Hydrothermal alteration of chevkinite-group minerals:Part 2. Metasomatite from the Keivy massif, Kola Peninsula, Russia

Chevkinite-(Ce) in a mineralized quartz-epidote metasomatite from the Keivy massif, Kola Peninsula, Russia, underwent at least two stages of low-temperature alteration. In the first, it interacted with hydrothermal fluids, with loss of Ca, Fe, LREE and Si and strong enrichment in Ti. The altered chevkinite was then rimmed and partially replaced by a zone of ferriallanite-(Ce) and davidite-(La), in turn rimmed by a zone of allanite-(Ce) richer in the epidote component. The allanite zone was in turn partially replaced by rutile-titanite-quartz assemblages, the formation of titanite postdating that of rutile. Aeschynite-(Y), aeschynite-(Ce) and REE-carbonates are accessory phases in all zones. The hydrothermal fluids were alkaline, with significant proportions of CO2 and F. At various alteration stages, the Ca, Si ± Al activities in the fluid were high. Formation of the aeschynite is discussed in relation to its stability in broadly similar parageneses; it was a primary phase in the unaltered chevkinite zone whereas in other zones it formed from Nb, Ti, REE and Th released from the major phases

Hydrothermal alteration of chevkinite-group minerals: products and mechanisms:Part 1. Hydration of chevkinite-(Ce)

Samples from Russia and Scotland are used to examine the interaction of the REE-Ti silicate chevkinite-(Ce) with hydrothermal fluids. Altered zones in crystals are distinguished by using areas of low intensity on backscattered-electron images, low analytical totals, increasingly large departures from stoichiometry and, in some cases, the presence of micropores. Initial alteration of the chevkinite results in strong Ca enrichment. With increasing degrees of alteration, Ca abundances drop sharply, as do those of the REE, Fe and Si. In contrast, Ti levels increase strongly, usually accompanied by higher Nb ± Th levels. The most altered zones contain up to 36 wt.% TiO2 and the formula cannot be expressed in the standard chevkinite formula. In detail, samples follow different alteration trends, presumably reflecting different P, T, f O2 and fluid composition. The Ti enrichment may have been related to a reaction front of dissolution-reprecipitation passing through the outer zones of the original chevkinite, leaving behind a reprecipitated Ti-enriched phase which may or may not be chevkinite

Hydrothermal alteration of a chevkinite-group mineral to a bastnäsite-(Ce)-ilmenite- columbite-(Fe) assemblage:interaction with a F-, CO2-rich fluid

The results are presented of a textural and mineral chemical study of a previously undescribed type of hydrothermal alteration of chevkinite-(Ce) which occurs in a syenitic pegmatite from the Vishnevye Mountains, Urals Region, Russia. The progressive alteration of the chevkinite to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage through a series of texturally complex intermediate stages is described and electron microprobe analyses are given of all the major phases. Unusual Nb ± Th-rich phases formed late in the alteration sequence provide evidence of local Nb mobility. The main compositional fluxes are traced, especially of the REE, HFSE, Th and U. It appears that almost all elements, with the exception of La, released from the chevkinite-(Ce) were reincorporated into later phases, such that they did not leave the alteration crust in significant amounts. The hydrothermal fluids are inferred to have been F- and CO2-rich, with variable levels of Ca activity, and with fO2 mainly between the nickel-nickel oxide and magnetite-hematite buffers. This occurrence represents a new paragenesis for a columbite-group mineral

Dovyrenite Ca6Zr[Si2O7]2(OH)4 a new mineral from skarned carbonate xenoliths in basic-ultrabasic rocks of the Ioko-Dovyren Massif, Northern Baikal Region, Russia

Dovyrenite, simplified formula Ca 6 Zr[Si 2 O 7 ] 2 (OH) 4 , occurs as an accessory mineral in vein skarns developed in carbonate xenoliths in subvolcanic layered plagiodunite-troctolite series in the Ioko-Dovyren Massif of Proterozoic age, Northern Baikal Region, Buryatia, Russia. Dovyrenite is a late mineral of altered pyroxene and melilite-monticellite skarns. Associated minerals are Zr-bearing phases: fassaitic pyroxene, perovskite and hydrogarnets; and also monticellite, vesuvianite, diopside, foshagite, brucite, calzirtite, tazheranite, baghdadite, apatite, calcite, native bismuth, sphalerite, selenian galena, clausthalite, safflorite, rammelsbergite, pyrrhotite, pentlandite, valleriite, laitakarite, nickeline, nickel-skutterudite. The average structure of dovyrenite is orthorhombic, space group Pnnm, with subcell parameters A = 5.666(16) Å, B = 18.844(5) Å, C = 3.728(11) Å, V = 398.0(2) Å 3 and Z = 1. Dovyrenite shows a new type of modular structure with stacking of the tobermorite-like and the rosenbuschite-like layers parallel to (010). Single-crystal structural data point to an incompletely occupied Ca(2) site from the rosenbuschite module which is confirmed by microprobe analyses: ZrO 2 16.47, SiO 2 32.83, TiO 2 0.14, HfO 2 0.16, Cr 2 O 3 0.01, CaO 43.87, FeO 0.25, MgO 0.13, MnO 0.02, Nb 2 O 3 0.03; total 99.38 wt% with calculated H 2 O. The empirical formula is (Ca 5.73 Fe 0.03 Mg 0.02 ) σ5.78 (Zr 0.98 Hf 0.01 Ti 0.01 ) σ1 Si 4 (O 13.56 OH 0.44 ) σ14 (OH) 4 . The presence of two types of OH group in the dovyrenite structure is corroborated by FTIR and Raman spectroscopy. Dovyrenite is an optically positive biaxial mineral: α 1.659(2), β 1.660(2); γ 1.676(2); 2Vz 30(5)° (measured), 28° (calculated). The coexistence of monticellite, foshagite and dovyrenite points to a narrow interval of crystallization 560-630°C under subvolcanic conditions (P < 10 8 Pa)

Assessment of the Relationship between the Shape of the Lateral Meniscus and the Risk of Extrusion Based on MRI Examination of the Knee Joint

<div><p>Background</p><p>Meniscus extrusion is a serious and relatively frequent clinical problem. For this reason the role of different risk factors for this pathology is still the subject of debate. The goal of this study was to verify the results of previous theoretical work, based on the mathematical models, regarding a relationship between the cross-section shape of the meniscus and the risk of its extrusion.</p><p>Materials and Methods</p><p>Knee MRI examination was performed in 77 subjects (43 men and 34 women), mean age 34.99 years (range: 18–49 years), complaining of knee pain. Patients with osteoarthritic changes (grade 3 and 4 to Kellgren classification), varus or valgus deformity and past injuries of the knee were excluded from the study. A 3-Tesla MR device was used to study the relationship between the shape of the lateral meniscus (using slope angle, meniscus-cartilage height and meniscus-bone angle) and the risk of extrusion.</p><p>Results</p><p>Analysis revealed that with values of slope angle and meniscus-bone angle increasing by one degree, the risk of meniscus extrusion raises by 1.157 and 1.078 respectively. Also, an increase in meniscus-cartilage height by 1 mm significantly elevates the risk of extrusion. At the same time it was demonstrated that for meniscus-bone angle values over 42 degrees and slope angle over 37 degrees the risk of extrusion increases significantly.</p><p>Conclusions</p><p>This was the first study to demonstrate a tight correlation between slope angle, meniscus-bone angle and meniscus-cartilage height values in the assessment of the risk of lateral meniscus extrusion. Insertion of the above parameters to the radiological assessment of the knee joint allows identification of patients characterized by an elevated risk of development of this pathology.</p></div