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Julgoldite from Tafjord, Sunnmøre. Contribution to the Mineralogy of Norway, No

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Julgoldite from Tafjord, Sunnmøre. Contribution to the Mineralogy of Norway, No Julgoldite from Tafjord, Sunnmøre. Contribution to the Mineralogy of Norway, No. 67 KJARTAN BRASTAD Brastad, K.: Julgoldite from Tafjord, Sunnmøre. Contribution to the Mineralogy of Norway, No. 67. Norsk Geologisk Tidsskrift, Vol. 64, pp. 251-255. Oslo 1984. JSSN 0029-196X. Julgoldite occurs in open fissures in a Precambrian granitic gneiss situated between Tafjord and Fjøra. Mineral growth in the fissures started under medium or low grade conditions and was completed, after a break, under very low grade conditions. The julgoldite-bearing association consists of calcite + quartz ± chlorite ± laumontite ± babingtonite ± julgoldite ± hematite ± titanite. Julgoldite crystals are very scarce and are found as small inclusions in calcite crystals or in equilibrium with calcite + laumontite ± titanite. The composition of this julgoldite (s.s.) is intermediate, ranging from 62% to 86% of the julgoldite 2 component, and with an average composition corresponding to the formula (Ca7.88Nao.17) ((Fe +, Fe3+) .62 MO".osM!lo.27) (Fe3+5.15Al2.25) (Siu.76Alo2404 .21. 9.59) (OH)t2.SO-t6.4t· The large cell size of 3 3 3 2 this mineral from Tafjord is indicative of extensive substitution of Fe + for Fe3+ and hence a low oxidation state. Kjartan Brastad, Mineralogisk-Geologisk Museum, Sarsgt. l, 0562 Oslo 5, Norway. During the construction of a new road tunnel lar for all the fissures, although there are some between Tafjord and Fjøra in the Sunnmøre dis­ differences in mineral assemblages from one fis­ trict of western Norway, some mineralized open sure to the next. The crystallization sequence fissures were discovered. Most of these were de­ consists of at !east four associations of minerals, stroyed by the construction work, but small whose age relationships can be established be­ pieces of the material were found by mineral cause of overgrowth. These associations are: collectors who sent some of it to the Mineralogi­ l. Orthoclase+ calcite I+ hastingsitic amphi­ cal-Geological Museum in Oslo for identifica­ bole ± apatite ± biotite ± ilmenite ± tion. This material contained a number of un­ magnetite ± zircon. common minerals, e.g. babingtonite, bavenite, Il. Calcite Il+ quartz ± chlorite ± laumontite datolite, stilpnomelane and julgoldite, in addi­ ± babingtonite ± julgoldite ± haematite ± tion to a number of common low-temperature titanite. minerals. Julgoldite, which is the Fe-dominant Ill. Chlorite ± stilpnomelane ± magnetite ± equivalent of pumpellyite, is very rare and pre­ apophyllite ± laumontite ± epidote ± viously only described from the Långban area of tremolite/actinolite ± pyrite ± galena ± Sweden (Moore 1971), and from two localities in sphalerite. Scotland (Livingstone 1976). This study concerns IV. Stilpnomelane ± chlorite ± datolite ± thau­ the mode of occurrence and chemistry of this masite ± bavenite. mineral in Tafjord. Two other minerals, axinite and prehnite, have Geological setting and petrography also been identified but it is not clear to which of these associations they are related. The wallrock for these mineral occurrences is a The julgoldite-bearing association (Il) consists light red, granitic Precambrian gneiss with low or mainly of calcite, quartz and chlorite. Laumon­ medium grade mineral assemblages. The normal tite, babingtonite, titanite, haematite and julgol­ mineral assemblage is K-feldspar + plagioclase + dite are minor constituents. Julgoldite and hae­ quartz + hornblende ± biotite ± Feffi-oxides. matite are commonly found as inclusions in, re- The gneiss encloses also a number of meta­ . spectively, calcite and quartz, but in one sample eclogites with relics of assemblages formed dur­ julgoldite is also found outside calcite together ing high pressure metamorphism (Brueckner with calcite, laumontite and minor titanite. Cal­ 1977). cite from this association is transparent, colour­ The general sequence of crystal growth is simi- less, and has well-developed crystals, O.l mm- 252 K. Brastad NORSK GEOLOGISK TIDSSKRIFT 3 (1984) surrounding gneisses, while associations Il, Ill 6 Zolotukhin et al., 1966 V and IV formed under much lower grade condi­ • Moore, 1971 '"" tions. Pumpellyites are characteristically formed • :�o� v Livingstone, 1976 Liou, under very low grade conditions (Winkler 19�9) • o 1979 • • Nystrøm, 1983 and Fe-rich pumpellyites are generally consld­ . o;..... " Cortesogno et al., o 1984 ered to have formed under lower P, T-conditions 30 • Tafjord � than Al-rich ones (Seki 1961, Coombs et al. 1976, Liou 1979). The Tafjord julgoldite is com­ positionally not very different from some Fe-rie� pumpellyites (F ig. l) and has probably approxl­ mately the same conditions of formation. Lack of 20 diagnostic mineral equilibria and ig ora ce of � � the chemical system forming the TafJord Julgol­ dite make it impossible to define the conditions of formation doser than to those of the low P, low T part of the very low grade of Winkler oD o "' (1979). 10 o• 'll o • o o .o Terminology o o The general formula for pumpellyite group min­ erals can be written as: W8)4YsZ12056-n(OH)n, 2 10 20 % AI203 where W is Ca; X is Mn, Mg, Fe +, Fe3+, Al; Y is Fe3+, Al; Z is Si and nis 12-16 (A llmann & Fig. J. Al/Fe-plot of julgoldites and some Fe-rich pumpellyites. 1971, 1973, 1973). FeO= Fe'"'. Donnay Passaglia & Gottardi Substitution of Al for Si and Na and K for Ca increase n still further, while substitution of Ti in Y position could reduce n to less than 12. The > 10 cm in diameter. Chlorite is dark green and data of Passaglia & Gottardi (1973) show, occurs as very fine grained masses. Aggregates of however, that these substitutions are sel dom sig­ well developed bladed chlorite crystals, 1-2 mm 2 nificant. The Fe +, Fe3+ dominant member of in diameter, are also common. The quartz crys­ this group, julgoldite, was first described by tals exhibit large size variations, ranging from 1- (1971). 2 50 Moore Moore proposed that 'julgoldites mm to ca. cm. Large crystals are usually 2 are defined when Fe( +,3+) in X and Fe3+ in Y subhedral while smaller ones are euhedral. Bab­ each occupy greater than fifty mole % of their ingtonite occurs as euhedral black crystals, 0.2-2 positions'. Passaglia & Gottardi (1973) have pro­ mm in diameter. Laumontite and titanite crystals posed a different nomenclature where minerals are euhedral and colourless, with diameters be­ with the pumpellyite structure are named pumpel­ tween O.l-l mm. Julgoldite grains are long pris­ lyite if Al is predominant in the Y-position and matic, either single crystals or fanshaped inter­ julgoldite if Fe3+ is predominant. The julgoldites growths of single crystals with a maximum length 2 are further denoted Fe +- or Fe3+-julgoldite de­ of 1-1.5 mm. The crystals are subhedral with a pending on which ion dominates the X-position. black, submetallic lustre. Haematite inclusions 2 The Tafjord julgoldite (Ca7.88 Nao.17) ((Fe +, are thin reddish blades with maximum length < l Fe3+h.62 Mn0.05 Mgo.21) (Fe3+5 .75 Ah.25) (Si11 .76 mm. Alo.24 04 .21_ 9.59) (OH)l2.!ro-16.41 has distinct Fe The first association of minerals is clearly 3 3 dominances in both X and Y positions and would formed under somewhat special conditions, as is therefore be classified as julgoldite by either of seen from chloritization of amphibole and biotite the above definitions. and the strong corrosion of calcite and magne­ tite, while minerals belonging to association Il, Ill and IV seem to have formed in uninterrupted Analytical data sequence under similar conditions. Association I probably formed under nearly the same ondi­ X-ray powder diffraction data and chemical ana­ � tions as the medium grade assemblages m the lyses were made on a mineral separation obtained NORSK GEOLOGISK TIDSSKRifT 3 (1984) Julgoldite from Tafjord 253 Table l. X-ray diffraction data for julgolditc from Tafjord, and for two julgolditcs from the literature (1: Moore 1971: Il: 1971). < 2. Livingstone The lists of d1.11 are not complete for d hkl-values refer to the Tafjord julgoldite. dnbs dl dn hk l d,." dl dn hk l 9.59 9.57 9.71 200 2.558 2.568 2.574 420 8.88 8.82 8.85 001 2.489 2.491 2.501 711 7.01 7.07 ZOI 2.473 6.11 6.14 6.16 201 2.433 4.809 4.80 4.817 400 2.370 2.385 222 4.739 Ill 2.323 4.479 2.294 4.439 4.43 002 2.242 2.240 2.247 802 4.232 2.218 2.209 2.213 004 4.087 4.068 311 2.194 620 3.845 3.84 3.859 202 2.149 2.158 2.159 422 3.588 2.121 2.125 3.507 3.494 4o2 2.102 3.305 2.034 2.039 3.158 1.949 1.948 1.952 6o4 3.064 3.059 3.068 402 1.903 1.902 1.911 622 2.941 2.950 2.958 003 1.881 1.885 1.888 821 2.885 2.881 220 1.626 1.627 730 2.857 021 1.621 1.622 711 2.773 2.778 2.780 s12 1.606 1.605 1.607 120() 2.710 2.717 2.724 221 1.512 1.517 1.519 040 2.674 2.680 2.675 Tl3 by dissolution in cold diluted HCl of calcite age analysis (Table 3) give, according to these containing julgoldite inclusions. For accurate dif­ limitations, a possible range for n from 12.8 to fraction data, a Guinier camera with FeKa radi­ 16.4. The compositional range shown by the ation (A.= 1.93728 Å) was used, with Pb(N03h analyses fills up most of the gap previously exist­ as internal standard. The observed d-values ing between the most iron-rich pumpellyite re­ (Table l) and calculated unit-cell dimensions ported (Zolotukhin et al.
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