Sb 3+ ,As 3+ ) S3 O 12 (O,OH,&) S3 , is a member of the dumortierite group that has been found in pegmatite, or alluvial deposits derived from pegmatite, at three localities: Greenbushes, Western Australia; Voron'i Tundry, Kola Peninsula, Russia; and Szklary, Lower Silesia, Poland. Holtite can contain >30 wt.% Sb 2 O 3 , As 2 O 3 , Ta 2 O 5 , Nb 2 O 5 , and TiO 2 (taken together), but none of these constituents is dominant at a crystallographic site, which raises the question whether this mineral is distinct from dumortierite. The crystal structures of four samples from the three localities have been refined to R 1 = 0.02À0.05. The results show dominantly: Al, Ta, and vacancies at the Al(1) position; Al and vacancies at the Al(2), (3) and (4) sites; Si and vacancies at the Si positions; and Sb, As and vacancies at the Sb sites for both Sb-poor (holtite I) and Sb-rich (holtite II) specimens. Although charge-balance calculations based on our single-crystal structure refinements suggest that essentially no water is present, Fourier transform infrared spectra confirm that some OH is present in the three samples that could be measured. By analogy with dumortierite, the largest peak at 3505À3490 cm À1 is identified with OH at the O(2) and O(7) positions. The single-crystal X-ray refinements and FTIR results suggest the following general formula for holtite: Al 7À[5x+y+z]/3 (Ta,Nb) x & [2x+y+z]/3 BSi 3Ày (Sb,As) y O 18ÀyÀz (OH) z , where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z 4 y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al 5.83 (Ta,Nb) 0.50 & 0.67 BSi 2.50 (Sb,As) 0.50 O 17.00 (OH) 0.50 and Al 5.92 (Ta,Nb) 0.25 & 0.83 BSi 2.00 (Sb,As) 1.00 O 16.00 (OH) 1.00 for holtite I and holtite II respectively. However, the crystal structure refinements do not indicate a fundamental difference in cation ordering that might serve as a criterion for recognizing the two holtites as distinct species, and anion compositions are also not sufficiently different. Moreover, available analyses suggest the possibility of a continuum in the Si/(Sb + As) ratio between holtite I and dumortierite, and at least a partial continuum between holtite I and holtite II. We recommend that use of the terms holtite I and holtite II be discontinued.
Textural relationships and compositional variations of (Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxides and associated minerals from low-to moderately fractionated, mixed (NYF + LCT) anatectic pegmatites, which belong to the Julianna pegmatitic system, exposed at Piława Górna, Lower Silesia, Poland, were studied to elucidate the geochemical evolution of the pegmatite-forming melt. The primary Nb-Ta oxides crystallized in the following sequence: (1) columbite-(Fe) evolving to columbite-(Mn), associated with Nb-and Ta-bearing ilmenite-pyrophanite + almandine-spessartine garnet + Ti-poor schorl, (2) columbite-(Fe), (3) tantalite-(Fe), (4) Ti-and Sn-bearing ixiolite ± ferrowodginite ± Nb-and Ta-bearing cassiterite ± columbite-(Fe), all crystallizing along with Ti-enriched schorl evolving to dravite. The most characteristic features of the mineral assemblages are the reversed Mn-Fe fractionation trends and increasing Ca concentrations at the final stage of crystallization. These compositional variations are explained by a possible pre-emplacement contamination of the pegmatite-forming melt and re-equilibration of biotite and plagioclase from the early-crystallized outer zones of the pegmatites with rapidly cooling pegmatitic melt. Local changes in melt composition were of minor importance. Late alterations, such as chloritization and muscovitization of biotite or albitization of Ca-rich plagioclase were significant only at the hydrothermal stage.
Fe 2+-and Mn 2+-rich tourmalines were used to test whether Fe 2+ and Mn 2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe 2+ (~2.3 apfu), and substantial amounts of Fe 3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe 3+ (no delocalized electrons) and Ti 4+ to the Z site and the amount of Fe 2+ and Fe 3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X (Na 0.9 Ca 0.1) Y (Fe 2+ 2.0 Al 0.4 Mn 2+ 0.3 Fe 3+ 0.2) Z (Al 4.8 Fe 3+ 0.8 Fe 2+ 0.2 Ti 4+ 0.1) T (Si 5.9 Al 0.1)O 18 (BO 3) 3 V (OH) 3 W [O 0.5 F 0.3 (OH) 0.2 ] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe 2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X (Na 0.9 Ca 0.
The newly dis cov ered Julianna peg ma titic sys tem from the Pi³awa Górna Quarry (the Góry Sowie Block, Sudetes, NE mar gin of the Bo he mian Mas sif) is de scribed in terms of geo log i cal set ting, pe trog ra phy and de scrip tive min er al ogy. The sys tem repre sents the larg est peg ma titic oc cur rence in the Pol ish Sudetes and con sists of a com plex net work of cogenetic rare-el e ment gra nitic pegmatites that in truded into tectonized am phi bo lite as dis cor dant dikes. The pegmatites range from bar ren and weakly zoned to tex tur ally well-dif fer en ti ated ones that are com posed of a fine-grained bor der zone, coarse-grained wall zone, graphic and blocky feld spar in ter me di ate zones and a quartz core. Uni di rec tional and skel e tal so lid i fi ca tion tex tures are well-de vel oped. The Julianna pegmatites con sist of rock-form ing plagioclase (£An 39 ), microcline, quartz and bi o tite accom pa nied mostly by ac ces sory to mi nor mus co vite, tour ma line, gar net and beryl. They crys tal lised from anatectic melt of hy brid NYF (ni o bium-yt trium-flu o rine) + LCT (lith ium-ce sium-tan ta lum) geo chem i cal char ac ter is tics. Pegmatites with a low to mod er ate de gree of frac tion ation, that dom i nate in the Julianna sys tem, bear NYF-sig na ture ac ces sory min er als, such as al la nite-(Ce), colum bite-, euxenite-and samarskite-group min er als, fergusonite-(Y) and gadolinite-(Y). How ever, rare dikes that at tained a very high de gree of frac tion ation con tain typ i cal min er als of LCT-sig na ture in clud ing tour ma lines of the elbaite-olenite-rossmanite se ries, le pido lite, lithiophilite, spodumene, Cs-rich beryl and pollucite.
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