American Mineralogist, Volume63, pages1053-1073, 1978 Newbiopyriboles from Chester,Vermont: II. Thecrystal chemistry ofjimthompsonite, clinojimthompsonite,and chesterite, and the amphibole-mica reaction DevIo R. Vnsr-nNleNn CHnRlns W. BunNHepr Department of Geological Sciences,Haruard Uniuersity C ambridge, M assachus e t ts 02 I 38 Abstract The crystalstructures of jimthompsonite,clinojimthompsonite, and chesteritehave been solvedand refinedusing three-dimensional X-ray intensitydata. Jimthompsonite and clino- jimthompsonitecontain I-beams composed of triplesilicate chains and wide octahedral strips. In jimthompsonitethese l-beams are stackedlike those of orthopyroxenesand ortho- amphiboles,while the l-beamsof clinojimthompsoniteare stacked like thosein clinopyrox- enesand clinoamphiboles. Chesterite contains both double- and triple-chain I-beams alternat- ing in theb directionand assembled in theorthopyroxene-orthoamphibole stacking sequence. The unnamedmineral that occursas lamellaein chesteriteis presumablythe monoclinic analogof this structure. The new mineralsare very similar to other low-calciumpyriboles with respectto several crystal-chemicalattributes, including polytypism, chain rotation,chain warping,lamellar orientation,and cationordering patterns. Examination of theoutermost polyhedra of chain silicateI-beams suggests that A-chain rotationsin orthorhombicpyriboles are a necessary consequenceof edge-sharingbetween tetrahedra and octahedra. As intermediatereaction products,the new structuresdelineate 4 reactionpath from anthophylliteand cummingtoniteto talc.This amphibole-to-micamechanism is not simple, but ratherproceeds by reconstructionofdouble-chain I-beams to form alternatingdouble and triplechains, pure triple chains, and finallycontinuous sheets of silicatetetrahedra. Introduction thompsonite,and chesteritewill then be presentedin Of the many new mineralsdescribed each year, turn. Comparisonsof biopyribole bond distancesand mostare not silicates,and thosethat areusually bear angles, stacking, exsolution directions, chain warp- no clearrelationship to the importantrock-forming ing, cation ordering, and chain rotations will follow. mineralgroups. The fortuitousdiscovery of four new Particular attention is given to chain rotations in mineralsthat are intimatelyrelated to pyroxenes, orthorhombic pyriboles, becausethis topic has re- amphiboles,and micas is thusunusual (Veblen, 1976; ceived so much attention in recent years and remains Veblen and Burnham, 1975,1976; Veblen et al., a point of controversy. In the final section of this 1977),and the insightsinto biopyribolecrystal chem- paper we discussamphibole-mica reactionsin light istry that theyprovide are profound. A previouspa- of the new minerals. perof this seriesdescribed physical the andchemical Model derivations characteristicsof thenew minerals (Veblen and Burn- ham, 1978).This paperdescribes their crystalstruc- Idealizedpyribole l-beamspossess either mirror or turesand presentsa "comparativeanatomy" of the c-glidesymmetry. Those containing chains that could low-calciumbiopyriboles. be assembled from an even number of pyroxene We beginby describingthe developmentof struc- chains("even chains") are bisectedby mirrors paral- tural modelsfor the new biopyriboles.The crystal lel to (010), while those with chains that could be structurerefinements of jimthompsonite,clinojim- assembledfrom an odd number of pyroxene chains ("odd chains") are cut by c-glidesparallel to (010). I Present address: Departments of Geology and Chemistry, Both even- and odd-chained ideal I-beams further Arizona State University, Tempe, Arizona 85281. contain 2-fold axes passing through the M-sites and 0003404x/78/l r r2-1053$02.00 1053 1054 VEBLEN AND BURNHAM: BIOPYRIBOLES Table l. Crystal data and intensity measurementconditions Burnham, 1978,Table l). The similar intensitydistri- butions on O-levelb-axis precessionphotographs of Cl1no Jinthofrpsonlte j imthoEpsonite* Chesteri te chesterite,anthophyllite, and enstatite (Fig. I ) in- dicate that in projection down b thesestructures are Maxlnun crysEal 0.10x0.11x0.52 0.05x0.08x0-40 0.06x0- 10x0.35 closely related. On this basis alone the new mineral was assumedto be a pyribole. Analogous reasoning volume,** 0.0044 Crystal 0.0011 0.0013 was used by Warren (1929) and Warren and Modell Llnear absorption 21.0 2l.O 20.5 (1930) to solve the first amphibole structures from coefflcient, cm-1 their relations to the pyroxenes. Crystal axis for c c c data collectlon Of the spacegroups consistentwith the chesterite 20 scan speed lo/nln. Lo/rir. r /m1n. diffraction symbol (A21ma, Am2a, Amma), only Total backgtou.d 40 sec. 80 sec. 40 sec. countlng tine A2rma is acceptableas pyribole symmetry; mirrors parallel to (100) are inconsistent with pyribole I- * Clinojinthonpsonite crystal data refer Lo the entire cleavage beam topology and linkage operations. The metric fragment. Diffraction was fr@ two lanellae (about 102 of the f ragnent) in jinthompsonlte. requirements and l-centering of the cell, and the ** calculated by absorption correction program, taking account of presenceparallel to (010) of both mirrors and c-glide planes all bounding crystal, planes in the chosen space group, led to the con- clusion that the chesteritestructure must consistof parallel to the b-axis,but this rotational symmetry is both even- and odd-chainedl-beams, alternating in destroyed if the two chains of an I-beam differ in the 6 direction. There were two possibilities:mixed some way. Ideal odd-chained l-beams (triple, quin- single and quadruple chains or mixed double and tuple, etc.) are thus symmetricallyequivalent to ideal triple chains.The latter was consideredmore likely. pyroxene I-beams, while even-chained I-beams Soon after the development of the chesterite (quadrupfe, sextuple, etc.) have the same linkage model, jimthompsonite was discovered.With b = characteristicsas amphibole l-beams. These consid- 27A (Veblen and Burnham, 1978,Table I ) and space erations were instrumental in the formulation of group Pbca (the same as orthopyroxene), it was clear structural models for the Chesterpyriboles. that if the reasoningbehind the chesteritemodel was The first new mineral to be recognizedwas chester- correct, this mineral should consistof triple-chainI- ite. Like the orthopyroxenesand orthoamphiboles, beams arranged in the same fashion as the single- it was observedto have a= l8 and c= 5VqA,and its chain I-beamsof orthopyroxene.Like chesterite,jim- 45A b-axis is an integral multiple of 9A (Veblen and thompsonite closelymatched the orthopyribole (iOl) diffraction intensity distribution (Fig. l). Clinojimthompsonite and the unnamed mineral a* were later discoveredas lamellaein jimthompsonite and chesterite.The fact that their a dimensionsand B angles(Veblen and Burnham, 1978,Table I ) are close to those of low-calcium clinopyroxenesimmediately suggestedthat they are monoclinic polytypescontain- ing triple chains and mixed double and triple chains respectively. Experimental The nearly colorlesssingle crystals used for X-ray intensity measurementare elongated in the c direc- tion, as a result of their excellentprismatic cleavages. Approximate dimensionsof the crystalsare listed in Table l. Becausethe Chester pyriboles are inter- grown with each other, jt is not easyto obtain mono- Fig. l. O-levelD-axis precession photographs ofenstatite (En), phase crystals of a size suitable for data collection. anthophyllite(An), jimthompsonite (Jt), and chesterite(Ch). The similarities in intensity distribution between the four minerals The chesteritecrystal appeared to be truly mono- first led to the conclusion that chesterite and jimthompsonite are phase, but was small. The jimthompsonite crystal pyriboles. The vertical direction is a*, and c* is horizontal. gave extremely weak diffractions from anthophyllite VEBLEN AND BURNHAM: BIOPYRIBOLES 1055 and chesterite,and precessionphotographs showed minimumobservable intensity criteria were the same light streaks in the b direction, probably indicating as thosedescribed by Burnhamet al. (1971),except some chain-width disorder. During intensity mea- that the minimumobservable intensity for clinojim- surement, these streaks interfeied with (ftkO) reflec- thompsonitewas setat 3o7,rather than 2ot. Unob- tions with h/2 + k : 2n * l, which were observedto served reflections were excluded from the least- be systematicallyabsent on precessionfllms. These squaresnormal equations matrices in all cases.Initial structure factors were therefore removed from the scalefactors were obtained from a Wilson plot pro- data set, and structure factor calculationsusing the gram(Hanscom, 1973). refined structure later showed that none of theseF"s The three structureswere refined with the full- have a magnitude above the minimum observable matrix least-squaresprogram RnlNr (Finger,1969), level. The data for clinojimthompsonite were col- usingscattering factors given by Cromerand Mann lected from two (100) lamellae in a crystal of jim- (1968)for Mg2+,t"'+, Si'+, and O1-; anomalous thompsonite. Each lamella was about 2p wide, and dispersioncorrections from Cromer and Liberman they were in the same crystallographicorientation. (1970)were applied to the scatteringcurves during The diffractionsfrom this crystalwere sharp, with no the finalstages of therefinements. The small amounts evidenceof streaking. of Ca'+, Al3+, and Mn2* were ignoredin the site Intensity data were measuredusing a Picker Fe,cs- occupancyrefinements. I four-circle diffractometer and Nb-filtered