An Occurrence of a Modulated Serpentine Related to the Greenalite
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American Mineralogist, Volume 74, pages 637441, 1989 An occurrenceof a modulatedserpentine related to the greenalite-caryopiliteseries SrnpnnN GuccnNnprpr Department of Geological Sciences,University of Illinois at Chicago,Chicago, Illinois 60680, U.S.A. SruncBs W. Berr.nv Department of Geology and Geophysics,University of Wisconsin, Madison, Wisconsin 53706, U.S.A. Ansruct Transmission electron microscope (rnvr) and X-ray powder-diffraction studies indicate that material originally described as the Zn- and Mn-rich serpentine miheral baumite contains predominantly submicroscopic coherent intergrowths of 7-A and l4-A phases. The 7-A phasesinclude at least two polytypes (group A and either group B, C, or D) of lizardite and a modulated l: I layer silicate similar to those of the greenalite-caryopilite series.The l4-A phaseincludes a dominant chloite-Ibb structure.A chrysotile-like phase is present also, although it is rare. Semiquantitative chemical analysesindicate that all phases are Zn and Mn rich, but crystal-chemicalarguments are used to suggestthat the greenalite-caryopilite-like phase is relatively Al poor. The modulated l:l layer silicate difers structurally from greenaliteand caryopilite by having island-like domains of about 30 A (vs. 21.3-23.3 A for greenalite and 16.7-17.2 A for caryopilite). Accompanying veinlets appear to be lizardite-1Z altering to chlorite-Ibb and chloile-Iba, with these phaseschemically distinct from those more directly associatedwith the modulated l: I layer-silicatephase. Ixtnonucrrox over a foot in length, baumite comprised no more than 25o/obyvolume. The chamositeand baumite brecciahosts Frondel and Ito (1975) describedbaumite, a new ser- white willemite (crystallites 1 x I cm in size) and white pentine mineral, and "brunsvigite," a variety of chlorite, granular calcite, along with franklinite, acmite (:aegir- from specimensfound at the Buckwheat dump at Frank- ine), and stilpnomelane. The stilpnomelane is brown, lin, New Jersey.They also reported a "pennine" from a probably Fe rich, and varies in texture from fine to coarse franklinite ore specimen.These minerals contain unusu- grained. Locally, stilpnomelane may be enclosedby frie- ally large amounts of MnO (5.5 to 12.3 wto/o)and ZnO delite, which occurs as late-stagecrystals in fractures. (4.75 to 9.6 wto/o).Baumite occursas fine-grained,dense, Baumite is of special interest becauseit is possibly a black masses"up to a foot in diameter" and is brownish modulated structure as a result of its reported low Al yellow in thin section.The "brunsvigite" occursas green- content and its high tetrahedral Si and octahedral Fe * ish-black radial crystals in low-temperature hydrother- Mn + Mg * Zn. Samplesstudied (Harvard Museum no. mal veinlets crossingthe baumite. The "brunsvigite" and 114072:type material labeled "baumite" and an unnum- "pennine" describedby Frondel and Ito are more prop- bered specimen labeled "brunsvigite" from J. L. Baum) erly designatedmanganoan zincian chamosite and clino- did not contain any of the clinochlore crusts.The present chlore, respectively,based on the reported chemical anal- paper concludesthat both the baumite and the chamosite yses.Both chloritescontain substantialamounts (13.1 to are submicroscopic, coherent intergrowths of 7-A and l4-A 14.0 wto/o)of alumina, and baumite apparently contains phases plus small amounts of other materials. It is lesseramounts (6.60 wto/o).Bayliss (1981) argued that noted, furthermore, that the material does contain a new baumite should be redefinedas a manganoanferroan liz- mineral similar to those of the greenalite-caryopilitese- ardite-lT, after he confirmed that the powder pattern as ries. However, this mineral cannot be documented ade- presentedby Frondel and Ito could be indexed on a 1T quately to give it new speciesstatus becauseit is inti- cell. This suggestion,however, was not approved by the mately mixed with other phases. IMA (E. H. Nickel, personalcommunication, 1988). Unpublished work (P. Dunn, personalcommunication, ExpnnrrvruNTAl, METHoDS AND INTERpRETATIoN 1988) indicated that a clinochlore does occur associated with baumite, but this material has not been studied. Baumite Dunn re-examined the original material and found X-ray study. Over 40 Debye-Scherrerpatterns were ob- baumite to occur as a breccia cement in a low-tempera- tained from different submillimeter-sized particles of ture cavity or vein filling. Although the original masswas baumite. Most particles were cut from thin sectionsso as 0003-004x/89/0506-0637$02.00 637 638 GUGGENHEIMAND BAILEY: MODULATED SERPENTINE Fig. l. Lattice fringes in optical (rur,r) images show 7-A and 14-A phasescommonly intergrown as aligned plates on the (001) plane. ChrysotileJike grains (note ringJike feature)were rare. The upper portion ofthis photograph,therefore, is more representative of the sample than the lower portion. to include only microscopically homogeneousmaterial. series or a physical mixture of 7 A and 14 A phases. The particles were mounted on glass fibers, centered in Support for both interpretations can be found in the ac- 114.6-mm-diameter film cameras,and exposedto filtered companying rrrvr study. A few difraction patterns show FeK" X-radiation. even-order00/ reflectionsas doublets,apparently because All patterns contain a strong 7-A diffraction line and a of compositional diferences between the 7-A and l4-A medium-intensity l4-A Hne. The ratio of the two inten- phaseswithin someparticles. Nearly all patternsalso show sities is approximately constant in patterns from most diffraction lines of strong to medium intensity at 1.54 A particles, although in some cases,the 7-A hne is en- and 1.60 A, which can be indexed as douoof chlorite and hanced.The non-00/ lines show considerablesimilarities douoof a greenalite-caryopilite-like phase, respectively. between diffraction pattems also, but with some differ- Heating particles at 500 "C increasesthe intensity of the encesin the weaker lines. Two patterns contained a line l4-A hne and decreasesall other lines. at about 9.5 A that is interpreted as indicating the pres- Transrnissionelechon microscopy(TEM) study. Sam- enceofa talc-like phase. ples of baumite were prepared by standard ion-thinning The l4-A hne indicates that chlorite is a major con- techniquesusing Cu grid supports. Also, powder mounts stituent of baumite, but the structural type could not be were made by dispersingfinely abradedmaterial on holey identified becauseofthe overlap with the patterns from C film supported on a nylon grid. Material was examined one or more 7-A phases.It was observed that the even in a rnol roocx microscope with a Tracor Northern Bps orders ofthe l4-A spacing (001 /: 2n) are consistently attachment. stronger than the odd orders. Calculations show that no The ion-thinned grids showedthe variable morphology distribution of heavy and light atoms between the octa- and textures of the mineral assemblagesof the samples. hedral interlayer sheetand the octahedralsheet ofthe 2: I Although most of the grains have platy morphology, some layer of a chlorite structure can account for the ob- sample areas exhibit chrysotileJike rolls (Fig. l). Most servedintensity ratios. For example,increasing the heavy platy grains are intergrowths of 7-A and, l4-A phases atom content ofthe interlayer decreasesthe 001 intensity aligned parallel to the (001) planes. In one case,a 2l-A relative to 002, but increasesthat of 003. The observed repeat was observedin the diffraction pattern, suggesting intensities require either an interstratified 7- and l4-A a regularity in intergrowth at the unit-cell level. This phase, GUGGENHEIM AND BAILEY: MODULATED SERPENTINE 639 however, was extremely rare. Unlike the X-ray study, no phasewith a 9.5-A spacingwas noted by rnvr. Electron-diffraction patterns ofgrains showing a platy morphology and containing c* characteistically show the superposition of two nets. One net could be attributed to a l4-A chlorite and the other to a 7-A greenalite-cary- opilite-like serpentine. Like greenalite-caryopilite (cf. Guggenheimetal.,1982, Fig. 5C), this 7-A phasehas a modulation that producessatellite reflectionsadjacent to the reflections normally observedin serpentinesof ideal l: I structure, thereby making identification definitive. In zt,t all caseswhere the 00/ diffraction patterns were compos- ites, the l4-A reflectionseither superimposedirectly over the reflections of the 7-A phase or fall exactly halfway between.Therefore, any differencesin chemical compo- sition betweenthe 7-A phaseand the l4-A phaseare not observed by comparing c-axis spacings.Reflections on 0kl-type nets with k + 3n are generally streaked along l the row lines because of random layer-stacking se- I v* quences. A For one grain, however, the diffraction pattern showed Crl sharp k + 3n reflections superimposed on the streaks. The modulated 7-A phaseis dominant in this grain, and Fig. 2. Electron-diffractionpattern showing the superposi- it is apparent that the lateral dimensions of this phaseare tion of two hol-typenets, one 7-A (lizarditeof groupB, C, or : largerthan thoseofthe l4-A phase.The l4-A 00/ reflec- D) andone 14-A(chlorite- Ibb) phase.Both phases have B 90'' : : tions could be removed from the pattern by tilting the c chlorite,I lizardite. sample stage,thereby allowing the identification of the polytype of the Z-A modulated phase. The positions of the sharp k + 3n reflections require a oneJayer c repeat only be characterizedas a member of groups B, C, or D with B : 90o,and this uniquely identifies the 7-A mod- (Bailey, 1988), with group A serpentineeliminated as a ulated phase as related to the 1f polytype of group C. possibility. This is the structure on which the modulated greenalite One observed h)l pattern is an exception in that the superlatticeis based(Guggenheim et al., 1982).The over- chlorite can be indexed on a lattice with B : 97oand not lap of reflectionsprevented the accuratedetermination of on a lattice with B : 90'. Overlap on this pattern with lateral cell dimensions a* and D*, but such determina- reflections from the intergrown 7-A phase prohibits the tions were possible from the hk}-type patterns (see be- use ofthe ftOl intensities to characterizethe chlorite fur- low).