Victor G. Thomas

Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstones.

When water is confined to nanocavities, its quantum mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.

Hydrosilicate liquids in the system Na2O-SiO2-H2O with NaF, NaCl and Ta: Evaluation of their role in ore and mineral formation at high T and P

Consideration of the existence of hydrosilicate liquids (HSL) in nature can help in understanding the accumulation and transport of some mineral- and ore-forming components at the transition from magmas to hydrothermal fluids. We studied the experimental formation of HSL using a base system Na2O-SiO2-H2O with addition of NaF, NaCl and metallic Ta. The interaction between quartz and aqueous solution, performed at 1.5 kbar and 600°C and followed either by cooling or by quench, showed that the formation of HSL occurred when initial Na2O exceeded 2 wt %. Neither NaF nor NaCl have a significant effect on the formation of HSL. The HSL concentrates F, whereas Cl partitions into the aqueous fluid. With addition of Ta to the system, the HSL becomes metal-enriched. Natural analogs of experimental HSL can be found among “melt/fluid” inclusions entrapped in quartz and other minerals of miaroles in granite pegmatites and raremetal granites. The HSL is a novel medium enabling extreme concentrations of lithophile ore metals at the magmatic-hydrothermal transition.

Incipient ferroelectricity of water molecules confined to nano-channels of beryl

Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole–dipole coupling and suppress the ferroelectric order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H2O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole–dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie–Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices.

Pulsation processes at hydrothermal crystal growth (Beryl as example)

Abstract The experiments on beryl hydrothermal growth showed that spontaneously formed growth zonality in crystals can be explained by sporadic temperature fluctuations at ∼0.7°C amplitude. Model experiments under normal conditions proved that such temperature fluctuations in the growth medium can be generated in hydrothermal growth processes. They are due to the pulsation character of heat–mass transfer in autoclave. The model is suggested to explain the hydrothermal crystal growth as a result of the instant local overcooling of the diffusion layer when they are in contact with the cold portions of hydrothermal solution.

Formation and properties of hydrosilicate liquids in the systems Na 2 O-Al 2 O 3 -SiO 2 -H 2 O and granite-Na 2 O-SiO 2 -H 2 O at 600°C and 1.5 kbar

In order to determine the mechanisms of formation and properties of natural hydrosilicate liquids (HSLs), which are formed during the transition from magmatic to hydrothermal mineral formation in granitic pegmatites and rare-metal granites, the formation of HSLs was experimentally studied in the Na2O-SiO2-H2O, Na2O-Al2O3-SiO2-H2O, and Na2O-K2O-Li2O-Al2O3-SiO2-H2O systems at 600°C and 1.5 kbar. It was shown that the sequential extension of composition does not suppress HSL formation in the systems and expands the stability field of this phase. However, HSLs formed in extended chemical systems have different structure and properties: the addition of alumina induces some compression of the structure of the silicate framework of HSLs, which results in a decrease in water content in this phase and probably hinders the reversibility of its dehydration. It was demonstrated that HSL can be formed by the coagulation of silica present in a silica-oversaturated alkaline aqueous fluid. It was supposed that the HSL formed during this process has a finely dispersed structure. It was argued that anomalous enrichment in some elements in natural HSLs can be due to their sorption by the extensively developed surface of HSL at the moment of its formation.

Tairus Hydrothermal Synthetic Sapphires Doped with Nickel and Chromium

© 1997 Gemological Institute of America he history of hydrothermally grown synthetic corundum began nearly 40 years ago, when Laudise and Ballman (1958) produced the first colorless synthetic sapphire. Their technique, when modified, also can be used to grow rubies (Kuznetsov and Shternberg, 1967). Until recently, however, attempts to grow blue sapphire by the hydrothermal method had failed. This problem is significant because flux-grown and flame-fusion synthetic sapphire crystals typically have uneven color distribution. In blue flame-fusion material, an intense blue hue is usually concentrated in thin outer zones of boules, while most of the bodycolor is much lighter. In blue flux-grown synthetic sapphires, color inhomogeneity is apparent as a sequence of blue and colorless angular growth zones that conform to the crystal shape. Such color inhomogeneity makes cutting difficult and greatly decreases the yield of faceted material. To address this market gap, the authors investigated the use of hydrothermal growth techniques to produce blue crystals that would be free from the defects of synthetic sapphire grown by other methods. This problem was approached in two ways: (1) by doping hydrothermal synthetic sapphire with iron (Fe2+) and titanium (Ti4+) ions (as proposed by, for example, Burns, 1981); and (2) by using other doping impurities that could reproduce the blue color of sapphire. Research into the development of a commercial technique by the first approach is still ongoing. However, the second approach has led to success: The use of nickel (Ni2+) as a dopant has resulted in crystals of “sky” blue synthetic sapphire, from which thousands of faceted stones as large as 4 ct have been produced by the Tairus joint venture and marketed internationally since 1995. The next step in the development of this technique was to produce sapphires of different colors by varying the concentrations of Ni2+, Ni3+, and chromium (Cr3+). At this time, the Tairus research group has created the technology T Researchers of the Tairus joint venture have developed the technology for the hydrothermal growth of gem-quality crystals of synthetic sapphire. They have achieved a broad spectrum of attractive colors by varying the concentrations of Ni2+, Ni3+, and Cr3+. Comprehensive testing of 18 faceted synthetic sapphires and 10 synthetic crystals, representing the range of colors, revealed a typical set of internal features that are specific to hydrothermally grown crystals (microscopy), as well as the presence of OH– and various carbon-oxygen groups in the sapphire structure (infrared spectroscopy). The reactions of the test samples to ultraviolet radiation and visible light were also found to be useful in distinguishing the Tairus synthetic sapphires from their natural counterparts and other synthetics. TAIRUS HYDROTHERMAL SYNTHETIC SAPPHIRES DOPED WITH NICKEL AND CHROMIUM

2D modeling of the regeneration surface growth on crystals

A physical model is proposed to describe the growth of regeneration surfaces (flat crystal surfaces that are not parallel to any possible faces). According to this model, the change in the growth rate of a regeneration surface during its evolution and the decrease in the number of subindividuals forming the growth front can be explained by the implementation of two types of geometric selection: within each subindividual (the absorption of rapidly growing faces by slowly growing ones) and between subindividuals (when subindividuals absorb each other). A numerical modeling of the growth of the regeneration surface (30.30.19) of potassium alum crystals showed quantitative agreement between the model proposed and the experimental data.

2D modeling of regeneration surface growth on a single-crystal sphere

This paper investigates the evolution of a sphere produced from a single crystal potassium alum in course of its regeneration, using numerical 2D-simulation in the kinematic model, which describes the growth of the regenerating surfaces.The modeling results demonstrate a qualitative agreement between the predictions of the kinematic model and real processes of sphere regeneration. It is shown that the face arising on the regenerating surface of a sphere may grow either more slowly or more rapidly than the surrounding surface. In the latter case, the face interacts with the regeneration surface and disappears from the sphere surface before intersecting in edges with neighboring faces. The influence of the input model parameters on the numerical modeling results is analyzed. It is established that the roughness parameters of the initial surface of a single-crystal sphere significantly affect the surface evolution during regeneration.

THz–IR spectroscopy of single H2O molecules confined in nanocage of beryl crystal lattice

We have measured the terahertz–infrared (3–7000 cm−1) spectra of the optical conductivity of iron-doped single crystals of beryl, (Mn,Fe):Be3Al2Si6O18, that contain lone water molecules isolated within nanometer-sized cages formed by the ions of beryl crystal lattice. By comparing the spectra with those of dehydrated crystals, we exclude phonon resonances and reconstruct the spectra determined exclusively by vibrations of the water molecules. At liquid-helium temperatures, well-known intramolecular H2O modes are observed above 1000 cm−1 and accompanied with satellite resonances that are combinations of intramolecular and external vibrations of H2O molecules. At terahertz frequencies, a broad bump centred around 20 cm−1 (at 5 K) is observed with three rather narrow resonances at its high-frequency shoulder (38, 42 and 46 cm−1). The origin of these low-energy excitations is discussed.

Micro-sectoriality in hydrothermally grown ruby crystals: the internal structure of the boundaries of the growth sectors

This study explores the fine structural details of the boundaries between the growth micro-sectors in a ruby crystal grown hydrothermally on a nonsingularly oriented (01) seed. The samples were examined using IR-spectroscopy and HRTEM-analysis, demonstrating that the interfaces of the micro-sectors serve as ‘traps’ for OH-groups often observed in grown crystals. Counter to what has previously been reported, a significant proportion of these OH-groups is incorporated into ruby crystals in an orderly manner, forming diaspore-like layers growing epitaxially on the corundum (012) lattice planes. The tensions on the boundaries between the micro-sectors result in local increase of internal pressure, making the diaspore-like phase stable. The assumption is made that the mechanism discussed in the article can explain the occurrence of OH-groups in the structure of nominally anhydrous minerals, such as MgSiO3 (akimotoite) which is structurally similar to corundum.