The thermal behaviour of a fluor-elbaite from Minas Gerais (Brazil) was investigated at room pressure through in situ high-temperature X-ray powder diffraction (HT-XRPD), until the breakdown conditions were reached. The variations of fluor-elbaite structural parameters (unit-cell parameters and mean bond distances) were monitored together with site occupancies, and two main internal reactions were identified: the thermally-induced Fe oxidation process counterbalanced by (OH)– deprotonation, which starts at 500 °C (773 K), followed by a partial intracrystalline Fe–Al exchange between the octahedrally-coordinated Y and Z sites. The fluor-elbaite breakdown reaction occurs between 850 °C (1123 K) and 900 °C (1173 K). The breakdown products were identified at room temperature by XRPD and the breakdown reaction can be described by the following reaction: tourmaline → B-bearing mullite + hematite + spinel + B-poor (Na, Li, H2O)-bearing glass. Boromullite itself was not observed in the final heating products, and the B-bearing mullite from the breakdown reaction exhibited unit-cell parameters a = 7.5382(2) Å, b = 7.6749(2) Å, c = 2.8385(1) Å, V = 164.22(1) Å3 (space group Pbam) consistent with an approximate Al8.5B1.5Si2O19 composition.

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(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’.

The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition 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 was investigated using both in situ High-Temperature X-Ray powder diffraction (HT-pXRD) and ex situ X-Ray single-crystal diffraction (SC-XRD) on two single crystals previously heated in the air up to 750 and 850 °C. The first occurrence of mullite diffraction peaks allowed us to constrain the breakdown temperature of Mnbearing elbaite at ambient pressure, at 825 °C. The breakdown products from the HT-pXRD experiments were cooled down to ambient temperature and identified via pXRD, represented by B-mullite and γ-LiAlSi 2 O 6 . A thermally induced oxidation of Mn 2+ to Mn 3+ was observed with both in-situ and ex-situ techniques; it started at 470 °C and is assumed to be counterbalanced by deprotonation, according to the equation: Mn 2+ + (OH) -→ Mn 3+ + O 2-+ 1/2H 2 . At temperatures higher than 752 °C, a partial disorder between the Y and Z sites is observed from unit-cell parameters and mean bond distances, possibly caused by the inter-site exchange mechanism Y

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The thermal behaviour of an uvite from San Piero in Campo (Elba Island, Italy) was investigated at room pressure through in-situ High-Temperature X-Ray Powder Diffraction (HT-XRPD), until the breakdown conditions were reached. The variation of uvite structural parameters (unit-cell parameters and mean bond distances) was monitored together with site occupancies and we observed the thermally-induced Fe oxidation process counterbalanced by (OH)− deprotonation, which starts at 450°C and is completed at 650°C. The uvite breakdown reaction occurs between 800 and 900°C. The breakdown products were identified at room temperature by X-Ray Powder Diffraction (XRPD) and the breakdown reaction can be described by the following reaction: tourmaline → indialite + yuanfuliite + plagioclase + “boron-mullite” phase + hematite.