03615nas a2200325 4500008004100000245011000041210006900151260001200220300000800232490000600240520260900246653001602855653002102871653001302892653002402905653001502929100002702944700002102971700002102992700002303013700002203036700002403058700002103082700001603103700002603119700001603145700002103161700002203182856008503204 2019 eng d00aThe Atomic Arrangement of Cr-rich Tourmaline from the #1 Mine, Balmat, St. Lawrence County, New York, USA0 aAtomic Arrangement of Crrich Tourmaline from the 1 Mine Balmat S c06/2019 a3980 v93 a

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.

10aBalmat mine10achromium-dravite10aNew York10aSt. Lawrence County10atourmaline1 aDannenberg, Steven, G.1 aDi Paolo, Devany1 aEhlers, Alix, M.1 aMcCarthy, Kyle, P.1 aMancini, Mark, T.1 aReuter, Matthew, B.1 aSeth, Dennis, M.1 aSong, Zihui1 aValladares, Maria, I.1 aZhu, Xuanfu1 aHughes, John, M.1 aLupulescu, M., V. uhttps://www.mdpi.com/2075-163X/9/7/398https://www.mdpi.com/2075-163X/9/7/398/pdf