Mn-Bearing Purplish-Red Tourmaline From The Anjanabonoina Pegmatite, Madagascar

A gem-quality purplish-red tourmaline sample of alleged liddicoatitic composition from the Anjanabonoina pegmatite, Madagascar, has been fully characterised 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)(Na-0.41.Ca-0.35(0.24))(Sigma 1.00)(Y)(Al1.81Li1.00Fe0.043+Mn0.023+Mn0.122+Ti0.004) Al-Z(Sigma 3.00)6 [(T)(Si5.60B0.40)(Sigma 6.00)O-18] (BO3)(3) (OH)(3) (W)[(OH)(0.50)F0.13O0.37] (Sigma 1.00) which corresponds to the tourmaline species elbaite having the typical space group R3m and relatively small unit-cell dimensions, a = 15.7935(4) A, c = 7.0860(2) A and V = 7.0860(2) A(3).Optical absorption spectroscopy showed that the purplish-red colour is caused by minor amounts of Mn3+ (Mn2O3 = 0.20 wt.%). Thermal treatment in air up to 750 degrees C strongly intensified the colour 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'.