Aluminium Phosphates in Muscovite-Kyanite Metaquartzites from Passo Di Vizze (Alto Adige, NE Italy)

Eur. J. Mineral. 1996, 8, 853-869

Aluminium phosphates in muscovite-kyanite metaquartzites from Passo di Vizze (Alto Adige, NE Italy)

GIULIO MORTEANI* and DIETRICH ACKERMAND**

*Lehrstuhl für Angewandte Mineralogie, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching, Germanyl **Mineralogisch-Petrographisches Institut, Universität Kiel, Olshausenstraße 40, D-24118 Kiel, Germany

Abstract: A metaquartzite horizon bearing aluminium phosphates occurs over several kilometres in the Penninic Lower "Schieferhülle" units of the Tauem Window at the Passo di Vizze (Alto Adige, NE Italy). It contains the following types of mineral association: A) pale-blue lazulite, colourless crandallite-goyazite, apatite, tourmaline, B) dark-blue lazulite, colourless svanbergite-goyazite, bearthite, apatite, celestite, and C) staurolite, chlorite and tourmaline. Additional minerals in all cases are quartz, muscovite, kyanite and accessory Ti-bearing hematite. As sociation-types A and B are the common ones, type C is found only as a thin layer on the surface of coarse quartz pebbles embedded in types A or B. Staurolite is never found in contact with the phosphates. 2+ 2+ Electron microprobe analyses show that lazulite, (Mg,Fe )Ah(OH)2(P04h, has XMg [= Mg/(Mg+Fe )] ranging between 0.95 and 0.75 in association-type A, and between 0.65 and 0.55 in type B. (Sr,Ca)-bearing AI phosphates are a quatemary solid-solution series between crandallite-goyazite and woodhouseite-svanber gite (Ca,Sr)AbH1-x[(OH)6(P04h-x{(P04)(S04}x], and a binary one between bearthite and goedkenite, (Ca,SrhAI(OH)(P04h. The quatemary phase of type A has an XSr [= Sr/(Sr+Ca)] in the range of 0.35 to 0.95 and an Xp [=(P-I)/(P-l) +S] in the range of 0.45 to 0.90, indicating a main solid solution series between crandallite and goyazite; in the association-type B this phase has an XSr in the range of 0.95 to 0.99 and an Xp in the range of 0.29 to 0.90, showing a solid-solution series between svanbergite and goyazite. Bearthite is found in association-type B only and has a composition c10se to Cal.lSro.9AI(OH)(P04h. Tour maline in type A shows a variation of XM from core to rim from 0.15 to 0.55 and in type C from 0.48 to g 2+ 0.70. Staurolite has an XMg [= Mg/(Mg+Fe +Zn)] of 0.25. Apatites are, if euhedral and set in the ground mass, in all cases Sr-rich, whereas the small and anhedral apatites found in contact or inc1uded in lazulite are Sr-poor. Lazulite blasts are often irregularly rimmed by (Sr,Ca)-bearing AI phosphates and show inc1usions of anhedral apatite. Bearthite is never found in contact with celestine and kyanite. On the basis of textural evidence lazulite is older than the (Sr,Ca)-bearing AI phosphates and than bearthite. As established by previous studies the maximum P-T conditions of metamorphism in the studied area were 5502C and 10 kbar. The protolith of the studied rocks was most likely a sabkha-like sediment with Na leached by flash flood waters.

Key-words: bearthite, celestine, crandallite-goyazite, Italy, lazulite, metaquartzite, Pas so di Vizze, svanbergite-goyazite, Tauem Window.

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