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Witherite (BaCO3) crystals have been synthesized by mixing BaCl2·2H2O and NaHCO3 aqueous solutions; their growth morphology is characterized by single and twinned individuals both [001] elongated and limited by {110} prism and {hhl} bipyramids. When quartz single crystals are used as substrates, heterogeneous nucleation of witherite occurs onto the {101̅0}, {101̅2}, and {011̅2} forms of quartz and {001} witherite lamellae grow along the [100], [001], and [121̅] directions of quartz. This witherite/quartz three-dimensional epitaxy is the first evidence that two-dimensional epitaxial layers of quartz can form as well on witherite nucleating from aqueous solutions containing Na-metasilicate. As it is known, nano-polycrystalline structures, the “silica biomorphs”, are precipitated from such solutions: this implies a very high nucleation frequency of witherite. We propose that this is caused by the lowering of the specific surface energy of witherite for the two-dimensional localized adsorption of silica chains.

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’.

In this work, we revisited the theoretical morphology of baryte for determining the character (flat, stepped, and kinked) and the different profiles (or terminations) of the crystal faces, and presented an accurate ab initio study of the structures and surface energies at 0 K of the low-index (001), (011), (210), (101), (100), (010), (201), (012), and (211) baryte faces, by using the hybrid Hartree–Fock/density functional B3LYP Hamiltonian. According to the surface energy values, the stability order of the baryte faces was found to be (210) ≅ (001) < (211) < (101) < (010) < (011) < (201) ≅ (100) < (012). Then, the equilibrium shape of baryte was drawn and compared with the previous ones obtained at an empirical level. This is a preliminary work, which is necessary to successively evaluate the influence of water and/or impurities adsorption on the most important baryte surfaces, both at equilibrium and during growth. Indeed, to evaluate these adsorption effects on the crystal morphology, it is fundamental an in-depth analysis of the structure and energetics of the crystal faces acting as substrates.