocusAf = -300 to -700A). t0t2 WHITE AND HYDE: LEUCOPHOENICITE trfi r'.-.r'SrdiirlIG{|: Microtwinning :r.,:.., : :.disttibiiioa Potlllph Perymprph .''::,.:., ol.5i.r'-. s The electron diffraction pattern of a twinned crystal containsreflections