A gem-quality purplish-red tourmaline sample of alleged liddicoatitic composition from the Anjanabonoina pegmatite, Madagascar, has been fully characterized using a multi-analytical approach to define its crystal-chemical identity. Single-crystal X-ray diffraction, chemical and spectroscopic analysis resulted in the formula: X(Na0.410.35Ca0.24)Σ1.00 Y(Al1.81Li1.00Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00 ZAl6 [T(Si5.60B0.40)Σ6.00O18] (BO3)3 (OH)3 W[(OH)0.50F0.13O0.37]Σ1.00 which corresponds to the tourmaline species elbaite having the typical space group R3m and relatively small unit-cell dimensions, a = 15.7935(4) Å, c = 7.0860(2) Å and V = 7.0860(2) Å3. Optical absorption spectroscopy showed that the purplish-red color is caused by the minor amounts of Mn3+ (Mn2O3 = 0.20 wt%). Thermal treatment in air up to 750 °C strongly intensified the color of the sample due to the oxidation of all Mn2+ to Mn3+ (Mn2O3 up to 1.21 wt%). Based on infrared and Raman data, a crystal-chemical model regarding the electrostatic interaction between the X-cation and W-anion, and involving the Y-cations as well, is proposed to explain the absence or rarity of the mineral species “liddicoatite”.

Mn-bearing purplish-red tourmaline from the Anjanabonoina pegmatite, Madagascar

Tempesta, Gioacchino;
2021

Abstract

A gem-quality purplish-red tourmaline sample of alleged liddicoatitic composition from the Anjanabonoina pegmatite, Madagascar, has been fully characterized using a multi-analytical approach to define its crystal-chemical identity. Single-crystal X-ray diffraction, chemical and spectroscopic analysis resulted in the formula: X(Na0.410.35Ca0.24)Σ1.00 Y(Al1.81Li1.00Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00 ZAl6 [T(Si5.60B0.40)Σ6.00O18] (BO3)3 (OH)3 W[(OH)0.50F0.13O0.37]Σ1.00 which corresponds to the tourmaline species elbaite having the typical space group R3m and relatively small unit-cell dimensions, a = 15.7935(4) Å, c = 7.0860(2) Å and V = 7.0860(2) Å3. Optical absorption spectroscopy showed that the purplish-red color is caused by the minor amounts of Mn3+ (Mn2O3 = 0.20 wt%). Thermal treatment in air up to 750 °C strongly intensified the color of the sample due to the oxidation of all Mn2+ to Mn3+ (Mn2O3 up to 1.21 wt%). Based on infrared and Raman data, a crystal-chemical model regarding the electrostatic interaction between the X-cation and W-anion, and involving the Y-cations as well, is proposed to explain the absence or rarity of the mineral species “liddicoatite”.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11586/361805
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