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Sovruenr K-Feldspar Conadion Mineralogist Vol. 30, pp.201-214 (1992) BAVENO-TWINNED ALBITE FROM MONT SAINT-HILAIRE, OUEBEC R. PETER RICHARDS WaterQuolity Loborotory, Heidelberg College, 310 East Market Street,T{fin, Ohio 44883,U.S.A. T. SCOTT ERCIT Mineral SciencesSection, Canadian Museum of Nature, Ottawa,Ontario KIP 6P4 ABSTRACT dolomiteet b6hoite.On trouve aussi a Saint-Hilaire,mais moinscouramment, l'abite maclde selon la loi deBaveno pyriteet synchysite,et avec Baveno-twinnedalbite (Ane) crystals were found in en associationavec sid6rite, considerablenumbers in a large pocket openedin May biotite,zircon, astrophyllite, aegyrine, calcite et ancylite. 1988 at the Poudrette Quarry, Mont Saint-Hilaire, Mots-clds:plagioclase, albite, macle, Baveno,mont Quebec. They are associatedwith aegirine, microcline, Saint-Hilaire, two earlier generationsof albite, serandite,leifite, and Qu6bec. other lessabundant constituents,but grew directly only on microclineand earlieralbite crystals.The twins do not INTRoDUC"TIoN exceed2 mm in size, but are morphologically complex and well suited for goniometricstudy. The habit of these In May, 1988, a large crystal-filled pocket was Baveno twins is different from that of Baveno-twinned openedin the wall of the Poudrette at Mont orthoclase,and from of untwinned Quarry that albite from the pocket produced samepocket. All of the sixteentwinned crystalsmeasured Saint-Hilaire, Quebec.This out- are Baveno-R (021) twins. Baveno twins found in a standingspecimens of seranditeand leifite, together sephratepocket in May 190 are of a larger size, but with a rich assemblageof other minerals, which morphologically similar to those from the earlier find. included some small, colorless heart-shaped These twins occur with quartz, microcline, sphalerite, twinned crystalsthat collectorsdid not recallhaving epididymite, elpidite, catapleiite, dolomite and behoite. seenbefore. Thesecrystal$ turned out to be albite Baveno-twinnedalbite also has been found in smaller twinned on the Bavenolaw. In May 1990,a second quantities Mont at Saint-Hilaire in association with pockei containing Baveno twins was encountered. siderite, pyrite and synchysite,and with biotite, zircon, astrophyllite,aegirine, calcite and ancylite. Theseare an order of magnitudelarger than those from the first pocket, and are a distinct pink color. Keywords: plagioclase,albite, twinning, Baveno twin, This paper describes the paragenesisand the Mont Saint-Hilaire, Quebec. morphology of theseBaveno twins, which are quite different from the usual Bavenotwins displayedby Sovruenr K-feldspar. Nous avonsddcouvert plusieurs cristaux d'albite (Ans) GEoLoGICAL SSTTINc AND PARACENESIS macldsselon.la loi de Bavenodans une large g6ode ouverte en mai, 1988 dans la carribre Poudrette, au mont Mont Saint-Hilaire lies about 40 km east of Saint-Hilaire (Qu6bec). Ces cristaux sont associdsi aegyrine, microcline, deux g6ndrations anterieures d'al- Montreal, and is one of the Monteregian Hills, a bite, sdrandite,leifite, et autres phasesaccessoires, mais seriesof igneousstocks of Cretaceousage that lie seuls les cristaux de microcline et d'albite pr6coce ont along a west-to-easttrend, and have been inter- servide substrat.La taille desindividus macl6sne ddpasse preted as a seriesof static vents of mantle-derived pas 2 mm; leur morphologie est complexeet propice i materials and heat sources (Currie et al. 1986) une 6tude goniomdtrique. Ils diffbrent morphologique- associatedwith the St. Lawrencerift system(Currie ment des macles de Baveno que I'on trouve dans les 1970). Descriptions of the geology of the cristaux d'orthoclase,et des cristaux non macl6sd'albite Monteregian Hills and of Mont Saint-Hilaire in de la mOmeg€ode. Les seizecristaux qui fait ont I'objet particular found in Currie (1970, 1983), de mesures sont des macles de Baveno-R (021). can be Les (1984), individus macl6s qui ont et6 decouverts dans la g6ode de Greenwood & Edgar and. Currie et al. mai, 1990 sont plus gros, mais semblablesdans leur (1986). morphologie. Ils montrent une associationavec quartz, The Poudrette Quarry is excavated in rocks microcline, sphal6rite, €pididymite, elpidite, catapl6iite, primarily of the East Hill suite, which represents 207 208 THE CANADIAN MINERALOGIST the last intrusive event, and consistsof a pipe of however, crystalswere typically in the range of 2 nepheline and sodalite syenites, phonolites, and to 5 mm across, with a maximum of 8 mm. intrusive breccias,with many xenoliths of the older Materialsavailable to us from this pocket comprise igneousand sedimentaryrocks. Well-formed crys- about 10-20 Bavenotwins, collectedby M. Picard tals of a wide variety of minerals have been and J. van Velthuisenof the CanadianMuseum of recovered from pockets in these rocks, at the Nature. Material from this pocket obtained by PoudretteQuarry and the adjacentDeMix Quarry. other collectorsmay also contain Bavenotwins. More than 250 specieshave been identified at the The paragenesisof the 1990pocket is relatively quarries,many of them rare. The mineralsof Mont simple,with earlylarge microcline crystals followed Saint-Hilaire are described by Mandarino & by smoky quartz and pink albite. The earliestalbite Anderson(1989), Horvdth & Gault (1990),in many crystals are platy albite-Carlsbadtwins, and the papersreferenced therein, and by Fisher & Glenn Baveno twins are of the epitactic type described (1989). below. Later mineralizationincluded sphalerite and The pocket openedin May 1988was encountered galena, elpidite, platy twinned epididymite, in the southeasternwall of the quarry, at the micaceous catapleiite, dolomite and, rarely, be- contact between hornfels and nepheline syenite. hoite. The pocket was a pipeJike structure, as much as Baveno-twinnedalbite also occurson a specimen 60 cm in diameter, rising steeplyinto the wall of of twinned siderite purchased by RPR from a the quarry for at least 4 meters. The pocket specimendealer. The data on the locality do not containedlarge crystalsof aegirine, seranditeand indicatewhich quarry the specimencame from. On leifite (Robinson & King 1989),twinned mangan- this specimen,the crystalsare of the auto-epitactic neptunite, willemite, rhodochrosite, sphalerite, type, are relatively crudely formed, and lack the polylithionite, microcline, and albite, including infilling faces seen on other examples. Their abundant Baveno twins to 2 mm across. A appearance is similar to Figure 2F. They are preliminary report on theseBaveno twins wasgiven associated with siderite and minor pyrite and by Richards (1989). Most of the researchfor this synchysite. paper was performed on specimens from this A single Baveno-twinnedalbite occurs in auto- pocket, i.e., on approximately 2@ Baveno twins. epitactic orientation on a platy crystal of albite Much more material from the pocketis in the hands collected by RPR from the Poudrette Quarry in of collectorswho visitedMont Saint-Hilairearound June 1990.Associated minerals are zircon, biotite, this time. In all probability, thousandsof Baveno- acicular astrophyllite, and late-stage aegirine, twinned albite crystals were recovered. The se- calcite, and ancylite. This crystal is very similar quenceof mineral depositionin this pocket, based morphologically to those from the 1988pocket. on overgrowth relationships,is shown in Table l. The pocket encounteredin May 1990was hosted BAvENoTwrNNnrc rN PLAcrocLAsE by a microcline-rich pegmatitic phase of the nephelinesyenite, near its contact with a sodalite Baveno twins are normal twins, i.e., twins by syenite (M. Picard, pers. comm.). The pocket reflection, with twin plane and usually composition contained abundant albite and microcline, with pl4ne {021}.In orthoclase,the two planes(021) and Iesseramounts of quartz, and uncommonaccessory (021) are equivalent and produce twins with species.Baveno-twinned albite constituted much identicalinterfacial angles.In plagioclase,these two less than lVo of the euhedral albite crystals; planes are not equivalent becauseof the triclinic symmetry, and twins on (021) have different interfacial anglesacross the twin plane than twins on (021). Twins on (021) are designatedas right TABLE1. PARAGENESISOF THE MINERALASSEMBLAGE IN THE 1988POCKET. Baveno twins, or Baveno-R twins, and those on (021)are designatedleft Bavenotwins, or Baveno-L Aegirin€ - Albh€1r twins. Micrcclln€ Baveno twins of plagioclase occur sparingly Albtt€ 2r Baveno-twinned ahite among the phenocrystsseen in thin sections of Mangan-nepluniio rarely albite Serandits igneousrocks, and evenmore among Polylithionito crystals in veins and pockets in igneous and Fhod6hrosits Leiltt€ metamorphic rocks. Smith (1974) reviewedmany Willemits presented Sphalo.tt€ earlier studies, and summary tables to indicatethat Bavenotwins generallyconstitute less ' Parageneslsls basod on overgrowthrolaionshlps obsepsd In sampl€s than l0q0 of the twinned plagioclase crystals trom th6 pockef. (1977) T Albile 1 ensists ot largo pldy crystals, usually Ctrlsbad twinned. Albit€ 2 observed.Vincz6n6 Szeber6nyi and Vincze- @nslsts ot crusts ol mlcrocryslalllnecrys1als. Szeber6nyi (1972) reported that Baveno twins BAVENO.TWINNEDALBITE. MONT SAINT-HILAIRE 209 comprisefrom 3 to 15Voof all twinnedphenocrysts identified, and the orientation could thereby be (An66to An65)in someandesites in Hungary. Both deduced.At this stage, both twin memberswere Baveno-Rand Baveno-L twinning were observed, exposedto theX-ray beam,and by takinga number with Baveno-Rinvariably more common. of photographsat various settingsof the drum, the Baveno twinning
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