%0 Journal Article %J Minerals %D 2019 %T The Atomic Arrangement of Cr-rich Tourmaline from the #1 Mine, Balmat, St. Lawrence County, New York, USA %A Dannenberg, Steven G. %A Di Paolo, Devany %A Ehlers, Alix M. %A McCarthy, Kyle P. %A Mancini, Mark T. %A Reuter, Matthew B. %A Seth, Dennis M. %A Song, Zihui %A Valladares, Maria I. %A Zhu, Xuanfu %A Hughes, John M. %A M. V. Lupulescu %K Balmat mine %K chromium-dravite %K New York %K St. Lawrence County %K tourmaline %X

Chromium-bearing tourmalines are rare. Chromium-rich tourmaline from the northwestern part of the Adirondack Mountains in the Adirondack Lowlands is among the most chromium-rich tourmalines found to date. The mineral, with >21.0 wt. % Cr2O3, is from the marble-hosted talc–tremolite–cummingtonite schist in the #1 mine in Balmat, St. Lawrence County, New York. The atomic arrangement of the sample (a = 16.0242(3) Å, c = 7.3002(2) Å) was refined to R1 = 0.0139. The composition, from chemical analyses and optimization of the formula, is X(Ca0.22Na0.69K0.01) Y(Cr3+1.68Mg0.80Ti0.13V0.06Mn0.02Fe0.02Li0.29) Z(Al3.11Cr3+1.18Mg1.70Fe0.01) T(Si5.93Al0.07) B3O27 OH3.99 F0.01. There has been extensive debate over the ordering of Cr3+ between the tourmaline Y and Z octahedral sites. Recent work has suggested that, at low concentrations (<~1.03 apfu), the substituent Cr3+ is ordered into the Y-site, whereas, at greater concentrations, the substituent is disordered over both octahedral sites. An analysis of nine recently published, high-precision structures of chromium-bearing tourmaline, in combination with the Adirondack tourmaline, suggests that structural changes to the Y-site at low concentrations of Cr3+ induce changes in the Z-site that make it more amenable to incorporation of the Cr3+ substituents by increasing <Z–O>. The bond lengths change to lower the bond-valence sum of Cr3+ in the Z-site of the chromium-dravite, making that site more amenable to the substituent. Calculations suggest that the Z-site begins to accept substituent Cr3+ when the bond valence sum of that ion in Z reduces to a value of ~3.36 valence units.

%B Minerals %V 9 %P 398 %8 06/2019 %G eng %U https://www.mdpi.com/2075-163X/9/7/398https://www.mdpi.com/2075-163X/9/7/398/pdf %N 7 %! Minerals %R 10.3390/min9070398