• journal_title：Mineralogical Magazine
• Contributor：A. Pieczka ; E. S. Grew ; L. A. Groat ; R. J. Evans
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2011.075.2.303
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：75
• issue：2
• firstpage：303
• section：Articles

The Szklary holtite is represented by three compositional varieties: (1) Ta-bearing (up to 14.66 wt.% Ta₂O₅), which forms homogeneous crystals and cores within zoned crystals; (2) Ti-bearing (up to 3.82 wt.% TiO₂), found as small domains within the core; and (3) Nb-bearing (up to 5.30 wt.% Nb₂O₅,) forming the rims of zoned crystals. All three varieties show variable Sb+As content, reaching 19.18 wt.% Sb₂O₃ (0.87 Sb a.p.f.u.) and 3.30 wt.% As₂O₃ (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.% As₂O₃ and 10.23 wt.% Sb₂O₃) with (As+Sb) > Si. The last phase is potentially a new mineral species, Al₆□B(Sb,As)₃O₁₅, or Al₅□₂B(Sb,As)₃O₁₂(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 Si⁴⁺ by Sb³⁺+As³⁺ at the Si/Sb sites and of Ta⁵⁺ by Nb⁵⁺ or Ti⁴⁺ at the Al(1) 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 ∼390–380 Ma.