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Insights Into Astrophyllite-Group Minerals

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Insights Into Astrophyllite-Group Minerals 1 Volume 41 February 2003 Part 1 The Canadian Mineralogist Vol. 41, pp. 1-26 (2003) INSIGHTS INTO ASTROPHYLLITE-GROUP MINERALS. I. NOMENCLATURE, COMPOSITION AND DEVELOPMENT OF A STANDARDIZED GENERAL FORMULA PAULA C. PIILONEN§ AND ANDRÉ E. LALONDE Ottawa–Carleton Geoscience Centre, Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada ANDREW M. MCDONALD Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada ROBERT A. GAULT Mineral Sciences Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada ALF OLAV LARSEN Norsk Hydro ASA, Research Centre Porsgrunn, Porsgrunn, Norway ABSTRACT The composition of 135 samples of astrophyllite-group minerals from 15 localities has been established by EMPA, ICP–AES, FTIR, TGA, thermal decomposition, NRA and Mössbauer spectroscopy. A standardized general formula has been developed and [10]–[13] + [10] [6] 2+ 3+ is of the form A2BC7D2T8O26(OH)4X0–1, where A = K, Rb, Cs, H3O , H2O, Na or Ⅺ; B = Na or Ca; C = Mn, Fe , Fe , Na, Mg, or Zn; [6]D = Ti, Nb, or Zr; [4]T = Si or Al, X = ␾ = F, OH, O, or Ⅺ. Data acquired by Mössbauer spectroscopy, thermodynamic approximations, and EMP analyses have been used to demonstrate that F orders at the ␾(16) site and does not occur at the two general OH sites within the O sheet. On this basis, formulas of the eight species of astrophyllite-group minerals 3+ have been redefined. Results from Mössbauer spectroscopy indicate Fe /Fetot values in the range from 0.01 to 0.21, correspond- ing to 0.05 to 0.56 apfu Fe3+, confirming that Fe2+ is the dominant valence state for iron in the structure. Minerals from silica- oversaturated and -undersaturated alkaline intrusions are distinct in chemical composition. In oversaturated rocks, the dominant member of the group is astrophyllite sensu stricto, which occurs as a late-stage postmagmatic phase, enriched in Rb, Fe2+, Ti, Si and F. In contrast, undersaturated intrusions, in particular Mont Saint-Hilaire, Quebec, show the greatest diversity in species and range in chemical composition. Kupletskite-subgroup samples are enriched in Na, Mn, Fe3+, Zn, Zr and Nb, whereas astrophyllite- subgroup samples are enriched in K, Ca, Fe2+, Ti, Zr and Al. Enrichment of kupletskite-subgroup samples in Fe3+, Mn and Nb suggests crystallization under more oxidizing conditions than those of the astrophyllite subgroup. Incorporation of Nb into the § Current address: Mineral Sciences Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada. E-mail address: [email protected] 001 vol 41#1 février 03 - 01 1 3/24/03, 10:37 2 THE CANADIAN MINERALOGIST structure and the formation of Nb-bearing kupletskite and niobokupletskite are the result of the substitution M(1)2+ + M(2,3)2+ + (Zr,Ti) + F ⇔ M(1)Na + M(2,3)Fe3+ + Nb + O. Keywords: astrophyllite-group minerals, kupletskite subgroup, astrophyllite subgroup, structural formula, crystal chemistry. SOMMAIRE Nous avons établi la composition de 135 échantillons de minéraux du groupe de l’astrophyllite provenant de 151 endroits au moyen de la microsonde électronique, des analyses ICP–AES, spectrométrie infra-rouge avec transformation de Fourier, analyse thermogravimétrique, décomposition thermique, analyse par réactions nucléaires, et spectroscopie de Mössbauer. Nous proposons [10]–[13] + [10] une formule générale standardisée, A2BC7D2T8O26(OH)4X0–1, dans laquelle A = K, Rb, Cs, H3O , H2O, Na ou Ⅺ; B = Na ou Ca; [6]C = Mn, Fe2+, Fe3+, Na, Mg, ou Zn; [6]D = Ti, Nb, ou Zr; [4]T = Si ou Al, X = ␾ = F, OH, O, ou Ⅺ. Les données acquises par spectroscopie de Mössbauer, approximations thermodynamiques, et analyses à la microsonde électronique ont servi à démontrer que le F est ordonné au site ␾(16) mais non aux deux sites OH au sein du feuillet d’octaèdres. Ainsi, nous redéfinissons la formule des huit espèces de minéraux du groupe de l’astrophyllite. Les résultats obtenus par spectroscopie de Mössbauer 3+ 3+ indiquent des valeurs Fe /Fetot entre 0.01 et 0.21, ou bien entre 0.05 et 0.56 atomes de Fe par unité formulaire, confirmant ainsi que le Fe2+ est prédominant dans la structure. Les minéraux provenant de complexes ignés sursaturés et sous-saturés en silice sont distincts en composition chimique. Dans les roches sursaturées, le membre dominant du groupe est l’astrophyllite sensu stricto, qui se présente comme phase tardive post-magmatique, enrichie en Rb, Fe2+, Ti, Si et F. En revanche, les complexes intrusifs sous-saturés, et en particulier le Mont Saint-Hilaire, Québec, fait preuve d’une plus grande diversité dans les espèces et dans leur variabilité en composition chimique. Les échantillons du sous-groupe de la kupletskite sont enrichis en Na, Mn, Fe3+, Zn, Zr et Nb, tandis que les échantillons du sous-groupe de l’astrophyllite sont enrichis en K, Ca, Fe2+, Ti, Zr et Al. L’enrichissement des minéraux du sous-groupe de la kupletskite en Fe3+, Mn et Nb découlerait d’une cristallisation sous conditions plus oxydantes que dans le cas des minéraux du sous-groupe de l’astrophyllite. L’incorporation du Nb dans la structure et la formation de la kupletskite niobifère ou bien de la niobokupletskite résultent de la substitution M(1)2+ + M(2,3)2+ + (Zr,Ti) + F ⇔ M(1)Na + M(2,3)Fe3+ + Nb + O. (Traduit par la Rédaction) Mots-clés: minéraux du groupe de l’astrophyllite, sous-groupe de la kupletskite, sous-groupe de l’astrophyllite, formule structurale, chimie cristalline. INTRODUCTION crystal-chemical nomenclature for the astrophyllite group of minerals, and (2) describe the chemical varia- Astrophyllite sensu stricto is a Fe-dominant alkali tions observed, particularly with respect to Na, Mn, Fe, titanosilicate first discovered in 1844 in a nepheline Zn, Mg, Ti, Zr, Nb and F, in the various parageneses syenite pegmatite on the island of Låven, part of the and localities. This is the first in a series of papers deal- Larvik complex of the Oslo Rift Valley, southeastern ing with the crystal chemistry and paragenesis of Norway, and later described as a “brown mica” (Weibye astrophyllite-group minerals; details regarding crystal- 1848). Astrophyllite was later named and formally de- structure variations, Mössbauer spectroscopy, and scribed by Scheerer (1854) and Brøgger (1890). Eight paragenesis will be presented in forthcoming papers. species of astrophyllite-group minerals (AGM) are known; the most recently discovered, niobokupletskite, BACKGROUND INFORMATION was described from Mont Saint-Hilaire (MSH; Piilonen et al. 2000). Astrophyllite-group minerals have been Members of the astrophyllite group have been docu- described from many alkaline intrusions, most com- mented from a number of alkaline intrusions including, monly as accessory or rock-forming minerals in quartz most importantly, Mont Saint-Hilaire (Quebec), Strange or nepheline syenites, alkaline granites and their associ- Lake and Seal Lake (Labrador), the Khibina and ated pegmatites, but also from metamorphic rocks such Lovozero massifs, Kola Peninsula (Russia), Ilímaussaq, as nepheline syenite gneiss and riebeckite gneiss. Kangerdlugssuaq, Narssarssuk and the Werner Bjerge Although the existence of astrophyllite has been complex (Greenland), and Mount Rosa, Pikes Peak known since the late 19th century, considerable debate (Colorado). Following Brøgger’s original description in still exists regarding the anionic scheme, method of cal- 1890, there have been relatively few detailed crystal- culation of the general formula, and contrasts in com- chemical studies of the astrophyllite group. The crystal positional trends between over- and undersaturated structure of magnesium astrophyllite was first deter- alkaline. In this paper, we present the results of an ex- mined by Peng & Ma (1963), and the structure of tri- tensive study on the chemical variations observed in clinic astrophyllite (sensu stricto) was later determined astrophyllite-group minerals from a large number of by Woodrow (1967), who described it as a layered over- and undersaturated alkaline intrusions (Table 1). titanosilicate with strong affinities to biotite. Until re- Our main objectives are to (1) establish a systematic cently, most compositions reported in the literature were 001 vol 41#1 février 03 - 01 2 3/24/03, 10:37 NOMENCLATURE AND FORMULA OF ASTROPHYLLITE-GROUP MINERALS 3 done as part of larger studies on the petrogenesis of the undersaturated rocks were found to be characterized by alkaline complexes in which they occur. higher Al, Ca, Mg, Mn, OH, F, K, Na and Zr compared Ganzeyev et al. (1969) were among the first re- to those from oversaturated rocks. No attempt was made searchers to specifically investigate the isomorphous to distinguish between Mn-dominant species and Fe- substitution of alkali cations such as Rb, Cs and Li for dominant species, which display strongly contrasting K in the interlayer. Similarly, Chelishchev (1972) car- chemical characteristics within a single petrogenetic ried out the only known experimental work on environment. astrophyllite-group minerals, examining the extent of Layne et al. (1982) carried out electron-microprobe ion exchange of K with Na, Rb and Cs in an aqueous analyses on astrophyllite from silica-oversaturated and fluid under supercritical conditions (400 to 600°C). -undersaturated pegmatite dikes at Bagnæsset and Results indicate increasing Na-for-K substitution with Kramers Island, Kangerdlugssuaq (East Greenland) and increasing temperature, the high temperatures (600°C) were the
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