Liddicoatite, Ca(Li2Al)Al6(Si6O18)(BO3)3(OH)3OH, a rare, long-anticipated OH-dominant species of the tourmaline supergroup

Abstract

Liddicoatite, ideally Ca(Li2Al)Al6(Si6O18)(BO3)3(OH)3OH, is a new mineral species of the tourmaline supergroup and the OH-dominant analogue of fluor-liddicoatite. The name liddicoatite was originally applied to a species that was subsequently renamed fluor-liddicoatite following the 2011 tourmaline-supergroup nomenclature. The new holotype material was found in an elbaite-subtype granitic pegmatite at Dolní Rožínka, Czech Republic, where it occurs as a late-stage metasomatic phase in the graphic unit. Liddicoatite forms microscopic domains, up to 1 mm across, within a zoned colorless to pale green rim surrounding black tourmaline. The compositional evolution of the zoned crystal is from Fe-rich fluor-elbaite in the core through fluor-liddicoatite to liddicoatite, followed by elbaite in the outermost rim.

Liddicoatite is colorless to very pale green, with vitreous luster, white streak, and conchoidal fracture. It is uniaxial (–), has a Mohs hardness of 7–8, a calculated density of 3.035 g/cm3, and an excellent compatibility index (1–KP/KC = 0.039). It is trigonal, space group R3m, with unit-cell parameters a = 15.8277(3) Å, c = 7.0985(1) Å, V = 1540.04(6) Å3 and Z = 3. The crystal structure was refined to R1 = 1.45 % using room-temperature data collected with MoKα radiation. Crystal-chemical analysis obtained using electron probe microanalysis, laser ablation inductively coupled plasma mass spectroscopy, Raman spectrometry, and crystal structure refinement resulted in the following empirical formula:(XCa0.426Na0.373□0.201)Σ1.000(YLi1.498Al1.436Fe0.051Mn0.015)Σ3.000Al6Z(TSi5.793B0.207)Σ6.000O18(BO3B)3(VOH)3(WOH0.659F0.385)Σ1.044.

Liddicoatite is a hydroxy-species belonging to the calcic group of the tourmaline supergroup. It is closely related to fluor-liddicoatite through the substitution WF → WOH, and to elbaite, rossmanite, and darrellhenryite through coupled substitutions involving X-, Y-, and W-ions. The Dolní Rožínka occurrence indicates that liddicoatite is a rare late-stage product of highly evolved pegmatitic systems, requiring not only Ca availability but also sufficiently low effective F activity to stabilize W(OH) dominance. Liddicoatite was approved by the IMA-CNMNC (IMA 2025-047). The name is retained from the former species name, originally honoring Richard T. Liddicoat (1918–2002), gemologist and former president of the Gemological Institute of America.