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The Atomic Arrangement of Cr-rich Tourmaline from the #1 Mine, Balmat, St. Lawrence County, New York, USA

1
Department of Geology, University of Vermont, Burlington, VT 05405, USA
2
New York State Museum, Research and Collections, 3140 CEC, Albany, NY 12230, USA
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(7), 398; https://doi.org/10.3390/min9070398
Received: 16 May 2019 / Revised: 24 June 2019 / Accepted: 27 June 2019 / Published: 28 June 2019
(This article belongs to the Special Issue Minerals of the Southern Grenville Province)
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Abstract

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. View Full-Text
Keywords: tourmaline; chromium-dravite; Balmat mine; St. Lawrence County; New York tourmaline; chromium-dravite; Balmat mine; St. Lawrence County; New York
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Dannenberg, S.G.; Di Paolo, D.; Ehlers, A.M.; McCarthy, K.P.; Mancini, M.T.; Reuter, M.B.; Seth, D.M., Jr.; Song, Z.; Valladares, M.I.; Zhu, X.; Hughes, J.M.; Lupulescu, M.V. The Atomic Arrangement of Cr-rich Tourmaline from the #1 Mine, Balmat, St. Lawrence County, New York, USA. Minerals 2019, 9, 398.

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