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 ,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 ,and intergrowths of the mineralswith anthophylliteand are petrographically distinctive. The minerals are biopyribolesand are chemicallyintermediate between anthophyllite and .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 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 precessionphotographs of enstatite (En), has so far precludedmeasurement of its physical Fig. l. 0-level a-axis anthophyllite (An), (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 .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 -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} 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 consistsof severalminerals so finelyintergrown that c = 5.2895(4)A the cleavageis not apparent;these specimens are v = 1776.0(1)A3 oftenfibrous. PM Structurally,the chesteriteand jimthompsonite z = 4[ (Ms,Fe) 7S18O22(OH)2] cleavagesare analogousto the {210}cleavage of the orthopyroxenesand orthoamphiboles.Because the monoclinicpolymorphs from Chesteroccur only as Sanford,personal communication). The block may lamellae,their cleavagescould not be observed,but bea sectionof a continuousblackwall zone, or it may on structuralgrounds one would predict a {120} be part of an isolatedpod. The simplifiedzoning cleavagefor the unnamedmineral and a {110}cleav- sequenceobserved in this block is: (l) chlorite;(2) agefor clinojimthompsonite. fibroustalc; (3) fibroustalc, jimthompsonite, clino- properties jimthompsonite,chesterite, the monoclinicanalog of Optical chesterite,anthophyllite, and cummingtonite;(4) Optical data for jimthompsonite,chesterite, and anthophyllite,cummingtonite, chesterite, the mono- anthophyllitefrom Chesterwere obtained by usinga clinic analogof chesterite,and actinolite;(5) acti- spindlestage (Wilcox, 1959).The jimthompsonite noliteand massivetalc. Magnetite is foundthrough- andchesterite crystals were the sameones used for X- out asan accessorymineral, and cummingtonitehas ray intensitymeasurement. The crystalswere first been observedas lamellaein actinolite and an- mountedwith theirb axescoincident with the spindle thophyllite. axis, preciseorientation being achievedby directly Actinoliteand anthophyllitecommonly occur as remountingthe crystals from theirX-ray goniometer- orientedintergrowths on planesnear (100), attaining head mounts.Optic axial angles(2V,) for sodium lengthsof 5 cm.Three similar occurrences of tremo- light weremeasured directly by spindlerotation, and lite and anthophyllitefrom the Gouverneurmining dispersionwas observed in blue-filteredlight from an districtin New York havebeen interpreted by Rosset incandescentbulb. Principal indicesof refraction al. (1968) as representingretrograde alteration of werethen measured for sodiumlight by mixingindex VEBLEN AND BURNHAM: NEW BIOPYRIBOLES 1003 oils until a matchwas obtained (Becke line method); Table 2. Optical properties of anthophyllite,chesterite, and the indexof the matchedoil was measuredwith an jimthompsonitefrom the Chesterwall zone Abb6 refractometercalibrated with oils matchedto Jirthonpsonite n(fluorite)and <.r(quartz).The crystalswere then re- Anthop[yll1te Chesterite mountedwith theirc axescoincident with the spindle = c= L.620 d = r.ofI d f.ou) axis,and the indiceswere checked. Optic axialangles a- 1.530 B = r.632 B = r.626 = y = I- 1. 5l+r Y r.5l+o r.533 calculatedfrom the indicesof refractionagree with & X=a the observedvalues of 2V, withinthe estimated preci- c Z= c Z= e 2\I 2''I = 62o sion of the measurements,The consistencyof these x x measurementswas further checked by measuringre- NegatiYe Negative r>v, veak r>v, veah tardation ratios of the componentsof multiphase grainsin thin sectionwith a Berekcompensator. Estirated errors: refr. ind.: lO.OOO5, 2V: 12o Optical properties(Table 2) canbe usedto differ- entiatebetween anthophyllite, chesterite, and jim- jimthompsoniteare intergrown on a scaletoo fineto thompsonitefrom the Chesterwall zone,but com- be optically resolved,or that there are still more parison of these data with the numerous structuralordering schemes or areasof disordered determinationsfor anthophyllitegiven by Rabbitt chain sequence.Most grains,however, are optically (1948)indicates that opticalmethods are probably uniform,and intergrowthsusually exhibit optically- not the bestdeterminative means for distinguishing sharp boundariesbetween anthophyllite, chesterite, theseminerals at other localities.Anthophyllite and andjimthompsonite. gedriteshow a widerange of opticalproperties, and the new mineralswill probablyshow similarvaria- Chemicalanalysis tionswith compositionalchanges. The resultsof electronmicroprobe analyses in- Clinojimthompsoniteoccurs only as thin lamellae dicatethat the major-elementcompositions of the andwas not suitablefor opticalmeasurements. How- new mineralsare intermediatebetween those of an- ever,the extinction angle NAc = 10owas measured. thophylliteand talc. Owing to the intergrownnature The new mineralsclosely resemble amphiboles in of the newminerals, pure samples could not besepa- thin section.In (001)sections the lowercleavage an- ratedfor wateranalysis. glesare revealed in someinstances, but thecleavage is Analyses(Table 3) were performedon an auto- oftenindistinct or fibrous.The minerals usually occur matedMAC Model5 electronmicroprobe. All speci- togetherin (010)intergrowths, however, leading to a mensand standardswere carbon-coated. Operating strikingappearance under crossed polars for crystals voltagewas l5 kV, with a referencecurrent of 300 with b near the plane of the section(Fig. 2a-c)s. mA. A diopside-jadeiteglass was usedas the stan- Becausepyribole exsolution features are usuallyre- dardfor Si,Al, Mg, Ca,and Na; naturalaenigmatite, strictedto planesnear (100) and (001),there should ilmenite,forsterite, chromite, rutile, and orthoclase be little difficultyin recognizingthese intergrowths; wereused for Fe,Mn, Ni, Cr, Ti, and K, respectively. anthophyllite-gedriteexsolution on (010) could, Countingtime was20 secondsfor both background however,cause confusion. When the intergrowths andpeaks; counting was stopped when the number of haveb inclinedmore steeply, but not normal,to the counts exceeded30,000. Compositions were cor- sectionplane, they can take on a softly stripedap- rectedby the alpha-matrixmethod of Benceand Al- pearance(like a multi-coloredcandy cane) in crossed bee (1968),using alpha factors calculatedas de- polars(Fig. 2d). scribedby Albeeand Ray (1970). Portionsof somegrains possess interference colors The microprobeanalyses show that the Chester intermediatebetween those shown by anthophyllite, pyribolescontain Mg, Fe, Mn, Ca, Al, Si, and Na; chesterite,and jimthompsonite,resulting in streaks the elementsNi, Cr, Ti, and K are not presentin parallelto (010)(Fig. 2a). These intermediate optical measurableamounts. Energy scans performed with propertiessuggest that anthophyllite,chesterite, and an Eonx 707A energy-dispersiveanalyzer on a Cam- ecaMS46 microprobeconfirmed the absenceof sig- nificantamounts of unexpectedelements. The jim- e photographof suchan intergrowthis on the cover A color of analysisis the averageof 1l analyzed Science,October 28, 197'l(Veblen et al., 1977).This photograph thompsonite also showsthe cleavageangles of anthophyllite,chesterite, and points on the samecrystal usedfor X-ray intensity jimthompsonite. measurement,plus three points from optically-identi- 1004 YEBLEN AND BIJRNHAM. NEW BIOPYRIBOLES

Fig. 2. Chesterbiopyriboles in thin section.Scale bars represent0.1 mm. (a) Sectionparallel to (001) olgrain consistingmostly of anthophyllite.The anthophylliteis beginningto transform to chesterite,producing streaksparallel to (010).The straightcrack parallelto (100) (arrowed) is probably a remanent "herringbone" twin plane, indicating that this grain was originally a monoclinic crystal.Crossed polars. (b) A (010) intergrowth of anthophyllite and chesterite,with the b axis near the plane of the section.Crossed polars. (c) A sharp (010) intergrowth of anthophyllite,chesterite, and jimthompsonite.The b axis is in the plane of the section,but no cleavageis visible.Crossed polars. (d) A (010) intergrowth of the pyriboleswith the b axis near the sectionnormal. Crossedpolars.

fied grains in polished thin section. The clinojim- ideal chemical formula for chesterite is M,rSi2o thompsonite analysisis the averageof four points on O,n(OH)6,and that for jimthompsonite and clinojim- one of the lamellae in the crystal used for intensity- thompsonite is MroSirrOrr(OH)n,where M refers to data collection; becausethe lamella width (2p) is Mg and Fe'+. The chesteriteformula is thus the sum narrower than the volume activated by the electron of the anthophyllite formula, M?SisOrr(OH)r,and the beam, this analysisis undoubtedly "diluted" by jim- jimthompsonite formula. Figure 3, a ternary plot of thompsonite, which is the host mineral. The chester- octahedral cations us. tetrahedral cations us. HOo.r, ite analysisis the averageof l4 points in thin section. shows that the new pyriboles ideally are chemically In addition, anthophyllite, foliated talc, and acti- intermediatebetween anthophyllite and talc and are nolite grains from the blackwall zone were analyzed, collinear with enstatite,anthophyllite, and talc. and the results given in Table 3 are averagesof 17 The analysesshow that small amounts of Mn and points from anthophyllite, five from talc, and three Ca substitutefor Mg and Fe; the Mn may be respon- from actinolite. sible for the light pinkish color, observablein hand The averageanalyses of chesterite,jimthompson- specimen, of the low-Ca pyriboles. Only clinojim- ite, and clinojimthompsoniteare consistentwith the thompsonite appears to contain octahedral Al, but refined structures(Veblen and Burnham, 1978).The the quality of the analysesmay not be good enough VEBLEN AND BURNHAM NEW BIOPYRIBOLES 1005 to rule out the presenceof someoctahedral Al in the otherminerals; small amounts of Al replaceSi in the tetrahedralsites of all thesepyriboles. If it is assumed that the Na shownin the analysesis restrictedto the A sitesof the minerals(which are analogousto the amphiboleA site),then thesesites in the analyzed anthophyllite,chesterite, jimthompsonite, and clino- jimthompsoniteare approximately 1,2/3,2 l/2, and 2 percentfilled respectively. The recalculationprogram of Brady (1974),used with analogousto thoseuti- chemicalassumptions Fig. 3. Chemistry of the Chester biopyriboles. The ideal lizedby Stout(1972) for the amphiboles,shows that compositions, derived from crystal structures and averaged Fe3+is not a majorcomponent of the pyribolesfrom electron microprobe analyses, are represented here on a molar Chester.When the averagedmicroprobe analyses of MO-SiOr-HOos ternary diagram, in which M representsdivalent Table3 arenormalized to the appropriatenumber of octahedral cations (primarily Mg and Fe) and water is given as HOo In this representation, enstatite, anthophyllite, chesterite, ". oxygens,and when it is assumedthat all Na is re- jinithompsonite, clinojimthompsonite, and talc are composition- strictedto A sites,the resultingFe3+/(Fe3++Fe2+) ally collinear. ratios are 0.04 in anthophyllite,0.00 in chesterite, 0.04 in jimthompsonite,and -0.05 in clinojim- minerals,and the clinojimthompsoniteaverage was thompsonite. calculatedfrom only four analysispoints; the SiO, From the averagedanalyses, there is no obvious differenceand other differencesare probably not sta- chemical differencebetween jimthompsonite and tisticallymeaningful. Because anthophyllite and cum- clinojimthompsonite.The largestapparent differ- mingtonitecan apparentlyoverlap in composition encesare in SiO, and AlrOr, but thereis overlapin (Rice el al., 1974),it is not surprisingthat jim- the valuesof individualanalysis points from the two thompsoniteand clinojimthompsonite can be compo- sitionallyundifferentiable. yieldedthe averages of Table 3. Average electron microprobe analyses of biopyriboles Theindividual analyses that from the Chester blackwall zone Table3 exhibitso,me scatter. The ratio(Si+Al)/(Si+ Al*Mg*Fe*Mn*Ca) isthe ratio of tetrahedralcat-

Anth ionsto tetrahedralplus octahedral cations, assuming that all Al is tetrahedral,that thereis no tetrahedral 56.15 57.95 ,7.78 '8.5' 6r. 3o )o.yo AfZ03 0.2\ 0.25 o.29 0.37 0.15 1.07 Fe3+,and that the Na is restrictedto the A site.A *FeO 16.03 tl+.11+ rz.2z 12.13 6,28 5.\9 I',1I1O 1.r5 O.99 o.72 0,7: 0.06 0.37 plot of this ratio for all the individualanalyses (Fig. l{so 23. !! 24.2t1 25.14 2\.93 28.00 considerableoverlap for anthophyllite, Ca0 0. 50 0, )+2 0.38 0.50 0.02 11.99 4) exhibits Na20 0.0l+ 0. 03 o.r2 o.1o o.ol+ jimthompsonite,and clinojimthompson- **H n chesterite, 2.r2 2.60 z.ez z.YJ 4.of 2.16 ite. It is not knownhow muchof the scatter,if any,is Total r00.07 r00,62 99.57 100.24 r00.1+5 roo.5r due to real compositionalvariations, and how much is the resultof analyticalerror. In spiteof the scatter, N@bers of Cations on the Basis of NOX OJrygens however,it is clearthat the new mineralsare more Nox 23 '7 34 34 22 23 silica-richthan anthophylliteand lessso than talc. Tetrahedral: si 7.96 19.9\ 11.91 1r.97 7.93 7.8't Becausethe new pyribolesare intermediatereaction At o.oU o.05 0.07 0.03 0.02 0,13 Total 8.00 19.99 rr.9g 12, oo 1 .g5 8.oo productsbetween anthophyllite and talc, it is con-

Octahedra"l: ceivablethat some of the chemicalscatter is real. Fe I. 89 )+.07 2.11 2.07 0,68 Variationsin the tetrahedral/(tetrahedral* octahe- I{n 0.11+ O.29 0.13 0.13 0.01 o.o)+ !E \.92 12.13 7.73 7.59 5.1+0 4.40 dral) ratio could resultfrom stuffingcations which Ca 0.08 0.15 0.08 o.l1 0.00 7.77 from A-1 0. 00 0. 00 0.00 0.05 0.00 0.04 are normally octahedralinto the A sites,or Tota.t 7.o3 16.95 10.05 9.9j 6.09 7.00 creationofoctahedral vacancies during the reactions. LartF ^ati^nc' Submicroscopiclamellae of other biopyribolescould Na O.01 0.02 o. 05 o. ob o. oo 0.06 alsocontribute significantly to theanalytical scatter.

*A11 was calculated F.2*, It is well established(e.g. Papike and Cameron, "" **Ihe uout of vater vas c€lculated. by assuing alf OH sites to 1976)that Fe, Mn, and Ca are usuallyconcentrated be filf ed {ith OHl- (Veblen @d Blmhu, 19?8 ). over Mg in the outer distortedM sitesof pyriboles, 1006 VEBLEN AND BURNHAM: NEW BIOPYRIBOLES

An rrlll.r lar to the distributionthat one would achieveby randomly analyzinga mixture of theseminerals, and becausethere are no obviousmaxima in the distribu- Ch .,lf, ... tionsan unwaryanalyst might easily explain them as compositionalvariations in a singlephase. The total JI distributionin Figure 4 is, however,clearly more silica-richthan that to be expectedfrom ideal an- thophyllite;that observationalone might alert the cjt . . 1.. petrologistto the presenceof phasesintermediate betweenthe amphibolesand the micas. Tc On balance,chemical data should prove most valu- able when the mineralspresent have alreadybeen At ',' identifiedby other means.The ambiguityof the mi- croprobedata emphasizesthat thesenew minerals must be definedprimarily on structuralrather than lowCo r,tlt.hll.l.rlrr.r . .r ' chemicalgrounds. Similar phaseswith significant amountsof Al would be evenharder to distinguish chemically,because the tetrahedral/octahedralratio ttttl tl o.52 0.54 0.56 o.58 for Al is determinedfrom microprobedata by usinga structuralassumption. Fig. 4. (Si+Al )/(Si+Al+ Mg+ Fe* Mn*Ca) ratios of individual electron microprobe analyses of anthophyllite, chesterite, jimthompsonite, clinojimthompsonite, talc, and actinolite from Powderdiffraction Chester. The number of analyses with a given ratio (rounded to Powder diffraction profiles for chesterite,jim- 0.001) is indicated by the height vertical of the line segments;the thompsonite,and clinojimthompsonitewere calcu- shortest lines represent one analysis. ldeal ratios, based on structural formulas, are indicated by the vertical line segments lated with the computerprogram Powo5 (Clark et below the horizontal lines. The line marked "low Ca" shows all the al., 1973),the sameprogram usedto calculatepat- analyses,excluding those of actinolite. The observed ratios are not terns for Geol. Soc. Am. Memoir 122 (Borg and evenly distributed about the ideal amphibole ratio. Smith, 1969).Using refinedatomic positions,iso- tropic temperaturefactors, and occupancies(Veblen andBurnham, 1978), the patterns were calculated for whileMg is concentratedin theinner more regular M CuKa radiation,assuming peak half-widths of 0.1l' sites.It might be expected,then, that the ratio Mgl 20 at 40o 20 and a scan speedof 2" 23 per inch. (Mg+Fe+Mn*Ca) wouldincrease fromanthophyl- Summariesof peaks with integrated intensities lite to chesteriteto jimthompsonite whenthese miner- greater than I percent of the largest intensity for alscoexist, because the ratio ofregularto distortedM chesterite,jimthompsonite, and clinojimthompsonite sitesincreases. Figure 5 showsthe Mgl(Mg*Fef Mn*Ca) variationsexhibited by theindividual probe analyses,and, even though there is significantscatter, An the expectedincrease in Mg over other octahedral ch cationsis clear. JI In spite of radical structural differences,an- thophyllite, chesterite,and jimthompsonitehave cjl ideal chemicalcompositions that are very closeto Tc eachother. Given the scatter demonstrated in Figures lowCo 4 and 5, whetherit be from analyticalerrors or real compositionalvariations, it wouldbe very difficult to rttttt differentiatethese pyriboles unambiguouslyfrom 0.65 0.70 0?5 0 80 0.85 0 90 chemicalmethods alone. The bottomlines of Figures Fig.5. TheMgl(Mg+Fe+Mn*Ca) ratiosof individualelectron 4 and 5 show the distributionof the tetrahedral/ microprobeanalyses of biopyribolesfrom Chester.Presentation is (tetrahedral* octahedral)and Mgl(Mgf Fe*Mn* the sameas for Fig. 4. Thereis a significantdegree of scatterin the analyses,but it can be seenthat in going from anthophylliteto Ca) ratios for all of the analyzedpoints, excluding chesterite to jimthompsonite and clinojimthompsonitethe thosefor actinolite.These total distributionsare simi- mineralsbecome increasingly magnesian. VEBLEN AND BURNHAM: NEW BIOPYRIBOLES 1007

Mhillitc dnthmpsonnc

Chcsr€ri'" Cilnojlmthompsonn.

Fig. 6. Calculated X-ray powder-diffraction profiles of anthophyllite (Finger, 1970), chesterite, jimthompsonite, and clinojimthompsonite. Patterns were calculated using refined atomic coordinates, isotropic temperature factors, and occupancies(Veblen and B urn ham, 1978). The patterns simulate random-orientation diffractometer traceswith Cui(a radiation, a peak half-width of 0.1I' 20 al 40o 20, and a scan speed of 2o per inch. The numbers beneath each trace are oN for CuKa. are listedin Table44. The powderdiffraction patterns Table 4 and Figure 6 suggestthat X-ray powder areplotted in Figure6, whichalso shows anthophyl- diffractioncould be usedto differentiatebetween the lite for comparison.The summarytables are in the variousbiopyriboles from Chester.The peaksbelow sameformat as that usedfor Geol.Soc. Am. Memoir 15" 20 (CuKa) may be particularly useful,because 122: 2THETA and D give the 20 and interplanar they are very intense,but higher-anglepeaks are re- spacingfor reflection(HKL), and I(INT) and I(DS) quired to separatejimthompsonite from clinojim- give the uncorrectedintegrated intensity and in- thompsonite.The calculatedpowder-diffraction pro- tegratedintensity corrected for Debye-Scherrerab- files might be usefulfor identifyingfine-grained run sorption,assuming that the peak can be resolved; productsof petrologicexperiments if the phasesare PEAK and I(PK) refer to the peak positionsand presentin largeenough amounts. It must be remem- peakheights of the patternproduced by summingall beredthat thesecalculated patterns assume random the diffractions,after eachhas beengiven a Cauchy orientationof the specimen,which will be difficultto profilewith the input half-width. achieve,given the prismaticor fibroushabit of all the minerals.Real intensitieswill differ from thesecalcu- lated onesbecause of preferredorientation. aTo obtaina Table4, copyof order DocumentAM-78-078 from Powder-diffractionphotographs of a mixture of the BusinessOffice, MineralogicalSociety of America, 1909 K Street,N.W., Washington,D.C. 20006.Please remit $1.00in the Chesterpyriboles estimated to contain60 percent advancefor the microfiche. anthophyllite,20 percentchesterite, and 20 percent l 008 VEBLEN AND BURNHAM: NEIII BIOPYRIBOLES jimthompsonite are significantlyless diagnostic than Smith are gratefully acknowledged. Financial support for this re- the powder calculations.A Debye-Scherrerfilm was searchwas provided by NSF grant GA-41415 to CharlesW. Burn- unable to resolvethe low-anglepeaks, The only lines nam. on a photograph Guinier that could be unambig- References uously assignedto minerals other than anthophyllite were the (200), (220), (620), and (2. 13.l of chester- Albee, A. L. and L. Ray (1970) Correction factors for electron ) probe microanalysis of silicates,oxides, carbonates,phosphates, ite atd: 9.35,8.59,3.08,and2.78A respectivelyand and sulphates.Anal. Chem.,42, 1408-1414. the (210) and (610) of jimthompsoniteat 8.82 and Bence, A. E. and A L. Albee (1968) Empirical correction factors 3.104. Small lamellae of the new minerals in an- for the electron microanalysis of silicates and oxides. J. Geol., thophyllite would be very difficult to recognize by 76, 382-403. power-diffraction techniques. Borg, L Y. and D. K. Smith (1969) Calculated X-ray powder patterns for silicate minerals. Geol. Soc. Am. Mem. 122. Summary: identification of the Chester pyriboles Brady, J. B. (1974) Coexisting actinolite and hornblende from west-central New Hampshire. Am Mineral., 59, 529-535. Although differences among the low-calcium Burnham, C. W. (1962) Lattice constant refinement. CarnegieInst. biopyriboles from Chester have been demonstrated Wash Year Book, 61, 132-135. optically, chemically,and by powder diffraction, it is Chidester, A. H., M. P. Billings and W. M. Cady (1951) Talc investigations in Vermont preliminary report. U.S. Geol. Sun:. likely that positiveidentification of the new minerals Circ. 95. at other localities will require single-crystalX-ray Clark, C. M., D. K Smith and G. G. Johnson(1973\ A FortranIV techniques.Optical study may prove difficult, owing Program for Calculating X-ray Powder Pattents-Version 5. to the intergrown nature of the minerals, and the Rep. Dept. Geosciences,Pennsylvania State University. (1970) effectsof chemicalsubstitutions on the optical prop- Finger, L. W. Refinement of the of an anthophyllite. Carnegie Inst. llash Year Book, 68, 283-288. ertiesare not known. The intergrowthsare distinctive Gillson, J. L. (1921) Origin of the Vermont talc deposits,with a in thin section,however, and the cleavagesofthe new discussionon the formation of talc in general.Econ. Geol.,22, minerals when observedin non-fibrous occurrences 246-28s. may be seento intersectat angleslower than those of Johannsen,A. (l9ll) Petrographicterms forfield use.J. Geol., 19, 3t7-322. amphiboles. Chemically, the new minerals are all Papike,J. J. and M. Cameron (1976)Crystal chemistryof silicate very close to anthophyllite and cummingtonite, and minerals of geophysical interest. Reu. Geophys.Space Phys , 14, electron microprobe analysesshow too much scatter 37-80. to be definitive. Powder diffraction is useful only Phillips,A. H. and H. H. Hess(1936) Metamorphic differentiation when the minerals are individually present in large at contacts between serpentinite and siliceous country rocks. amounts. Am. Mineral.. 21. 333-362. Rabbitt, J. C. (1948)A new study of the anthophylliteseries. lz. The new mineralscan easilybe recognizedby their Mineral.. 33.263-323. distinctiveX-ray precessionphotographs, which pro- Rice, J. M., B. W. Evans and V. Trommsdorff(1974) Widespread vide both cell-parameterand symmetry information, occurrence of magnesiocummingtonite in ultramafic schists, and by electron diffraction patterns. Even the com- Cima di Gagnone, Ticino, Switzerland. Contrib. Mineral Pet- ponents of fine multiphaseintergrowths can be iden- rol., 43, 245-251. Ross,M., W. L. Smith and W. H. Ashton (1968)Triclinic talc and tified by thesetechniques. Single-crystal methods are, associated amphiboles from Gouverneur mining district, New in fact, the only means by which the lamellar mono- York. Am. Mineral., 53,751-769. clinic phasescan be recognized.Identification and Stout, J. H (1972) Phase petrology and mineral chemistry of study of thesenew mineralscan bestbe accomplished coexisting amphiboles from Telemark, Norway. J. Petrol., 13, by using a combination of X-ray, optical, and chem- 99-t45. Thompson, J. B. (1970) Geometrical possibilitiesfor amphibole ical methods, and not restricting oneself to a single structures: model biopyriboles. Am. Mineral , 55,292-293. technique. - (1978) Biopyriboles and polysomatic series.Am. Mineral., 63,239-249. Acknowledgments Veblen, D. R. (1976) Triple- and Mixed-chain Biopyriboles from We wish to give special thanks to James B. Thompson for his Chester, Vermont. Ph.D. Thesis, Harvard University, Cam- insight on many mattersrelating to biopyriboles.Discussions with bridge, M assachusetts. James F. Hays, Cornelius S. Hurlbut, Clifford Frondel, John B. - and c. w. Burnham (1975) Triple-chain biopyriboles: Brady, Richard Sanford, and Kenneth Shay were also helpful. newly discovered intermediate products of the retrograde an- Peter Buseck and John Armstrong of Arizona State University thophyllite-talc transformation, Chester, Vermont (abstr.). provided energy-dispersivemicroprobe analysis.This paper repre- Trans. Am Geophys Union,56, 1076. sentspart of a Harvard University Ph.D. Thesis (Veblen, 1976). - and - (1976) Biopyriboles from Chester, Vermont: the Critical reviews by Cornelis Klein, James J. Papike, and Joseph V. first mixed-chain silicates(abstr.). Geol. Soc. Am. Absnacts with VEBLEN AN,D BURNHAM: NEW BIOPYRIBOLES r009

Programs,8,1153. Wilcox,R.E.(1959)Useof thespindlestagefordeterminationof - a1d - (1978)New biopyribolesfrom Chester,Ver- principal indicesof refractionof crystalfragments. Am. Min- monl II. The crystal chemistryof jimthompsonite,clinojim- eral.,44, 1272-1293. thompsonite,and chesterite,and the amphibole-micareactioi. Am. Mineral.,63,in press. -, P. R, Buseckand C. W. Burnham(1977) Asbestiform ehainsilicates: new mineralsand structuralgroups. Science, /.98, Manuseriptreceiued, October /,3,1977; accepted 359-365. foi publication,April 2l' 1978'