Abstract
AbstractA 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(Na0.41□0.35Ca0.24)Σ1.00Y(Al1.81Li1.00Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00ZAl6 [T(Si5.60B0.40)Σ6.00O18] (BO3)3 (OH)3W[(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 colour is caused by minor amounts of Mn3+ (Mn2O3 = 0.20 wt.%). Thermal treatment in air up to 750°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’.
Highlights
Tourmaline is the most common and the earliest-formed boron mineral on Earth and has recently received increasing interest from the geoscience community (Grew et al, 2016; Dutrow and Henry, 2018; Henry and Dutrow, 2018)
The comprehensive multi-analytical approach of the present study shows that the gem-quality purplish-red colour of the hand specimen of elbaite from the Anjanabonoina pegmatite (Madagascar) is caused by Mn3+, which occurs as a minor element (Mn2O3 = 0.20 wt.%, corresponding to 0.02 apfu)
The bulk chemical composition of the present sample corresponds to elbaite, based on the electrostatic H+–Na+ repulsion, the more intense Fourier-transform infrared (FTIR) bands related to O1 could be ascribed to the rossmanite-type arrangements (YYY)–O1(OH)–X(□) rather than elbaite-type ones (YYY)–O1(OH)–X(Na)
Summary
Tourmaline is the most common and the earliest-formed boron mineral on Earth and has recently received increasing interest from the geoscience community (Grew et al, 2016; Dutrow and Henry, 2018; Henry and Dutrow, 2018). Red tourmalines are commonly given the varietal name ‘rubellite’, which refers to tourmalines with colours ranging from rose, dark pink to purplish red, but do correspond to several regular mineral species: elbaite, fluor-elbaite, rossmanite, fluor-liddicoatite (Pezzotta and Laurs, 2011) and oxy-dravite (Bosi and Skogby, 2013). The sample was analysed through SEM-EDS and surprisingly showed no trace of F; in addition, its composition did not fit ‘liddicoatite’ in terms of Na and Ca. In the present study, such a tourmaline sample from the Anjanabonoina pegmatite, central Madagascar, has been fully characterised by X-ray single-crystal diffraction, electron microprobe analysis, laser induced breakdown spectroscopy, micro-Raman, infrared and optical absorption spectroscopy, with the aim of defining its crystal-chemical identity and species. More details about the history of the 1500s to 2000s explorations and mining of the Malagasy tourmaline gem deposits can be found in Strunz (1979), Pezzotta (1999) and Dirlam et al (2002)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
