Tetragonal Analcime Fbom Southeastern Newfoundland

Canadian Mineralogist Vol. 18, pp. 73-75 (1980)

TETRAGONALANALCIME FBOM SOUTHEASTERNNEWFOUNDLAND

V.S. PAPEZIK AND PETER ELIAS Depafirnent ol Geology, Memorial University of Newloundland, St. lohn's, Newloundlanil AIB 3X5

ABsrRAcr North America, are.rather sparse in pre-Meso- zoic rocks of the northern Appalachians. In Pseudotrapezohedral twins of analcime, 1-Z mm the Atlantic provinces, metamorphic zeolites across, form spotty encrustations on fracture sur- have been reported from New Brunswick (Moss- faces in Late pyroclastic Precambrian felsic rooks man & Bachinski 1972) aad from western New- in the viciniE of a diabase dyke in Placentia Bay, foundland (Baker 1979). Minor occurrencesof southeastern Newfoundland. These crystals are tetragona.l, with known symmetry 141/acd, a 13.70, hydrothermal zeolites filling veins and coating c 13.68 A; optically (-), n, 1.484-1.497,n6 1,487, fractures are probably widespread, but as they birefringence 0.002-0.003, 2V" A-40", A2d (Saha are of little economic interest and usually do 1959) equals 1.84. Composition: SiOs 55.58, Al,O" not contain well'formed crystals,they are seldom 21.69, CaA nil Nago 12.89, K,O 0.03, HrO 9.14 reported (e,9., Papezik 1974). However, even (total 99.33). Formula: (Na14.s1K6.62)Alg.16Si3s.6 such apparently insignificant occurrencesshould Oee.18.06 probably HzO). The crystals were formed not be disregarded totally. For example, in the by the interaction of seawater with solutions de- central and southern Appalachians, thin veinlets rived from the felsic tuff, and were preserved near zeolites (mainly the dyke because it acted as a buttress against small filled with late hydrothermal shearing movements on joint zurfaces. laumontite) have been used in the evaluation of "fault capability", one of several critical Keywords: tetragonal analcime, trapezohedral factors in selecting suitable sites for nuclear crystals, Precambrian tuff, Newfoundland. Avalon (Butler 1977, Moody 1977). In this z.One. reactors brief note, we report the occurrence of analcime Sornuarnr in Late Precambrian rocks of the Avalon zone in southeasternNewfoundland. Les pseudo-trap6zoddres habituels d'analcime ma- cl6e de I i 7 mm de diamdtre forment des incrus- tations clairsem6es le long de diaclases dans les LocALrrY roches felsiques pyroclastiques du Pr6cambrien su- p6rieur, en bordure d'un dyke de diabase (baie The mineral was found by Peter Elias in Placentia, dans le Sud-Est de Terre-Neuve). De 1976 on the shore of Baker Cove at the head sym6trie connue 141/acd, a 13.7O, c 13.68 A. ces Bay (47"5CI@"N,54o06'32"W; Nat. cristaux sont optiquement n6gatifs of Placentia et variables en Islahd"). indices de r6fraction (nat 1.487, ne 1.484-L.487). Topogr. Series 1M/16 East, "Sound bir6fringence (0.002-0.003) et 2V, (04Oo). L'indice The area is underlain by steeply dipping felsic A2d (Saha 1959) est de 1.84. La composition, SiOg pyroclastic rocks of the Sound Island Forma- 55.58, AlrO' 21.69, CaO nil, NazO 12.89, K,O 0.03 iion of Late Precambrian age (OlDriscoll L977). et H:o 9.14 (total: 99.33) correspond i la formule In Baker Cove, the rock is a crystal-lithic tuff (Naro.*trG.or) All5.15sisz.osOsa.18.06 HoO. L'analcime containing l-3 mm crystals of sodis plagioclase probablement r6sulte de l'interaction de I'eau de and thin" flattened lenses'of fine-grained felsic mer et de solutions issues des tuffs felsigues; elle rocks, partly altered to albite, quartz' sericite, doit sa pr6servation i la proximit6 du dyke qui minor epidote, and cut by thin I'a protdg6e contre les cisaillements le long des Fe oxide and fractures. veinlets of calcite. At the analcime locality' the Clraduit par la R6daction) pyroclastic rock is cut by a steeply dipping dyke oi altered diabase less than 2 m wide, now Mots-clds. analcine t6tragonale, trap6zoidres, tuff consisting of albite, chlorite, hematite and abun' pr6cambrien, Terre-Neuve, zone de l'Avalon. dant leucoxene, with minor calcite and quartz. The analcime is found as pseudotrapezohedral INrnopucrroN crystals, locally forming encrustations 9n lhe Occurrences of zeolites, relatively common wutts of joints and fractures in the tuff (Fig. 1). and well known in Triassic rocls of eastern The crystals are relatively abundant within 3 m 73

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9! the dyke, but their- quantity decreaseswith possibly as a result of stress. Refractive indices distance; no crystals were found more than and specific gravity are given in Table 1. about 10 m from the dvke. The powder-diffraction pattern of the anal- cime agrees closely with publishd data (e.9., Pttyscer, PnopgntrEs PDF 19-1180). Single-crystal study by preces- sion techniques shows that the mineral is tetra- Crystals of analcime from 1 to 7 mm across gonal, space group l4Jacd. The space group are well developed but flattened parallel to and cell dimensions (Table 1) are in good the surface of the rock fracture and thus agreementwith the data for tetragonal analcimes "hemispherical". The trapezohedron is the only from various localities given by Mazzi & Galli form observed. In appearance,the crystals vary (1978); however,it is possiblethqt somecrystals from water-clear to milky with a reddish cast (or even different parts of some of the larger caused by small quantities of finely dispersed crystals) may have isometric or orthorhombic hematite. They adhere firmly to the fracture symmetry. walls but can be detached with a knife. Microscopic examination shows that the anal- cime is weakly anisotropic (birefringence 0.002- TABLI| 2. (:0MPOStl'IONOI BAKI|R(;OVli ANAI,(;tMti 0.003), and optically negative. It has a very fine lamellar twinning. Different parts of the Atoms pcr 96 o same 5-mm crystal were found to be uniaxial si02 55.58 si 32-93 or biaxial, 2V" from 4Oo. In view ranging 0 to Alzni 2t.69 Al 15.15 of the low birefringence, such anomalies may be caused by differences of less than 0.001 in CaO 0.00 (la the refractive index along the horizontal axes, Nar0 I 2.89 Nit 14.81 Kzn 0.03 K 0.02 TABLE1. pnopnnrrsboF BAKERcovE ANAIcTME ll2(J* 9. 14 oo Tet"agonal, f4./acd. a 13.70A, c 13.68A 1- 99. 35 llZO 18.0(r 420* 1.84 + 0.01, CuKcr (Avcragc of six (si+tr1'1 48.08 6L.487, e1,.484- t.487; 2Vx 0-400 spot :ln:! I yscs) (Na+K+2Ca) 14.83 S.G.(neas.)**2.260 + o.OU * By t,OT si/Al 2.77 S.G.(ca1c.) 2.286 l:lcctron-probe analysis by M. l3onardi, * 20 analcirne.rn - 20 siliconj3l; Saha (1959). Ccological Survcy of Canada, Ottawa. ** Beman baLance, average.of six fragnents. l12Otry .I. Vahtra, Mcmorial llnivcrsity.

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The A20 value (j.e., 20 analcimeaae- 20 sili- analysis, A. Roberts (Geological Survey of congsr!Saha 1959) was determined by triplicate Canada, Ottawa) for the single-crystal study, scanning of the interval between 76 and,79' 20 B. D. Sturman for part of the optical data and (Cu Ka) on three separatemounts, using silicon Cyril O'Driscoll and Wayne Muggeridge for a as an internal standard. The average A20 value hair-raising boat ride to and particularly from (1.84: Table l) lies in the middle of goup C Baker Cove. J, Vahtra produced the XRD scans, of Coombs & Whetten (1967), which includes W. Marsh took the photograph, and Cynthia "silica-poor analcimes. . . formed by direct pre- Neary and Glenys Woodland typed the manu- cipitation or by reaction of highly alkaline water script. The study was supported by NSET'C with sediment." grant 4-2131, which is gratefully acknowledged.

Cnrrr,rrsrny REFERENCES

An electron-microprobe analysis of tlte Baker BllrBn, D.F. (1979): Geology and Geochemistry ol Cove analcime is given in Table 2. Total water an Alkali Volcanic Suite (Skinnet Cove Forma- was determined separately by loss on ignition. tion\ in the Humber Arm Allochthon, Newtound' The analysis corresponds to Nao.raAlo."sSis,oeOo.Iand. M.Sc. thesis, Memorial Univ. of Newfound- 1.13 HrO, slose to the ideal formula NaAlSisOo. land. St. John's. HrO, with only a small excessof Si (Si/Al 2.17) BoNARDI,M. (1979): Compositional changesduring and a small deficiency of alkalis. The dominance electron probe analysis of dachiardite: contribu- of Na, very low K and the absenceof detectable tion to zeolite stndy. Geol, Assoc. Can. / Mineral. Ca may indicate that seawaterhad a prominent Assoc. Can, Program. Abstr. 4, 40. role in the formation of the mineral. However, Burrrn, J.R. (1977): Comments on zeolite forma- it is possible that during probe analysis, the tion in the Carolina Piedmont. Geol. Soc. Amet. original alkali content may have been modified Program Abstr, 9, 124'125, by differential volatilization under the electron (1967): Composi- beam in a vacuum (Bonardi 1979). Coortns, D.S. & Wnp1-reN, J.T. tion of analcime from sedimentary and burial metamorphic rocks. Geol. Soc. Amer. Bull. 78' OnrcrN 269-282. The analcime crystals were probably formed Lou, J.G. (1971): Analcime equilibria. Lithos 4, by the reaction of seawater with solutions rich 389-402. in Si and Al derived from the felsic tuff, at a Mpn, F. & Ger.rr, E. (1978): Is each analcime time when seawater was first able to penetrate different? Amer. Mineral- 69, 448-460. the fractures and at a temperature whish need of laumontite not have been much above the normal climatic Mooov, I.B. (1977): Implications the evaluation of fault capability. range (Liou l97I). The observed close spatial stability-Soc, on GeoL Amer. Program Abstr. 9, 168-169. relationship of the analcime crystals to the dia- base dyke is not likely to be the result of a Mossvr,eN, D.J. & Beomlsrr, D.f. (J972)z Tnolite thermal or chemical gradient. It is more prob- facies metamorphism in the Silurian-bvonian able that analcime crystals, perhaps formerly fold belt of northeastern New Brunswick. Can. l, more common throughout that part of the felsic Earth Sci. 9, t7O3-L709, pyroclastic rocks exposed to the action of sea- O:DRIscoLL, C.F. (19?7): Geological mapping.of water, may have been largely destroyedby small- the Sound Island map area, In Report of Activi- scale movements along joints and fractures; the ties 1976. Nfld. Dep. Mines Energy 77-1, 4347' dyke may have acted as a buttress, preventing PApEaK, V.S. (1974): Prehnite-pumpellyite facies sush small shearing movements in its vicinity metamorphism of Late Precambrian rocks of the and thus protecting the crystal-coated surfaces. Avalon Feninsula, Newfoundland. Can. Mineral' As the crystalscan be detachedfairly easily from t2, 463-468. the rock surface and probably would not survive (1959): Geochemical and X-ray investiga- a large number of freeze-thaw cycles,the locality Sene, P. of natural and synthetic analcites. Amer' unless further crystal- tion appears to be ephemeral, Mineral. 44' 300-313. coated fractures are exposed by shoreline ero' sion. AcrNowrsDcMENTs Received lune 1979, revised. manuscript accepted We thank M. Bonardi and J. Vahtra for the Iuly 1979.

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