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, hasbeen fully characterised using a multi-analytical approach to define its crystal-chemical identity. Single-crystal X-ray diffraction, chem-ical 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 betweenthe 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’.

Celata, BeatriceSapienza Università di Roma, Italy (författare)
Skogby, HenrikNaturhistoriska riksmuseet,Enheten för geovetenskap(Swepub:nrm)henrskog (författare)
Hålenius, UlfNaturhistoriska riksmuseet,Enheten för geovetenskap(Swepub:nrm)ulfhalen (författare)
Tempesta, GioacchinoUniversity of Bari "Aldo Moro", Italy (författare)
Ciriotti, MarcoUniversity of Turin, Italy (författare)
Bittarello, EricaUniverity of Turin, Italy (författare)
Marengo, AlessandraUniverity of Turin, Italy (författare)
Sapienza Università di Roma, ItalyEnheten för geovetenskap (creator_code:org_t)

Ingår i:Mineralogical magazine: Mineralogical Society85:2, s. 242-2530026-461X1471-8022

Av författaren/redakt...: Bosi, Ferdinando; Celata, Beatrice; Skogby, Henrik; Hålenius, Ulf; Tempesta, Gioacc ...; Ciriotti, Marco; visa fler...; Bittarello, Eric ...; Marengo, Alessan ...; visa färre...

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