To the Issue of the Presence of the Occurrence of Rare Earth Elements in the Structure of Chromium Pyrope Garnet

Rare earth elements (REE) in garnet are of interest in various fields of modern geology. The geochemistry of REEs in magmatic minerals is widely used in determining the distribution coefficients of crystal/melt and crystal/fluid, modeling the processes of melting and crystallization of magmatic rocks, studying deep mantle processes, age estimates and other issues of petrogenesis. The aim of the present work was a synthesis of a peridotite mineral association including the garnet containing REE at high pressure and high temperature. The initial sample consisted mainly of natural serpentine collected from ophiolites of the Eastern Sayan (Russia). As is known, the extreme stage of the regressive metamorphism of peridotites is serpentinization. It is depleted in calcium, but can recrystallize at high PT conditions into a harzburgite paragenesis, and at the initial stage of the experiment the chemical composition of the sample was a model harzburgite depleted in calcium and chromium, as well as a fluid of predominantly aqueous composition. As a source of chromium, chromite grains of 1–2 mm in size from peridotite xenoliths of the Udachnaya kimberlite pipe (Yakutia) were used. REE were added to the initial charge in the form of water-soluble salts. The experiment at a pressure of 5 GPa and temperature 1300 was performed on a multi-anvil high-pressure apparatus of the “split sphere” type (BARS) designed and developed at the V.S. Sobolev Institute of Geology and Mineralogy SB RAS. A container based on refractory oxide ZrO2 was used as a highpressure medium. The pressure in the cell before sample heating was estimated using the reference substances Bi and PbSe. The temperature was measured by a platinum-platinum-rhodium thermocouple PtRh30-PtRh6. The quenched was performed by switching off the voltage in the heater circuit. The experiment products contain an association of olivine + garnet + orthopyroxene + newly formed spinel. The predominant phase was olivine of a forsterite composition. A low-Fe orthopyroxene (1.49 – 1.68 wt% FeO) was found in elongated grains uniformly distributed throughout the sample. The newly formed spinel shows the faceted grains. The chromium content in the spinel significantly exceeds that of the initially added to the initial charge, 61.63 and 54.04 wt% Cr2O3, respectively. The garnet is characterized by a purple color, and was identified in the sample volume between olivine grains in the form of individual faceted crystals or their clusters. The largest garnets reached 0.5 mm in size. The synthesized garnet was determined as a high-Cr low-Ca pyrope variety. The contents of Cr2O3 and CaO are 10.15-11.21 and 0.06-0.11 wt%, respectively. The total content of REE in the garnet identified by the microprobe analysis is relatively high reaching 5-7 wt%. As a result of the work a mineral association corresponding to the peridotite paragenesis was obtained, including the subcalcic Crrich pyrope containing rare earth elements in significant amounts. It was estimated that their content in garnet mainly depends on the size of the ionic radius and, accordingly, on the atomic weight. This is consistent with the known facts about the preferable position of heavy REEs into the garnet structure compared to the light REEs.

MEASUREMENT OF THERMOPHYSICAL PROPERTIES RIGID FIBER INSULATION

Describes a mathematical model for processing the results of measurements of thermal conductivity of highly porous fibrous materials of thermal protection. The results and some methodological features of measuring the thermal conductivity of rigid thermal insulation based on refractory oxide fibers are presented. The possibility of measurements taking into account the anisotropy of properties is investigated. The stiffness of the thermal insulation at the fiber contacts is provided by the binder. Thermal conductivity was measured by the stationary method on cylindrical samples in a wide temperature range from 20 to 1700 °C in various gaseous media.

Applying CO2 and ytterbium fiber lasers to high-performance production of refractory oxide nanopowders

The main features of obtaining refractory oxide nanopowders using a repetitively-pulsed CO2-laser (10,6 µm) with an average radiation power of 500 W or a CW ytterbium fiber laser (1,07 µm) with a radiation power of up to 700 W are considered. In particular, the influence of pressure, buffer gas composition and average radiation power on the size of nanoparticles and the productivity of their obtaining were studied. Depending on the thermophysical properties of the material, in atmospheric pressure air the productivity of nanopowder synthesis varies from 15 – 23 g/h (YSZ) to 350 g/h (WO3). The mass yield of nanopowder obtained upon evaporation of one target is usually is 30 wt. % of the weight of the initial target. The obtained nanopowders contain weakly agglomerated nanoparticles of spherical shape. The average size of nanoparticles 11 – 20 nm weakly depends on their material. The most important features of using a CW ytterbium laser to obtain nanopowders of refractory oxides are their high transparency for radiation of 1,07 μm, as well as the spraying of many melt droplets. These features led to a reduction in the productivity of nanopowder production and its mass yield. On the other hand, the scattering of laser radiation in porous of the initial target and its concentration in some regions makes it possible to efficiently evaporate oxide targets from materials with a refractive index of more than 1.7 – 1.75. The transition to a repetitively-pulsed mode of radiation (pulse duration 120 μs, square waveform, and peak power 600 W), an increase in the spot diameter and the speed of beam movement over the target surface made it possible to significantly reduce droplet spattering and increase the yield of Nd: Y2O3 nanopowder from 9,7 to 30 wt.% of the weight of the initial target. However, a twofold decrease in the average radiation power led to the fact that the productivity of obtaining the nanopowder was only 15 g/h. Thus, to obtain nanopowders of refractory oxides, it is desirable to use a quasi-CW fiber ytterbium laser, which is specially designed for operation in a repetitively pulsed mode. The obtained nanopowders YSZ, Nd: Y2O3, Al2O3, etc. are used for the manufacture of ceramic solid electrolytes of the YSZ type and highly transparent laser ceramics.