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Download the Scanned 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 muscovite-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, illite 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.
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