Holtite and Dumortierite from the Szklary Pegmatite, Lower Silesia, Poland

The Szklary holtite is represented by three compositional varieties: (I) Ta-bearing (up to 14.66 wt.% Ta(2)O(5)), which forms homogeneous crystals and cores within zoned crystals; (2) Ti-bearing (up to 3.82 wt.% TiO(2)), found as small domains within the core; and (3) Nb-bearing (up to 5.30 wt.% Nb(2)O(5),) forming the rims of zoned crystals. All three varieties show variable Sb+As content, reaching 19.18 wt.% Sb(2)O(3) (0.87 Sb a.p.f.u.) and 3.30 wt.% As(2)O(3) (0.22 As a.p.f.u.) in zoned Ta-bearing holtite, which constitutes the largest Sb+As content reported for the mineral. The zoning in holtite is a result of Ta-Nb fractionation in the parental pegmatite-forming melt together with contamination of the relatively thin Szklary dyke by Fe, Mg and Ti. Holtite and the As- and Sb-bearing dumortierite, which in places overgrows the youngest Nb-bearing zone, suggest the following crystallization sequence: Ta-bearing holtite -> Ti-bearing holtite -> Nb-bearing holtite -> As- and Sb-bearing, (Ta,Nb,Ti)-poor dumortierite -> As- and Sb-dominant, (Ta,Nb,Ti)-free dumortierite-like mineral (16.81 wt.% As(2)O(3) and 10.23 wt.% Sb(2)O(3)) with (As+Sb) > Si. The last phase is potentially a new mineral species, Al(6)rectangle B(Sb,As)(3)O(15). or Al(5)rectangle(2)B(Sb,As)(3)O(12)(OH)(3), belonging to the dumortierite group. The Szklary holtite shows no evidence of clustering of compositions around 'holtite I' and 'holtite II'. Instead, the substitutions of Si(4+) by Sb(3+)+As(3+) at the Si/Sb sites and of Ta(5+) by Nb(5+) or Ti(4+) at the Al(l) site suggest possible solid solutions between: (1) (Sb,As)-poor and (Sb,As)-rich holtite; (2) dumortierite and the unnamed (As+Sb)-dominant dumortierite-like mineral; and (3) Ti-bearing dumortierite and holtite, i.e. our data provide further evidence for miscibility between holtite and dumortierite, but leave open the question of defining the distinction between them. The Szklary holtite crystallized from the melt along with other primary Ta-Nb-(Ti) minerals such as columbite-(Mn), tantalite-(Mn), stibiotantalite and stibiocolumbite as the availability of Ta decreased. The origin of the parental melt can be related to anatexis in the adjacent Sowie Mountains complex, leading to widespread migmatization and metamorphic segregation in pelitic-psammitic sediments metamorphosed at similar to 390-380 Ma.

Boron-Bearing Kornerupine from Fiskenaesset, West Greenland: A Reexamination of Specimens from the Type Locality

In 1884, Lorenzen proposed the formula MgAI2SiO6 for his new mineral kornerupine from Fiskenæsset and did not suspect it to contain boron. Lacroix and de Gramont (1919) reported boron in Fiskenæsset kornerupine, while Herd (1973) found none. New analyses (ion microprobe mass analyser and spectrophotometric) of kornerupine in three specimens from the type locality, including the specimens analysed by Lorenzen and Herd, indicate the presence of boron in all three, in amounts ranging from 0.50 to 1.44 wt.% B203, e.g. (Li0.04 Na0.01 Ca0.01) (Mg3.49 Mn0.01 Fe0.17 Ti0.01 Al5.64)Σ9.30 (Si3.67 Al1.02 B0.31)Σ5 O21 (OH0.99 F0.01) for Lorenzen's specimen. Textures and chemical compositions suggest that kornerupine crystallized in equilibrium in the following assemblages, all with anorthite (An 92-95) and phlogopite (XFe = atomic Fe/(Fe + Mg) = 0.028-0.035): (1) kornerupine (0.045)-gedrite (0.067); (2) kornerupine (0.038-0.050)-sapphirine (0.032-0.035); and (3) kornerupine (0.050)-hornblende. Fluorine contents of kornerupine range from 0.01 to 0.06%, of phlogopite, from 0.09 to 0.10%. In the first assemblage, sapphirine (0.040) and corundum are enclosed in radiating bundles of kornerupine; additionally sapphirine, corundum, and/or gedrite occur with chlorite and pinite (cordierite?) as breakdown products of kornerupine. Kornerupine may have formed by reactions such as: gedrite + sapphirine + corundum + B203 (in solution) + H20 = kornerupine + anorthite + Na-phlogopite under conditions of the granulite facies. Boron for kornerupine formation was most likely remobilized by hydrous fluids from metasedimentary rocks occurring along the upper contact of the Fiskenæsset gabbro-anorthosite complex with amphibolite.

New Data on Welshite, e.g. Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2 [Si2.8Be1.7Fe0.653+Al0.7As0.17O18], an Aenigmatite-Group Mineral

Electron and ion microprobe data on two samples of welshite from the type locality of Langban, Sweden, gave analytical totals of 99.38-99.57 wt.% and BeO contents of 4.82-5.11 wt.%, corresponding to 1.692-1.773 Be/20 O. Mossbauer and optical spectra of one of these samples gave Fe-[iv](3+)/Sigma Fe = 0.91, Fe-[iv](2+)/Sigma Fe = 0.09, and no evidence of Mn3+. The resulting formula for this sample is Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18], and that for the second sample, Ca2Mg3.8Mn0.12+Fe0.12+F0.83+Sb1.25+O2[Si2.8Be1.8F0.653+Al0.25As0.25O18], is related by the substitution involving tetrahedral and octahedral sites: 0.59([vi,iv])(Fe,Al)(3+) approximate to 0.42([vi])(Mg,Mn,Fe)(2+) + 0.21(Sb-[vi],As-[iv])(5+), i.e. 3([vi,iv]) M3+ = 2([vi])M(2+) + M-[vi,iv](5+). WelShite is distinctive among aenigmatite-group minerals in the high proportion of Fe 3+ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is >50% occupied by Be and that one octahedral site is >50% occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.