The Formation of Barite and Celestite through the Replacement of Gypsum

Barite (BaSO4) and celestite (SrSO4) are the end-members of a nearly ideal solid solution. Most of the exploitable deposits of celestite occur associated with evaporitic sediments which consist of gypsum (CaSO4·2H2O) or anhydrite (CaSO4). Barite, despite having a broader geological distribution is rarely present in these deposits. In this work, we present an experimental study of the interaction between gypsum crystals and aqueous solutions that bear Sr or Ba. This interaction leads to the development of dissolution-crystallization reactions that result in the pseudomorphic replacement of the gypsum crystals by aggregates of celestite or barite, respectively. The monitoring of both replacement reactions shows that they take place at very different rates. Millimeter-sized gypsum crystals in contact with a 0.5 M SrCl2 solution are completely replaced by celestite aggregates in less than 1 day. In contrast, only a thin barite rim replaces gypsum after seven days of interaction of the latter with a 0.5 M BaCl2 solution. We interpret that this marked difference in the kinetics of the two replacement reactions relates the different orientational relationship that exists between the crystals of the two replacing phases and the gypsum substrate. This influence is further modulated by the specific crystal habit of each secondary phase. Thus, the formation of a thin oriented layer of platy barite crystals effectively armors the gypsum surface and prevents its interaction with the Ba-bearing solution, thereby strongly hindering the progress of the replacement reaction. In contrast, the random orientation of celestite crystals with respect to gypsum guarantees that a significant volume of porosity contained in the celestite layer is interconnected, facilitating the continuous communication between the gypsum surface and the fluid phase and guaranteeing the progress of the gypsum-by-celestite replacement.

One-dimensional β–Ga2O3 nanostructures on sapphire (0001): Low-temperature epitaxial nanowires and high-temperature nanorod bundles

Well-aligned Ga2O3 nanowires were formed on the sapphire (0001) substrates at temperatures of 650–450 °C using a single precursor of gallium acetylacetonate via a vapor-liquid-solid (VLS) method. Structural analyses reveal that the well-aligned Ga2O3 nanowires are expitaxially grown on the sapphire (0001) with Ga2O3/ sapphire orientational relationship [201]||[0001] and [211]||[1120]. In addition, formation of the flowerlike Ga2O3 nanorod bundles at a temperature of 750 °C via the vapor-solid (VS) mechanism was also demonstrated. Instead of being catalysts in the VLS method, the Au nanoparticles are proposed to play a role in sinking the Ga vapor for forming the nuclei of Ga2O3 nanorods in the VS method.

Structural Relationships between QC and Meta-stable Crystalline Phases in Melt Spun Zr80Pt20 Ribbons

ABSTRACTTwo competing meta-stable crystalline phases, a bcc hyperstoichiometric β-Zr(Pt) (Im3m) superstructure and a non-stoichiometric fcc ε-Zr6Pt3O (Fd3m) phase, have been observed to coexist with a quasicrystalline (i) phase, respectively, in as-spun Zr80Pt20 ribbons with low oxygen content (184ppm mass O) and high oxygen content (4737 ppm mass O). Transmission electron microscopy (TEM) results show that the β-Zr(Pt) superstructure and i phase have a well defined orientational relationship, good crystallographic match and nearly identical stoichiometry. Icosahedral two-fold axes coincide with <110>, <111>, <112> and <113> axes in β-Zr(Pt); {110}β-Zr planes register with icosahedral {211111} fivefold or {221001} twofold planes. The observed orientational relationship and the space group (Im3m) preclude β-Zr(Pt) as a cubic approximant to the i phase. Both β-Zr(Pt) and the i phase are distorted; β-Zr(Pt) maintains a basic β-Zr Bravais lattice with an aperiodic superlattice. Morphologies suggest that the i phase forms first, followed by an easy nucleation of the β-Zr(Pt) on the surfaces of the i phase. Also, a similar orientational relationship and lattice match between ε-Zr6Pt3O and i phase has been revealed by TEM.

Stability of channels at a scalloplike Cu6Sn5 layer in solder interconnections

The thermodynamic stability of the solder channels at a scalloplike Cu6Sn5 layer formed between Sn-containing solders and Cu substrate was evaluated by studying the penetration behavior of the liquid solders into the grain boundaries of a Cu6Sn5 substrate. The orientational relationship between the grains of the Cu6Sn5 layer formed during reflow soldering was also analyzed using the electron backscattered diffraction technique. The results showed that liquid solders penetrate into the grain boundaries at an order of faster speed than the growth rate of the layer, which provided a direct evidence of thermodynamic stability of the channel.