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The Relative Stabilitv of Elpidite and Vlasovite: a P T

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The Relative Stabilitv of Elpidite and Vlasovite: a P T Canadian Mineralogist Vol. 23, pp. 577-582(1985) THE RELATIVESTABILITV OF ELPIDITEAND VLASOVITE: A P_T INDICATORFOR PERALKALINEROCKS KENNETH L. CURRIE AND EVA ZALESKI GeologicolSqney of Canada,601 Booth Street,Ottawa, Ontario KIA 0E8 ABSTRACT accessoryzircon. In peralkaline rocks, the ratio (Na + K)/Al excds 1, zircon may be rare or absent, The univariant reaction Na2ZrSi6Otr.3H2O (elpi- andZr appearsin a bewilderingvariety of minerals, dite; = 11ar7t5ino11(vlasovite) + 2 SiO2+ 3 H2O passes including in amphibolesand pyroxenes(up to seve- through the points 1000bars, 595 + 4'C and 2000 bars, ral percent ZrO). Among minerals that may appear &4 t 4"C. The reaction curve is unaffected by the presence as Zr-bearingphases in peralkalinerocks, vlasovite of excessNa in the vapor phase,but the substitution of Na2ZrSiaOlland elpidite Na2ZrSi6Orr.3HrOare of I N water HCI for decreasesthe stability field of elpidite specialinterest because for stoichiometriccomposi- by about 20'C. Elpidite is unstablerelative to keldyshite qtJartz+ Na2ZrSi2OTin highly chlorinated assemblages.Synthetic tions, elpidite:vlasovite +2 3 water. elpidite has smaller cell-dimensionsand a different space- Therefore, elpidite would be expectedin environ- group than natural elpidite, but syntietic vlasovite appears ments with relatively high activities of silica and essentiallyidentical to the natural mineral. Elpidite forms water, and vlasovite in environments with relatively from hig[-temperature hydrothermal fluids, whereas low aclivities of silica and water. However, both vlasovite forms as a magmatic accessoryor by metamor- minerals, although never found together, apparently phism of elpidite. The more common occurrenceof elpi- occur in similar environments,namely highJevel dite, relative to vlasovite, suggeststhan an aqueousphase peralkaline ganites and peralkaline nepheline peralkaline granitic separatesfrom magma only when crys- relatively commonly in tallization is virtually complete. syenites. Elpidite occurs peralkaline granites, for example at Lac Brisson, Keywords: elpidite, vlasovite, reaction curve, pressure- Quebec(Zajac et al. 1984)and Mount Champlain, temperatureindicator, peralkalinegranite. New Brunswick(R.F. Martin, pers.courm. 1983)in Canada, and also in peralkaline syenitesat Mont Saint-Hilaire, (Chao 1967)and Lovozero, SoMMAIRE Quebec U.S.S.R.(Semenov 1967), as well asin maficqtrartz- (Kapustin 1966).Vlasovite has been (elpi- bearingfenites La r€action univariante Na2ZrSi6O15.3H2O peralkalinegranile dite) = yar2r5iao11 (vlasovite)+ 2 sio2 + 3 H2o est en reportedin blocks of high-level 6quilibre aux points 1000bars, 595 i 4'C et 2000 bars, found as inclusionsin trachyteon AscensionIsland 644 t 4oC. La courbede r6actionn'est pas affect6epar la (Fleet & Cann, 1967), and from metamorphosed pr6senced'un excbsde sodiumdans la phasevapeur, mais agpaitic rocks of amphibolite grade at Kipawa, Que- le champ de stabilitd de l'elpidite d€croit d'environ 20oC bec (Gittins et al. 1973),as well as from nepheline quand on remplaceI'eau par une solution de HCI (1N). syenitesof the Lovozero somplex (Semenov1967). L'elpidite estinstable vis-i-vis la keldyshite(Na2ZrSi2O?) The similarity of the paragenesessuggests that some dans les mdlangestrds riches en chlore, L'elpidite synth6- feature other than silica or water activity must con- tique possedeune maille plus petite que cellede I'elpidite mine- groupe trol the mutually exclusivecharacter of these naturelle, et un spatial diff6rent, mais la vlasovite pressure synthdtiqueresemble beaucoupau min6ral naturel. L'elpi- rals. If this controlling factor is or tempe- dite se forme d partir de solutions hydrothermales i haute rature, then the comparatively simple chemical temperature, tandis que la vlasovite provient d'une phase characterof the minerals (Semenov1967) suggests magmatiqueaccessoire, ou par metamorphismede I'elpi that their relations rnight give a useful P-T indica- dite. L'elpidite est plus rdpandueque la vlasovite, ce qui tor. In order to test this hypothesis,:m experimen- iait penserqu'une phaseaqueuse ne ses€pare des magrnas tal study was undeflaken to determine the relative granitiques hyperalcalins que lorsque la cristallisation tire stabilitiesof elpidite and vlasovite. i sa fin; EXPERIMENTAL METHOD Mots-c6s: elpidite, vlasovite,courbe de r6action, indica- teur de pressionet de temperature,granite hyperalcalin. Both elpidite and vlasovitecan be readily synthe- sized (Christophe-Michel-L5vy1961, Baussy et al. INTRoDUCTIoN 1974),but previous studies produced little useful information on the relativeor absolutestabilities of In most rocks, the molecular ration (Na + K)/Al the minerals.Since the natural paragenesessuggest is less than or equal to l, and Zr occurs mainly in that silica activity is not the major factor control- s77 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/23/4/577/3446027/577.pdf by guest on 27 September 2021 578 THE CANADIAN MINERALOGIST ling the relationsof the minerals,we worked mainly TABLE2. RESULTS0F HYoR0'ItlERl'lALEXPERIMENTS with materialsof stoichiometricelpidite composition. (a) Flnid phe Fre watq We used different mixtures only to evaluate the Ru P(bars) T(c) Reactsts Produqts peralkalinity produce Na-Zr degreeof required to 5-t 2026 680 get vl + qtz silicatesrather than zircon in peralkalinecomposi- 7-2 m26 660 Bel vl + qtz 9-7 m26 650 gel vl r qtz tions. 54 2n26 AO gel el(+qiz) In all casesthe starting materialswere dried gels xt7 ?f26 635 *el et({itz) xl8 2n26 640 lel dl(+qtz) prepared from reagent-grade zirconyl chloride x20 2026 gr5 iel el+vl+qtz (ZrOCl.8 x21 2026 650 rel el(+qtz) H2O) as a sourceof Zr, sodium-stabilized xn 2026 648 tvl+gtz vl+qtz colloidal silica sol as a source of SiO2, and either xzt 2026 643 tvl+qtz el+vlrqtz x24 2026 638 *Yl{qtz reagent-gradeNaHCO3 or NaCl as a source of 64 l5L9 6140 tel Yl a qtz prepared u3 L5l9 6)0 gel vl + qtz sodium. Gels were by dissolving the 6-t l5l9 620 gel vl+(el) appropriateweight of zirconylchloride in 0.1 N HCI v2 L5l9 615 gel vl(+gtz + el) 8-l l'9 600 gel el(+vl + qtz) and adding the silica sol to it while stirring. For the 6-5 I5l9 600 gel el(+vl + qtz) initial experiments,weighed amounts of NaCl were xL2 l5l9 615 *eI el(rqtz) xl3 ljl9 620 rel dissolvedin this solution, and the materialeelled by xt4 lJl9 625 rel el{vl+qtz xt6 l'9 630 *el vI + qtz adding NH4OH. Hydrothermal experimentson this x26 l5t9 62t *vl+qtz el+vl+qtz material consistently produced keldyshite x27 1'9 62t rvl+qtz el+vl+qtz x29 t5l9 618 rvlrqtz e(+qtzJ Na2ZrSi2OTrather than elpidite or vlasovite. We )-6 tol, 600 gel vl(iqtz + el) prepared gel. t+4 l0r3 600 gel vl + qtz therefore a chloride-free The solution l8 t0l3 t80 gel e(+Stz) of zirconyl chloride and silica was gelledby adding ,7 tol, xo gel e(+qtz) u5 tol, 550 gel el(+vl + qtz) excessNH.OH. The gel was dried at 90oC for 12 54 tol Vto cel el(+Ctz) hours and fired at 900oCfor one hour, which quan- 34 t01, J00 tel e(rqtz) 44 l0l, 500 gel el(+gtz) titatively removed Cl by volatilization as NH4CI. ,2 l0l, 400 geI el(+gtz) u2 101, 400 tel el(rCtz) NaHCO, was then addedto the chloride-freemix, x5 l0l3 585 fel el and the mixture shakenin an agitator with plastic x7 l0l3 ,90 *el el( + qtz) x8 l0l3 595 el{Yl}qtz grinding balls for 30 minutes and heatedat 550oC x9 1013 59t *el vl + qtz for 4 hours to drive off COr. Gelswere made up so x30 101, 600 rvllqtz vl r qtz xrt 101, 59' rYllqtz el+vl{gtz that the calculated weight of the dried products xtt 101' 'EE rvl{qtz el(+qtz) 6-8 507 '80 6el vl r gtz(d) would be about 15-20 grams. The products were ,2 507 '80 gel el({qtz) rejectedif their weightdeparted from the calculated 9-t 507 560 gel e(rgtz) 507 %0 tel el(+qtz) weight by more than 0.02 grams. X-ray-diffraction 64 507 tt0 gel e(qtz) analysisof xl 507 546 *el el(*stz) the chloride-freestarting material showed xz 507 550 *el el weak linesof baddeleyite.Wet-chemical analysis of x3 507 554 *el el+yl+qtz x4 507 559 rel vl { qtz this material by S. Courville (Table 1) showedthat xt5 507 550 fvl+qtz it correspondedto the statedZr:Na:Si ratio within xu 507 557 rvl+qtz YI r qtz analytical error. (b) FluldphcelNNaoH The experimentswere conductedwith standard Blq P(bars) T(c) R€ctetg @ hydrothermaltechniques. About 0.2 gramsof start- 4-10 l0l3 600 get vl r qtz(el) ,-10 t0l3 550 gel el ing material were loadedinto thin-walled (0.1 mm) u9 101, 550 geI el gold tube, about 0.02 gramsof aqueousphase (dis- 3-9 l0l3 500 Eel el(+qtz) 4-7 1013 450 gel el(+qtz) tilled water, 1 N HCl, I N NaOH) added, and the ,7 t0L3 450 gel el(+qtz) capsulewelded and weighed.Experiments were made (c) FluidpharelNHcl in Ren641 test-tubebombs with graphite or stain- R@ Pbas) 19) R@ctilts Products lesssteel filler-rods. Temperaturewas measuredby 94 2026 700 gel vl r qtz chromel-alumelthermocouple inserted into drilled 8-5 2026 660 gel e(+vl + qtz) 7-9 2026 6It0 gel el({vl r qtz) wellsin the bombs, and is believedto be maintained 7-7 2026 620 tel el(avl r qta within 2'C during experimenis.Prqssure was meas- X'E 2026 640 rel vl I qtz x19 2126 630 *el Yl + qtz ured on a 30-cm Heise bourdon gauge, and is x51 2026 625 *dl el+Yllgtz x40 2A26 620 reI el(+qtz) x4l 2026 640 * vl+qtz Yl + qtz xttz 2026 630 lvl+gtz vl + qtz x4? 2026 620 el TABLE 1. CHEMICAL COMPOSITIONOF STARTING MATERIALS 8-8 1013 60 gel vl(+el r qtz) 94 l0l3 640 gel Yl + qtz Veiftt Mol 9-1 l0l3 620 gel vl r qtz(+el) 7-5 l0l3 600 tel e(+qtz) SlOz 59,Ot% 6.006 x44 101, t90 *el vl r qtz Na?o 10.25 1.0t0 x45 1013 5t0 *el vl + qtz rel ZrO 2 20.17 1.@0 x46 l0l3 570 el(+qtz) Hz0 10.16 3.5t4 x47 1013 590 rvlrqtz vl + qtz 0.04 0.006 x48 101, 58 *vl{qtz vt + gtz x5t 101, t75 *vlrqlz el+vligtz S.
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