Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

—The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological, and geochemical aspects

The nominally anhydrous, calcium-silicate garnets, grossular (Ca3Al2Si3O12), andradite (Ca3Fe23+Si3O12), schorlomite (Ca3Ti24+[Si,Fe23+]O12), and their solid solutions can incorporate structural OH-, often termed “water.” The IR single-crystal spectra of several calcium silicate garnets were recorded between 3000 and 4000 cm–1. Spectroscopic results are also taken from the literature. All spectra show various OH- stretching modes between 3500 and 3700 cm–1 and they are analyzed. Following the conclusions of Part I of this study, the garnets appear to contain local microscopic- and nano-size Ca3Al2H12O12- and Ca3Fe23+H12O12-like domains and/or clusters dispersed throughout an anhydrous “matrix.” The substitution mechanism is the hydrogarnet one, where (H4O4)4– ↔ (SiO4)4–, and various local configurations containing different numbers of (H4O4)4– groups define the cluster type. A single (H4O4) group is roughly 3 Å across and most (H4O4)-clusters are between this and 15 Å in size. This model can explain the IR spectra and also other experimental results. Various hypothetical “defect” and cation substitutional mechanisms are not needed to account for OH- incorporation and behavior in garnet. New understanding at the atomic level into published dehydration and H-species diffusion results, as well as H2O-concentration and IR absorption-coefficient determinations, is now possible for the first time. End-member synthetic and natural grossular crystals can show similar OH- “band patterns,” as can different natural garnets, indicating that chemical equilibrium could have operated during their crystallization. Under this assumption, the hydrogarnet-cluster types and their concentrations can potentially be used to decipher petrologic (i.e., P-T-X) conditions under which a garnet crystal, and the rock in which it occurs, formed. Schorlomites from phonolites contain no or very minor amounts of H2O (0.0 to 0.02 wt%), whereas Ti-bearing andradites from chlorite schists can contain more H2O (∼0.3 wt%). Different hydrogarnet clusters and concentrations can occur in metamorphic grossulars from Asbestos, Quebec, Canada. IR absorption coefficients for H2O held in hydrogrossularand hydroandradite-like clusters should be different in magnitude and this work lays out how they can be best determined. Hydrogen diffusion behavior in garnet crystals at high temperatures is primarily governed by the thermal stability of the different local hydrogarnet clusters at 1 atm.

Micro- and nano-size hydrogarnet clusters and proton ordering in calcium silicate garnet: Part I. The quest to understand the nature of “water” in garnet continues

The calcium-silicate garnets, grossular (Ca3Al2Si3O12), andradite (Ca3Fe23+Si3O12), and their solid solutions [Ca3(Alx,Fe1−x3+)2Si3O12], can incorporate various amounts of structural OH–. This has important mineralogical, petrological, rheological, and geochemical consequences and extensive experimental investigations have focused on the nature of “water” in these phases. However, it was not fully understood how OH- was incorporated and this has seriously hampered the interpretation of different research results. IR single-crystal spectra of several nominally anhydrous calcium silicate garnets, both “end-member” and solid-solution compositions, were recorded at room temperature and 80 K between 3000 and 4000 cm–1. Five synthetic hydrogarnets in the system Ca3Al2(SiO4)3-Ca3Al2(H4O4)3-Ca3Fe23+(SiO4)3-Ca3Fe23+(H4O4)3 were also measured via IR ATR powder methods. The various spectra are rich in complexity and show several OH- stretching modes at wavenumbers between 3500 and 3700 cm–1. The data, together with published results, were analyzed and modes assigned by introducing atomic-vibrational and crystal-chemical models to explain the energy of the OH- dipole and the structural incorporation mechanism of OH-, respectively. It is argued that OH- is located in various local microscopic- and nano-size Ca3Al2H12O12- and Ca3Fe23+H12O12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H4O4)4– ↔ (SiO4)4–, and various local configurations containing different numbers of (H4O4)4– groups define the cluster type. Some spectra also possibly indicate the presence of tiny hydrous inclusion phases, as revealed by OH- modes above about 3670 cm–1. They were not recognized in earlier studies. Published proposals invoking different hypothetical “defects” and coupled-substitution mechanisms involving H+ are not needed to interpret the IR spectra, at least for OH- modes above about 3560 cm–1. Significant mineralogical, petrological, and geochemical consequences result from the analysis and are discussed in the accompanying Part II (this issue) of the investigation.

Spectral Properties and Thermal Quenching of Mn-=SUP=-4+-=/SUP=- Luminescence in Silicate Garnet Hosts CaY-=SUB=-2-=/SUB=-MgMAlSi-=SUB=-2-=/SUB=-O-=SUB=-12-=/SUB=- (M=Al, Ga, Sc) -=SUP=-*-=/SUP=-

Multi-component silicate garnet ceramics CaY_2MgMAlSi_2O_12 comprising different cations M = Al, Ga or Sc in octahedral sites doped with Mn^4+ ions have been synthesized and studied as novel red-emitting phosphors aiming at warm white pc -LED applications. All synthesized phosphors exhibit Mn^4+ luminescence in rather deep red region, the shortest-wavelength spectrum of Mn^4+ luminescence (peak wavelength at 668 nm) being obtained for the host with the largest cation M^3+ = Sc^3+ in the octahedral site. The effect of increasing the energy of the emitting Mn^4+^2 E level with the size of the host cation in octahedral sites is supposed to be the result of decrease of the covalence of the “Mn^4+-ligand” bonding with increase of the interionic Mn^4+–O^2– distance. All studied phosphors demonstrate rather poor thermal stability of Mn^4+ photoluminescence with a thermal quenching temperature T _1/2 below 200 K, the lowest value being observed for the host with M = Sc. As expected, the decrease of the energy of the O^2––Mn^4+ charge-transfer state is observed with the increase of the M^3+ cation radius, i.e. with the increase of the O^2––Mn^4+ interionic distance. The thermal quenching temperature of Mn^4+ luminescence in the studied phosphors correlates with the energy of the O^2––Mn^4+ charge transfer state which is supposed to serve as a quenching state for Mn^4+ luminescence.

Chromaticity-Tunable and Thermal Stable Phosphor-in-Glass Inorganic Color Converter for High Power Warm w-LEDs

In this work, an aluminate silicate garnet phosphor, Y2Mg2Al2Si2O12:Ce3+ (YMASG:Ce3+), exhibiting strong and broad yellow-orange emission, was successfully synthesized. Attributed to the double cation substitution of YAG:Ce3+, which led to a compression effect, a redshift was observed with respect to YAG:Ce3+. More importantly, a transparent phosphor-in-glass (PiG) sample was obtained by incorporating the phosphor YMASG:Ce3+ into a special low-melting precursor glass. The energy dispersive spectrometer (EDS) mapping analysis of the as-prepared PiG sample indicates that YMASG:Ce3+ was successfully incorporated into the glass host, and its powders were uniformly distributed in glass. The photoluminescence intensity of the PiG sample was higher than that of the powder due to its relatively high thermal conductivity. Additionally, the combination of the PiG sample and a blue high-power chip generated a modular white LED with a luminous efficacy of 54.5 lm/W, a correlated color temperature (CCT) of 5274 K, and a color rendering index (CRI) of 79.5 at 350 mA.