AmericanMineralogist, Volume 66, pages 100-105,1981

Wonesite:a new rock-forming silicate from the Post Pond Volcanics,Vermont

FneNr S. Spean'. Ronpnr M. HazpN AND DoucLAS RUMBLEIII

Geophysical Laboratory, Carnegie Institution of Washington ll/ashington, D. C. 20008

Abstract

A new, Na-rich, trioctahedral layer silicate with the composition (Caoo*NaorroKo,or)orr" (Mg, ,noFeo77sMno o*Cro oorTb oroAL.ro), ,r4(Alr 534SL 466)8 0ooOro(OH,F)a has been discovered in the Post Pond Volcanics,Vermont. The mineral is similar in optical, chemical,and phys- ical properties to phlogopite, the major differencesbeing the high Na/K and smaller d-, (:9.574). Monoclinicunit-cell dimensions are a:5.312(3), b : 9.163(5),c : 9.825(6)A,F : 103.18(6)"and Y :465.6(5)A3. The mineral coexistswith phlogopite,talc, chlorite,cordier- ite, gedrite,anthophyllite, qvartz and plagioclase(An,o-ro), and is thought to be a stableequi- librium phaseunder metamorphicconditions. The mineral has beengiven the name wonesite in honor of David R. Wones.

Introduction qtartz, cordierite, gedrite, anthophyllite, chlorite, A sodium-rich, trioctahedral layer silicate inter- talc, wonesite,phlogopite, plagioclase(An,o .o), ru- mediate in composition between talc and Na, tile, and apatite.The amphiboles(gedrite and antho- (Mg,Fe)uAlrSiuOro(OH)ohas been discoveredin the phyllite) exist as discretegrains and as mutual inter- Post Pond Volcanics,Vermont. The mineral is simi- growths, and some are slightly retrograded to lar optically and chemically to phlogopite but con- chlorite. Type material has beendeposited at the Na- tains an excessof Na over K as the alkali cation. To tional Museum of Natural History, Washington, the authors' knowledge,there are no published re- D.C., under catalog# NMNH 145724. ports of a mineral similar in optical, physical,X-ray, Optical, physical,and chemicalproperties or chemical properties.The mineral has been given the name wonesitein honor of David R. Wones,and Wonesite was discoveredduring routine electron the name and mineral description have been ap- microprobe analysisof biotites from the Post Pond proved by the International Mineralogical Associa- Volcanics, and is closely associatedor intergrown tion. A briefaccount ofthis occurrencewas given by with phlogopite,talc, or both. Except where the two Spearer al. (1978). mineralsare in contact,the mineral is extremely dif- Severalsamples containing wonesite were col- ficult to distinguish from ordinary phlogopite under petrographic lected from the Ordovician Post Pond Volcanics in the microscope,owing to the similarity in optical properties. Pleochroismin wonesite is the southwestcorner of the Mt. Cube Quadrangle, New Hampshire and Vermont. The samplescome strong and is similar to that of coexistingphlogopite: from the staurolite-kyanitezone approximately 20 m c is pale brown and B and y are dark brown. In below the Siluro-Devonian unconformity. Temper- grains that are optically continuous intergrowths of ature estimatesfor the metamorphism in this area wonesiteand phlogopite, the wonesite is slightly paler phlogopite (Fig. rangefrom 485oto 535'C (Spear,1977). The rocks than the l); intergrown talc is phlogopite. are typically medium-grained(average grain size is much paler than either wonesiteor phlogo- approximately 0.5 mm) and include the minerals Textural relations between wonesite and pite (Fig. l) are very similar to those observedfor -paragoniteintergrowths (Eugster et al., 'Presentaddress: Department ofEarth andPlanetary Sciences, 1972,Fig.7). Typical "bird's-eye maple" extinction, MassachusettsInstitute of Technology,Cambridge, Massachusetts characteristicof nonbrittle micas, is observed in 02139. wonesite, and indices of refraction are similar to 0003-ffi4x/ I | /0 l 02-0I 00$02.00 100 SPEAR ET AL.: WONESITE t0l

Fig. l. (A) Photomicrograph of wonesite (Na), phlogopite (K), talc (Tc), chlorite (Ch), anthophyllite (An), and quartz (Qt) taken in plane-polarized light. The intergrown micas in the cent€r of the photograph are optically continuous. Wonesite is slightly paler than phlogopite and darker than talc. (B) Photomicrograph ofwonesite (Na), phlogopite (K), and cordierite (Cd) taken in plane-polarized light. The intergrown micas in the center of the photograph are optically continuous; wonesite is slightly paler than phlogopite. The box shows the area ofthe X-ray photographs C and D. (C) X-ray photograph, Na radiation. Intensity drops offin NW and SE corners of photograph as a result of the spectrometer geometry. (D) X-ray photograph, K radiation. Scale bar is 200 pm.

thoseof phlogopite:B : y : 1.608+0.002and a : relatively homogeneous in composition, both within 1544+0104,6= 0.06and 0o < 2V <5o. a single grain and from grain to grain in a single thin Single crystals of wonesite are irregular basal- section. Traverses across grain boundaries demon- cleavageplates of typical mica habit. Twinning is ob- strate that the compositional break between coexist- served in single-crystalphotographs with composi- ing wonesite and phlogopite is sharp. The Si/Al and tion plane(001) and twin axis[310], as is typical with Mg/Fe ratios of wonesite are intermediate to those of other biotites. Wonesite, phlogopite, and talc form coexisting talc, which is higher in these ratios, and macroscopicepitaxial intergrowths of the (001) phlogopite, which is lower. Wonesite is also alkali- plane. deficient, with approximately 1.0 alkali cation per Table I givesthe chemicalcompositions of coexist- formula unit (out of a possible 2.0 based on 22 oxy- ing wonesite,phlogopite and talc, as determined by gens) compared with coexisting phlogopite with l.z$- electron microprobe analysis.All three phases are 1.5 alkali cations. The talc also has unusually high t02 SPEAR ET AL: WONESITE

Table |. Representativeel€ctron microprobe analysesof cm-' and 3662cm-' is interpretedas indicative of the coexistingwonesite, phlogopite, and talc presenceof oH- in wonesite. A broad absorption

Z oxides band in the region around 3400 cm-' may indicate

wonesite Phlogopi te 1a1C minor HrO in the interlayer position (George Ross-

si02 48,53 40.28 60.75 man, personalcommunication). AI2O3 13.73 17.70 2.87 The compositionof wonesitecan be describedfor TiO2 0.77 0.77 0.06 Cr203 0.08 0.07 0.0 the most part in terms of the hypothetical end-mem- 14gO 22.rL 19.61 27.34 Fe0* 7,02 8.87 4,25 ber NarMguAlrSLOro(OH)oand the substitutions K Mn0 0.04 0.04 0.03 cao 0.04 0.0 0.01 e2 Na, Fe ? Mg, Al"'+ Alt" C Mg + Si, tr + Si c Na2O 3,O7 0.50 o,32 K20 0.85 7.84 0,17** Na * Al'" and F e OH (Figs. 2 and 3). There is a Ff 0.15 0.26 0.11 small amount of K Na substitution in wonesite, Totalft 96.39 95.94 95.92 P Cations based on 22 orygens Na/(Na + K) typically being 0.85-0.95.There is sl e.,,ool- -^- s.zo+I - more than 30 percentsubstitution of the type Al'' + : -^; > 8.000 ; ^:. ) 8.000 '..* alrV L.)ic J z.zto ) 3'133) LtVr 0.620\ 0.660\ Al'" C Mg * Si, so that wonesitehas a substantial TI 0.074 | 0.07e I 3:333 Cr 0.008t-^-. 0.0041- l componentof the Na-eastoniteend member (seeFig. 5.874 5.ej4 ''"0 Mg ;'a;0 ) ;'i;0 ) ?'.1,,) 3). Another substitution,which resultsin octahedral Fe o.i?B I t.oat I Mn o.oo4) o.oo4) 3:i"J vacancies,is 2E + Tio* = 3R'*. Ca 0.004I 0.0 ) Na 0.790) 0.939 0.137l 1.550 Considerablesubstitution of the type n(Interlayer) K 0.145) 1.413J i,in)"' Fe/ (I'e + ug) 0.151 0.202 + Si'" a: Na + Al'" also occurs in wonesite. This type of substitutionis common in the alkali-deficient *r11 F6 -^nh,1raA ac Fa^ **K2O content of this taic is anonaTo\sTg high and nag represent and silica-rich dioctahedral micas, and bram- contamination with phfogopite. Most tafc anafgses shaw 0.02-0.O5 ft Z K2O. mallite. In fact, wonesitemight best be describedas +Fiuorine anafgsis bg R. Jones. ++ideal H2O content for phfogopite is 4.3f wt %, which wolLd "trioctahedral brammallite" and the corresponding bting totaTs up to apptoxinatelV fO, fr z. structuralformula is (Na,K),,,(Mg,Fe,Al)o(Sir- ur,Al,-,r)Oro(OH,F)owith Na > K and Mg > (Fe + Al). Wonesite,like illite, contains little or no inter- Al,O3 (2.5-3.7wt. percent)and NarO (0.3-0.6 wt. layer water,is not penetratedby organicliquids, does percent) contents.2Wonesite, phlogopite, and talc not expand irreversibly when heated, and does not from these rocks also contain 0.1-0.2 wt. percent when treated with glycol. Wonesite,therefore, fluorine, as determinedby electronmicroprobe anal- swell is both chemicallyand physically relatedto the sodic ysis (R. Jones,analyst, University of California, Los Angeles). illite brammallite. The reasonfor the alkali deficiencyin wonesiteis Infrared spectrawere obtained with a Perkin El- not known, but we believethat reportedvalues repre- mer model 180spectrophotometer (G. Rossman,an- equilibrium alkali contentsunder metamorphic alyst, California Insitute of Technology). A 2l-pm- sent If the alkali deficiencywere due to leach- thick cleavagefragment was positioned over a 200- conditions. ing from a more Na-rich phase,a completerange of pm-diameteraperture in a metal disk and mounted alkali contents would be expected;however, we at the focus of a 6x reflectingbeam condenser.The found that individual grains are relatively homoge- convergentnature of the infrared beam at the sample neousin compositioneven where in contactwith talc. ensuredthat componentsof the sample beam were Moreover,the systematicpartitioning of Na, K, and traveling in suchdirections that vibrational modesof vacanciesbetween wonesite, phlogopite, and talc OH- polarized normal to the cleavagefragment (Fig. 2) are evidencefor the attainment of equilib- could be excited. Absorption in the region of 3593 rium amongthe alkali cationsin thesephases.

2 Crystallography Talc analyses with Al2O3 and Na2O values of 2.36 to 3.42 and 0.33 to 0.88 wt. percent, respectively, have been reported by Unit-cell dimensionsdetermined on single crystals Shreyer and Abraham (1976), who speculated that the alkalies with an automatedfour-circle diffractometer(MoKa, might be contained within the tetrahedral layers in the centers of radiation, tr : 0.70926A; are a : 5.312(3),b - (pseudo-) the hexagonal rings. Possibly Na may be widely dis- 9.163(5),c : 9.825(QA, : 103.18(6)',and V : persed in the alkali layer of talc. The Al2O, may be accomodated F in the talc structure primarily by the substitution AlIvAlvI 465.6(5)43.The crystals are monoclinic, diffraction e SiMg. aspectC2/m, Z : 2. The mica is a lMd polymorph SPEAR ET AL,: WONESITE

(Mg,Fe)5 S isO2e(OH )a

20 v v v \2.O ro 09 08 07 0.6 0.5 0.4 03 02 0l0t o N02(N4g,Fe)6AlzSi6O2o(OH)4 Kz(Mg,Fe)6A12Si6O2o(OH)4Kz(Mg,Fe )6 Al2 S i6Ozo(OH )4 #f*

Fig. 2. (A) Compositions ofcoexisting wonesites (circles), phlogopites (squares), and talcs (triangles) plotted as a function ofNa/(Na + K) and number of alkali vacancies. Tie lines connect coexisting phases. Crossing tie lines are believed to reflect analytical unoertainty (error on Na and K : +0.05) and the fact that this plot is a condensation of the composition space. with stacking disorder (typical of ) present in Powder X-ray diffraction patterns of hand-picked most specimens.Single-crystal X-ray diffraction pho- micas from samplescontaining talc, wonesite, and tographshave neither streakingnor repeat distances phlogopite show three distinct peaks in the range greaterthan 9.5A alongc*. Wonesite,therefore, does 10.4-9.34.Treatment with ethyleneglycol had no ef- not appear to be a mixed-layer or alternating-layer fect on the positionsof thesepeaks. Similar triplets of structure with talc and Na-phlogopite layers inter- peaksappear in the ranges3.42-3.07,2.56-2.30, and mixedon an orderedl:l basis. 2.05-1.854and are interpretedas representingthe

Mg+Si=2Al No.(Mg,Fe).Al.Si 60zo(OH )4 Noz(M g,Fe)4 Al6SroO2e(OH )a

Y + o z

c, z

o 02 04 06 08 lo t2 t4 16 t8 20 K2(Mg,Fe). AlrS i6Ozo(OH)4 K. (Mg,Fe).Al.Si402o(OH)4

Fig. 3. Compositionsofcoexisting wonesites(circles), phlogopites (squares), and talcs (triangles),plotted as a function ofNa/(Na + K) and Tschermak'ssubstitution. The low-Al end-membcris phlogopiteand the high-Al end-memberis aluminouseastonite. Tie lines connectcocxisting phases. The one talc analysisthat plots at Na/(Na + K) : 9.75is anomalouslyhigh in K and may actually plot with the other talc analyses. 104 SPEAR ET AL: WONESITE

001, 003, 004, and 005 interplanar spacingsof three Table 2. Refined param€tersof wonesite;0O/ data only* distinct layer silicateswith d*, of 10.08(phlogopite), 9.62 (wonesite),and 9.354 (talc) (+9.95A). The Atom Occupancy 10.084 basal spacing compareswell with those re- Na 000 3(1) 0.55(10) ported by Wones (1963)for syntheticbiorites, which M1 0 rl2 rl2 0.8(4) 0.86 Mg, 0.14 Fe TT2 0 083 rl2 0.8(4) 0.86 Mg, 0.14 Fe havea d*, rangingfrom 10.108to 10.166A,and with T 0.s8 0 17 0.225(r) 0.8(4) 0.80 si, 0.20 Al o1 0.82 O 23 0.158(2) r.t (1) 1.00 oxygen natural biotites,which have a d*, ranging from 9.99 o2 0.52 0 0.158(2) r.7(1) 1.00 oxygen to l0.llA (Franzani and Schiaffino, 1963). The 03 0.63 0.17 0.393(2) r.2(3) 1.00 oxygen o4 0.13 0 0.193(2) 1.2(3) 1.00 oxygen 9.62A interplanar spacing correspondsto the doo, spacing obtained from single crystals of wonesite, *A71 x and g parameters wete fjxedi z parameters af Na, Mf and u2 are consxtaineU btl sqmexrg; occupancies of alf sites 9.57L. The smallestd-, of 9.35A has been assigned except Na wete fixed; ZOf : ZO2; ZO3 = ZO4; BT = BMf = BN2 = 0.38p6 = 0.5BOl = 0.5802 = 0.7803 = O-78O4 A total af sjx to talc (doo,of pure talc : 9.3lA). i,ara tat iboA ;n-t,t) ind Crystal imperfections and intergrowths with ?'. )na rh^;nrarl)1,a, ^--,1n:----r)ncq. phlogopite precluded a complete three-dimensional refinementof the crystal structureof wonesite.It was One-dimensionalstructural data on wonesite are possible,however, to collect (001) intensity data on consistentwith a 50 percent occupancyof the inter- one specimenthat was epitaxially intergrown with layer by Na, as suggestedby the electronmicroprobe phlogopite.This one-dimensionalrefinement was an- analyses.The interlayer thickness(r.e. the spacingbe- alyzed to provide information on the position and tween adjacentOl-02 layers: 2.zor. c'sin B) is electrondensity of each atom layer parallel to (001). 3.03A, comparedwith 3.37A for biotites. This differ- Intensitiesof all 0.0./ reflections,l/l < 14, were encein interlayer thicknessis comparableto the dif- measuredon an automatedfour-circle difractometer ferencein interlayer Na and K distancesin parago- (Finger et al., 1973),using MoKc radiation. Back- nite and muscovite (Burnham and Radoslovich, ground counting time and scanrate were adjustedto 1964). The tetrahedral-octahedral-tetrahedral yield a constantratio of intensity to its standard de- "sandwich" thicknessof wonesite(6.564) is similar viation (o,/I :0.02). Intensitieswere correctedfor to that of many biotites (6.60A,Franzini and Schiaf- absorptionby the flat plate crystal,130 x 130 x 30 fino, 1963).Furthermore, as noted above,the a arld b Fm (Jr,: 15.4cm-'). axesof coexistingwonesite and phlogopiteare identi- Reflectionsat +/ and -/ were averaged;of the 14 cal. It appears,therefore, that the only major struc- reflections,three were unobserved(l : 12, 13, and tural and compositionaldifferences between wonesite 14) and one (004) had an irregular background,per- and phlogopite are associatedwith the interlayer cat- haps due to interferencewith phlogopite difraction. ion. To a first approximation,the octahedral-tetrahe- One-dimensionalrefinement was completedwith the dral-octahedral "sandwich" layers are modules remaining l0 reflections.The first cyclesincluded re- (Thompson, 1978)common to both micas. finement of a scalefactor and the z positional param- eters of two oxygen and one tetrahedral cation Discussion atomic layers. Interlayer composition was con- Two synthetic Na-rich trioctahedral micas are re- strainedto be 50 percentNa and 50 percentvacancy, ported in the literature. Carman (1974) synthesized and temperaturefactors were constrainedto be equal to those of annite, as observedby Hazen and Burn- Table 3. Calculated and observedstructure factors for wonesite: ham (1973).After convergenceof positional parame- 00/ data onlv ters, all z's were constrained,and the scalefactor, an interlayer occupancy factor, and one temperature 'calc factor were refined.The ratios of temperaturefactors oDs were constrainedto be the same as those of annite 001 74.8 -74.6 and phlogopite (Hazen and Burnham,1973\. The re- oo2 19.4 -21.5 003 132.7 -133.1 sulting model (Table 2) is in agreementwith the l0 005 59.9 61.9 observedreflections (wR:6.3 percent;R: 12.5per- 006 47.5 to /, cent). One-dimensional 007 60.0 -59.3 refinements of biotites by 008 35.6 73.6 Franzini and Schiaffino (1963)yielded similar resid- 009 28.2 -29.I 00.10 34.5 3I.2 uals. Calculated and observedstructure factors are 00.11 32.6 -65.8 listed in Table 3. SPEAR ET AL.: WONESITE three different hydratesof Na-phlogopite (10.0, 12.0, only 100 m to the east) rather than wonesite.Speci- and 15.0A phases)with the nominal end-member mens containing phlogopite * talc but no wonesite composition of NarMgAlrSiuOro(OH)o.t1s found contain substantiallymore modal plagioclase(15-30 that the least-hydratedvariety has a maximum ther- percent of An,r-ro)than specimenscontaining wone- mal stability limit of 990oC at 2.0 kbar P",o, but that site (0-5 percent). this phase was not quenchable.Hewitt and Wones Acknowledgments (1975)synthesized a Na analogueof aluminous east- We thank R. Jonesfor performing fluorine analysesand T. C. onite, NarMg4AlrALSLOro(OH)o,which was also re- Hoering and G. Rossmanfor performing infrared analyses.The versibly hydrated to 12 and l5A phases.Wonesite is manuscript benefited greatly from the reviews and helpful dis- intermediatein octahedral Al betweenthe synthetic cussionson the chemistryand structureof wonesiteof L. W. Fin- micas of Carman and of Hewitt and Wones (seeFig. ger, D. Bish, J. B. Thompson,Jr., and H. S. Yoder, Jr. 2). Natural wonesitealso contains Fe, displays con- References siderablealkali deficiency,and doesnot exhibit mul- Burnham, C. W. and Radoslovich,E. W. (1964)Crystal structures tiple hydration states. ofcoexisting muscoviteand .Carnegie Institution of There are two reported natural occurrencesof WashingtonYear Book, 63,232-236. minerals resemblingwonesite in composition.Kulke Carman, J. H. (1974) Synthetic sodium phlogopite and its two (1976) described a mica from a metamorphosed hydrates: stabilities, properties and mineralogic implications. evaporite deposit in northern Africa that is very American Mineralogist, 59, 261-273. Eugster,H. P., Albee, A. L., Bence,A. E., Thompson,J. B.' Jr. and similar in compositionto the Na-phlogopite synthe- Waldbaum, D. R. (1972)The two-phaseregion and excessmix- sized by Carman (CaoorNa, ,nKo ou) (Mg, noAlo,o) ing properties of paragonite-muscovitecrystalline solutions. (Al,rrSLo8)Oro(OH)..The other occurrenceis from a Journal of Petrology,13, 147-179. metamorphosed ultramafic complex in the Alps Finger, L. W., Hadidiacos,C. G. and Ohashi, Y. (1973) A com- Carnegie (Keusen, 1912).Thecomposition of this mica (David put€r-automated,single-crystal X-ray diffractometer. Institution of WashingtonYear Book, 72, 69+699. Bish, personal communication) is very close to that Franzini, M. and Schiaffino,L (1963)On the crystal structure of of the Na-aluminous eastonitesynthesized by Hewitt biotites. Zeitschrift for Kristallografie, l19, 29'l-309. and Wones(1975). Hazcn, R. M. and Burnham, C. W. (1973) Crystal structuresof Thompson ( I 978; personalcommunication) specu- one-layer pblogopitc and annite. American Mineralogist' 58, lated on the existenceof Na-phlogopitein nature and 889-900. Hewitt, D. A. and Wones, D. R. (1975) Physical properties of suggestedthat natural examplesare rare because(l) somesynthetic Fe-Mg-Al trioctahedralbiotitcs. American Min- bulk compositionsnecessary for the formation of this eralogist,60, 85+862. phaseare rare and (2) Na-phlogopiteshould weather Keusen,H.-R. (1972)Mineralogie und Petrographiedes metamor- quickly to vermiculite.Thompson (1978, p.245, Fig. phen Ultrarnafitit-Komplexesvom Geisspfad(Penninische Al- Mineralogischcund PetrographischeMit- 5) speculatedthat Na-phlogopite should coexistwith pen). Schweizerische teilungen, 52,385478. talc, phlogopite, and edenite. In the occurrencede- Kulke, H.H.G. (1976) Metamorphism of evaporitic carbonate scribed here, talc and phlogopite coexistwith wone- rocks (NW Africa and Afghanistan) and the formation of lapis site,but no Ca-amphiboleis stablewith the Na-mica. lazuli (abstr.) International Geological Congress,25th' Ab- The stable amphiboles are the Ca-poor ortho- stracts.l. l3l. Abraham, K. (1976)Three-stage metamorphic amphiboles anthophyllite and gedrite. This difer- Schreyer,W. and history of a white-schistfrom Sar e Sang,Afghanistan, as part ence is probably due to the presenceof additional of a former evaporitedeposit. Contributions to and componentsin the natural example. Petrology,59, I I l-130. Wonesite in these rocks is believed to be a stable Spear,F. S. (1977)Phase equilibria ofamphibolites from the Post equilibrium phase under metamorphic conditions, Pond Volcanics,Vermont. CarnegieInstitution of Washington "16,613-619. based on the textures observedin thin section and Year Book, Spear,F. S., Hazen,R. M. and Rumblc D., III (1978)Sodium tri- the systematicpartitioning of alkali elements (Fig. octahedral mica: a possible new rock-forming silicate from the 2A,B) between wonesite, phlogopite, and talc. The Post Pond Volcanics, Vermont. Carnegie Institution of Wash- presenceof wonesitein the Post Pond Volcanics can ington Y€ar Book, 77, 808-812. be attributed to the unusual bulk composition. The Thompson, J. B., Jr. (1978) Biopyribolesand polysomatic series. Mineralogist, 63, 239-249. bulk compositionsof the samplesreported American here are Wones, D. R. (1963) Physicalproperties of synthetic biotites on high in Mg/Fe and contain a moderate amount of the join phlogopitc-annite. American Mineralogist' 48' 1300- NarO but very little CaO. Additional CaO would 1321. probably stabilize first plagioclaseand finally horn- Manuscript received,March 19, 1979' blende (thesephases are found in metavolcanicrocks acceptedforpublication, September 15, 1980.