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Perboeite-(Ce) and alnaperboeite-(Ce), two new members of the epidote-tornebohmite polysomatic series; chemistry, structure, dehydrogenation, and clue for a sodian epidote end-member

Paola Bonazzi, Giovanni O. Lepore, Luca Bindi, Christian Chopin, Tomas A. Husdal and Olaf Medenbach
Perboeite-(Ce) and alnaperboeite-(Ce), two new members of the epidote-tornebohmite polysomatic series; chemistry, structure, dehydrogenation, and clue for a sodian epidote end-member
American Mineralogist (January 2014) 99 (1): 157-169

Abstract

In yttrian fluorite from pegmatites of the Tysfjord granite, Norway, grayish-green to very pale green gatelite-like crystals occur along with millimeter-size aggregates of Y-silicates as a late primary phase; they are associated with allanite-(Ce), bastnasite-(Ce), and intimately inter- or overgrown by tornebohmite-(Ce). Sub- to euhedral crystals, up to 400 mu m in size, are chemically zoned between two near-end-member compositions that imply the existence of two new members of the polysomatic gatelite group, in which ET polysomes are composed of E modules with epidote-type structure alternating with T modules of tornebohmite-type structure. The two new minerals form a continuous solid-solution series, along which two crystals of intermediate compositions served for species definition. Their electron-microprobe analyses yield the empirical formulas (Ca (sub 1.00) Mn (sub 0.03) Na (sub 0.08) La (sub 0.51) Ce (sub 1.30) Pr (sub 0.16) Nd (sub 0.62) Sm (sub 0.10) Gd (sub 0.06) Dy (sub 0.03) Er (sub 0.01) Y (sub 0.06) Th (sub 0.01) ) (sub Sigma 3.97) (Al (sub 3.21) Fe (super 2+) (sub 0.79) ) (sub Sigma 4.00) Si (sub 5.01) O (sub 20) (OH) (sub 2) for perboeite-(Ce) [IMA 2011-55] and (Ca (sub 1.10) Mn (sub 0.03) Na (sub 0.20) La (sub 0.42) Ce (sub 1.14) Pr (sub 0.16) Nd (sub 0.60) Sm (sub 0.1 3) Gd (sub 0.07) Dy (sub 0.03) Er (sub 0.01) Yb (sub 0.01) Y (sub 0.12) Th (sub 0.02) ) (sub Sigma 4.04) (Al (sub 3.54) Fe (super 2+) (sub 0.40) Mg (sub 0.02) ) (sub Sigma 3.96) Si (sub 4.99) O (sub 20) (OH) (sub 2) for alnaperboeite-(Ce) [IMA 2012-54]. The respective end-member formulas are (super A) (Ce (sub 3) Ca) (super M) (Al (sub 3) Fe (super 2+) )Si (sub 2) O (sub 7) (SiO (sub 4) ) (sub 3) O(OH) (sub 2) , which requires Ce (sub 2) O (sub 3) 45.10, CaO 5.14, FeO 6.58, Al (sub 2) O (sub 3) 14.01, SiO (sub 2) 27.52, H (sub 2) O 1.65, total 100.00 wt%; and (super A) (Ce (sub 2.5) CaNa (sub 0.5) ) (super M) (Al (sub 4) )Si (sub 2) O (sub 7) (SiO (sub 4) ) (sub 3) O(OH) (sub 2) , which requires Ce (sub 2) O (sub 3) 40.86, CaO 5.58, Na (sub 2) O 1.54, Al (sub 2) O (sub 3) 20.31, SiO (sub 2) 29.92, H (sub 2) O 1.79, total 100.00 wt%. Cell parameters of perboeite-(Ce) and alnaperboeite-(Ce) for these crystals are a = 8.9277(6) and 8.9110(4), b = 5.6548(6) and 5.6866(2), c = 17.587(1) and 17.5252(7) Aa, beta = 116.475(8) and 116.300(5) degrees , V = 794.8(1) and 796.13(7) Aa (super 3) , respectively. Members of the perboeite-(Ce)-alnaperboeite-(Ce) solid solution are topologically identical to the minerals gatelite-(Ce) and vastmanlandite-(Ce). Structural data (space group P2 (sub 1) /m) were obtained for the holotype crystals and for several crystals with intermediate composition. Structural refinements of a crystal annealed step-wise in air confirm that most of Fe in M3 is divalent before heating and show that oxidation/dehydrogenation takes place mostly in the E module (M3 and H1). Perboeite-(Ce) derives from gatelite-(Ce) by the homovalent substitution [ (super M3) Fe (super 2+) --> (super M3) Mg]. Alnaperboeite-(Ce) derives from perboeite-(Ce) or gatelite-(Ce) by the coupled heterovalent substitutions [ (super A) Na (super +) + 2 (super M3) Al (super 3+) --> (super A) REE + 2 (super M3) (Fe (super 2+) or Mg)]. Tornebohmite-(Ce) associated with alnaperboeite-(Ce) is Na-free, whereas coexisting allanite is Na-bearing and shows the same coupled substitution between A and M sites as the one relating perboeite-(Ce) and alnaperboeite-(Ce) (Na (sub 0.5) Al <--> REE (sub 0.5) Fe (super 2+) ). This could suggest, although crystallographic evidence is inconclusive, that Na incorporation in the ET polysome occurs in the E module alone (A2 or A1 sites), leading to the sodian E end-member (super A) (CaREE (sub 0.5) Na (sub 0.5) ) (super M) (Al (sub 3) )Si (sub 2) O (sub 7) (SiO (sub 4) )O(OH). In any event, this new epidote end-member is needed to account for up to ca. 10 mol% of the composition of allanite-group minerals, in which Na (sub 2) O contents may reach 0.3 wt%. Sodium must be analyzed in epidote-supergroup and gatelite-group minerals.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 99
Serial Issue: 1
Title: Perboeite-(Ce) and alnaperboeite-(Ce), two new members of the epidote-tornebohmite polysomatic series; chemistry, structure, dehydrogenation, and clue for a sodian epidote end-member
Affiliation: Universita di Firenze, Dipartimento di Scienze della Terra, Florence, Italy
Pages: 157-169
Published: 201401
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 37
Accession Number: 2014-008047
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 11 tables
N64°55'00" - N69°17'60", E11°45'00" - E18°15'00"
Secondary Affiliation: Ecole Normale Superieure, FRA, FranceRuhr-Universitaet Bochum, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201406
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