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

Abstract

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 □ 0.35 Ca 0.24 ) Σ1.00 Y (Al 1.81 Li 1.00 Fe 3+ 0.04 Mn 3+ 0.02 Mn 2+ 0.12 Ti 0.004 ) Σ3.00 Z Al 6 [ T (Si 5.60 B 0.40 ) Σ6.00 O 18 ] (BO 3 ) 3 (OH) 3 W [(OH) 0.50 F 0.13 O 0.37 ] Σ1.00 which corresponds to the tourmaline species elbaite having the typical space group R 3 m and relatively small unit-cell dimensions, a = 15.7935(4) Å, c = 7.0860(2) Å and V = 7.0860(2) Å 3 . Optical absorption spectroscopy showed that the purplish-red colour is caused by minor amounts of Mn 3+ (Mn 2 O 3 = 0.20 wt.%). Thermal treatment in air up to 750°C strongly intensified the colour of the sample due to the oxidation of all Mn 2+ to Mn 3+ (Mn 2 O 3 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’.