Original paper

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

Morteani, Giulio; Ackermand, Dietrich

European Journal of Mineralogy Volume 8 Number 4 (1996), p. 853 - 870

50 references

published: Jul 29, 1996
manuscript accepted: Apr 9, 1996
manuscript received: Mar 4, 1994

DOI: 10.1127/ejm/8/4/0853

BibTeX file

ArtNo. ESP147050804016, Price: 29.00 €

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Abstract A metaquartzite horizon bearing aluminium phosphates occurs over several kilometres in the Penninic Lower "SchieferhÜlle" units of the Tauern 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. Association- 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. Electron microprobe analyses show that lazulite, (Mg,Fe2+)Al2(OH)2(PO4)2, has XMg [= Mg/(Mg+Fe2+)] ranging between 0.95 and 0.75 in association-type A, and between 0.65 and 0.55 in type B. (Sr,Ca)-bearing Al phosphates are a quaternary solid-solution series between crandallite-goyazite and woodhouseite-svanbergite (Ca,Sr)Al3H1-x[(OH)6(PO4)2-x{(PO4)(SO4}x], and a binary one between bearthite and goedkenite, (Ca,Sr)2Al(OH)(PO4)2. The quaternary phase of type A has an XSr [= Sr/(Sr+Ca)] in the range of 0.35 to 0.95 and an Xp [=(P-1)/(P-1)+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 close to Cai.iSro.9Al(OH)(PO4)2- Tourmaline in type A shows a variation of XMg from core to rim from 0.15 to 0.55 and in type C from 0.48 to 0.70. Staurolite has an XMg [= Mg/(Mg+Fe2++Zn)] of 0.25. Apatites are, if euhedral and set in the groundmass, in all cases Sr-rich, whereas the small and anhedral apatites found in contact or included in lazulite are Sr-poor. Lazulite blasts are often irregularly rimmed by (Sr,Ca)-bearing Al phosphates and show inclusions 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 Al phosphates and than bearthite. As established by previous studies the maximum P-T conditions of metamorphism in the studied area were 550ºC and 10 kbar. The protolith of the studied rocks was most likely a sabkha-like sediment with Na leached by flash flood waters.


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