AmericanMineralogist, Volume63, pages 100A-1009, 1978 Newbiopyriboles from Chester,Vermont: I. Descriptivemineralogy Dnvn R. VnsLeNllNo CHlnr-rsW. BunNHenr Departmentof GeologicalSciences, Haruard Uniuersity Cambridge,M assachusetts02 I 38 Abstract Four new magnesium-ironchain-silicate minerals have beenidentified from a metamor- phosedultramafic body near Chester,Vermont. They occur with anthophyllite,cummingto- nite,and talcbetween the chloriteand actinolite zones at theboundary ofthe body.The cell parametersof the mineralsare diagnostic:(l) jimthompsoniteis orthorhombic,Pbca, a : 18.6,b:27.2,c:5.30A;(2)clinojimthompsoniteismonoclinic,C2/c,a:9.87,b--27.2,c : 5.32A,0: 109.5";(3)chesteriteis orthorhombic,A2,ma,a:18.6,b:45.3,c:5.30A; (4) an unnamedmineral is monoclinic,A2/m, Am, or A2, a : 9.87,b : 45.3,c : 5.294,B : 109.7o.The physicaland opticalproperties are closeto thoseof low-Caamphiboles. The cleavageangles (37.8' and44.7")are lower than those of amphiboles,and intergrowths of the mineralswith anthophylliteand cummingtonite are petrographically distinctive. The minerals are biopyribolesand are chemicallyintermediate between anthophyllite and talc.The ideal chemicalcomposition for jimthompsoniteand clinojimthompsonite is(Mg,Fe),oSi,roor(OH)n, and that of chesteriteis (Mg,Fe),rSiroOEl(OH)6.The new mineralsmight easilybe confused with amphibolesif theirelectron microprobe analyses were considered alone. Introduction Veblenet al., 1977).Physical and opticaldata, unit- Physical similaritiesamong pyroxenes,amphi- cellparameters, electron microprobe chemical analy- boles,and micasled Johannsen(1911) to call these ses,and X-ray powderdiffraction patterns calculated mineralgroups collectively the "biopyriboles."Solu- from the refinedcrystal structuresare reportedfor tion of the major biopyribolestructure types later three of the new minerals.Nomenclature for these showedthat the similaritieswere not fortuitous,but three well-characterizedminerals is also presented. ratherwere a directresult of structuralsimilarities. Only preliminaryunit-cell dimensions and possible Thompson(1970) has pointed out that most amphi- spacegroups are reportedfor the fourth mineral, bolescan be thoughtof as alternatingslabs of mica whichremains unnamed. andpyroxene structure cut parallelto (010)along the All four minerals,which are structurally and chem- ideal C2/m mica a-glide planes and pyroxene c- ically intermediatebetween anthophyllite and talc, glides.These M (mica)and P (pyroxene)slabs, when werefound duringexamination of singlecrystals by assembledwith the properoperations, form the MP X-ray precessionphotography, and the crystalstruc- double-chainamphibole structure. Thompson (1978) turesof threeof them havebeen solved and refined has also suggestedthat slab mixtureswith different using diffractometer-measuredX-ray diffraction mica-pyroxeneratios might be found. Our work con- data. Two of the mineralsconsist of triple silicate firmsthis predictionand reveals the biopyriboles as a chainsconnected by octahedralcation strips,while coherentmineral family, comprising several distinct the othertwo arethe first known mixed-chain silicate but closelyrelated structure types. structures,containing both doubleand triplesilicate This paperdescribes four new mineralsand their chains.The structuralcrystallography is describedin occurrencein a talc quarry at Chester,Vermont a subsequentpaper (Veblen and Burnham,1978). (Veblenand Burnham, 1975,1976; Veblen, 1976l' Identificationof the new mineralsis not simple, becausethey occur together as fine intergrowths. I Present address:Departments of Geology and Chemistry, Ari- Nevertheless,the informationprovided in this paper zona State University, Tempe, Arizona 85281. shouldenable mineralogists and petrologiststo dis- 0003-0Mx/ 78 /09 l 0- I 000$02.00 VEBLEN AND BURNHAM: NEII BIOPYRIBOLES 1001 tinguishthe new mineralsfrom otherbiopyriboles, and alsofrom eachother. Nomenclature Names for three of the new mineralshave been approvedby the InternationalMineralogical Associ- ation Commissionon New Mineralsand Mineral Names.The orthorhombicand monoclinicminerals with b axesof -27 A that containonly triple silicate chains are named jimthompsoniteand clinojim- thompsonite,after Professor James B. Thompsonof HarvardUniversity. The orthorhombicmineral con- tainingmixed double and triplesilicate chains with b = 45A is namedchesterite, for the localitynear Ches- ter, Vermont.Although what is presumedto be the monoclinicanalog of chesteritehas beenidentified, its occurrenceas very fine intergrowthsin chesterite precession photographs of enstatite (En), has so far precludedmeasurement of its physical Fig. l. 0-level a-axis anthophyllite (An), jimthompsonite (Jt), and chesterite (Ch), propertiesand confirmationof either its chemical showing differencesin D* length. compositionor crystalstructure. Followingthe usageof Johannsen(l9ll) and fractometer,while those of the unnamedmineral Thompson(1970), biopyriboles are minerals that can weremeasured from precessionphotographs. be representedas mixturesof (010) pyroxeneand Spacegroups of the mineralslisted in TableI were mica slabs;pyriboles are the subsetof biopyriboles determinedby examinationof extinctioncriteria on excludingthe micas.In this structuralclassification, long-exposureprecession films. The spacegroup of talc is consideredas a memberof the micagroup. jimthompsonite(Pbca) is uniquelydetermined, but for selectingone ofseveral Cell parametersand spacegroups theothers are not; reasons diffraction-equivalentspace groups for eachmineral The new mineralswere first distinguishedfrom arediscussed in conjunctionwith the structuralcrys- amphibolesby theirb celldimensions. Figure I com- tallographyin a subsequentpaper (Veblen and Burn- pares0-level a-axis precession photographs of ensta- ham.1978). tite, anthophyllite,jimthompsonite, and chesterite. The photographsshow the effectsof differingb-axis Occurrenceand associations lengthsand exhibit the following extinction criteria: k The new mineralsall occurin the blackwallzonez : jimthompsonite, 2n 1 l missingfor enstatiteand of a metamorphosedultramafic body that is exposed : and k * I 2n * I missingfor anthophylliteand in the Carletontalc quarry nearChester, Vermont. chesterite. The geologyof the quarry has beendescribed by Table I comparesthe cell parametersand space Giflson (1927), Phillips and Hess (1936), and Chides- groupswith thoseof anthophyllitefrom Chester.The ter et al. (1951).The generalizedzoning sequence jim- celldimensions of anthophyllite,chesterite, and from country rock to ultramaficbody is: (l) mus- thompsonitewere determined by least-squaresrefine- covite-quartz-garnetgneiss; (2) altered gneiss;(3) ment with 84,95, and 98 measurementsrespectively biotiteand chloriteblackwall; (4) actinolite;(5) talc; from precisionback-reflection Weissenberg films, us- (6) talc,magnesite, and serpentinite. ing the programLcI-sQ (Burnham, 1962) with sys- The materialused in this study is from a single tematiccorrection terms for absorption,film shrink- block of blackwallfound on the quarry dump, al- age, and cameraeccentricity errors. All data for thoughsimilar material has been found in placeon jimthompsonite chesteriteand were obtainedfrom the northern wall of the flooded quarry (Richard the samecrystals used for X-ray intensitymeasure- mentsby remountingthe crystalsto obtaina second 2 The term "blackwall zone" is commonly used to describe the orientation.The clinojimthompsonitecell parameters chlorite- or biotite-bearing metasomatic reaction zones that are were refinedby a least-squaresmethod using twelve frequently found at the boundaries of metamorphosed ultramafic diffractionsmanually centeredon a four-circle dif- bodies. VEBLEN AND BURNHAM: NEW BIOPYRIBOLES Table l. Cell parameters and space groups of low-calcium chain tremoliteto anthophyllite.At Chesterthe anthophyl- silicates. Chester. Vermont lite is replacedby chesterite,the unnamedmineral, jimthompsonite,clinojimthompsonite, and fibroirs Orthorhomblc Monocllnlc talc.The new mineralsare thus part of a retrograde reactionsequence from anthophylliteto talc. J inthonpsonlte C11noj iEthoDpsonlte a = 18.6263(3)A a = 9.874(4)A Habit, color, cleayages,and partings b = 27.2303(6)A b = 27.24(3)L The newminerals occur as intergrowths parallel to c = 5.2970(3)A c = 5.316(3)A (010) form v = 2686.6(2)A3 B = 109.47(3)' in anthophylliteand cummingtonite,and v = 1347.(3.)A3 radiatingsprays of prismaticcrystals up to 5 cm long. The anthophylliteand the new mineralsare trans- Pbca c2l c parentand havethe samecolor, ranging from color- z = 4[ (Mg,Fe)r's112032(oH)4] z = 2[ (Mg,Fe)10s112032(oH)41 lessto very light pinkishbrown. All are colorlessin thin section. Chesterlte Unnaned mlneral Cleavageangles were measuredwith a reflection goniometeron the singlecrystals used for X-ray in- a = 18.6140(3)A a = 9.867A perfect b = 45,305(I)A b = 45.31A tensity measurements.Chesterite possesses c = 5.2966(3)A c = 5.292A, {ll0} cleavageintersectinBat44.To and 135.3o,while v = 4465.8(3)A3 B = L09.7" jimthompsonitehas perfect{210} cleavage at 37.8o v = 2227.A3 and 142.2".In addition,both mineralsbreak along and but thesedirections may be partings, A2,u A2ln, An, or 42 {100} {010}, z = 4[ (Mg,Fe)rrs120054(ott)6] {100}resulting from breakagealong fine monoclinic lamellaeand {010}from separationalong lamellae of another orthorhombic pyribole. The cleavages Anthophylllte shouldbe valuable diagnostic properties in well-crys- a = 18.5863(2)A tallizedspecimens, but somematerial from Chester b = 18.0649(2)A