Formation Of Silicate Minerals Research Articles

Geochemical Characteristics of Iron in a Sediment Core at 63°40′ E, Eastern Southwest Indian Ridge: Implications on Regional Hydrothermal Activities and Source Origin

Iron is a limiting nutrient in the marine biogeochemical cycle, and hydrothermal processes at mid-ocean ridges are well-known as one of its sources to the water column. However, a major portion of the hydrothermal iron is precipitated near the source and plays an essential role in oceanic elemental cycling. Here, we carried out a detailed study on the geochemical characteristics of Fe, using a sequential chemical extraction protocol, in a short sediment core collected from the eastern Southwest Indian Ridge (SWIR) to understand the iron association in individual mineral phases. Major and trace and rare-earth element concentrations, positive europium anomaly, and rare-earth fractionation show that the source components in the sediment core are composed of biogenic, local mafic, ultramafic, and hydrothermal origin. Solid-phase Fe speciation results indicate that >60% of Fe is associated with the Fe-oxides phase and indicate the hydrothermal plume particulates settled from the water column. A relatively low concentration of Fe associated with the pyrite and silicate (FeRes) phase suggests an erosion of sulphide and silicate minerals from the nearby vent field. The down-core variation reflects the transformation of primary ferrihydrite to more stable oxide mineral goethite/hematite and, to some extent, the formation of silicate minerals.

Nanomechanical Characterization Effect of Mica and Aging on Asphalt Binder

AbstractAsphalt concrete (AC) consists of approximately 95% aggregate, by weight. Of this 95%, about 6% is smaller than 0.075 mm in size (passing the #200 sieve and called filler). The fillers often contain mica, which is a formation of silicate minerals having perfect basal cleavage. Mica has been shown to reduce the strength of AC. This study evaluates the effects of mica on asphalt materials subjected to aging. Mica is combined with asphalt binders to make mastics. Mastics are aged at four different levels and tested by nanoindentation to determine modulus and hardness values. It is shown that mastic with no mica becomes much harder after long-term aging. Aged mastic with no mica is shown to be harder than the aged mastics with low concentrations of mica. Mastic with less than 5% mica in the fillers behaves similarly to binder. However, with a mica content of 7.5%, the modulus decreases after long-term aging. A mastic sample containing 5% mica is shown to have the highest modulus value. Therefore, low-...

Black chert-hosted manganese deposits from the Bistritei Mountains, Eastern Carpathians (Romania): petrography, genesis and metamorphic evolution

The East Carpathian manganese deposits (ECMD) occur in the Tulghes Group, a Cambrian–Ordovician island arc complex metamorphosed under greenschist facies conditions during the Silurian. The deposits consist of ferromanganese carbonate–silicate orebodies (Fe-rhodochrosite, Mn-amphibole, spessartine, stilpnomelane, rhodonite, pyroxmangite and tephroite), occurring within larger and lower-grade mineralized areas. Although hosted by organic matter-rich cherts (C org=1–1.5%), the ore is devoid of organic matter. The δ 13C carbonate values are from −3.5‰ to −15.7‰ (PDB) in the carbonate ore and −1.7‰ to +0.9‰ in the neighboring carbonate schists, while the hosting organic matter-rich cherts exhibit δ 13C org values from −22.7‰ to −27.8‰ (PDB). The ore shows positive Ce and negative Eu anomalies on chondrite-normalized plots. The manganese was supplied by both terrigenous and igneous sources stored in oxygen-deficient bottom water of a back-arc basin and precipitated as Mn carbonate and oxide. The metamorphism of ECMD is polyphase with contrasting tendencies. The formation of silicate minerals by decarbonation reactions was followed by the extensive replacement of silicate phases by Mn carbonate and, locally, by iron sulfide, as a result of reducing fluids circulation.

Microbial biomineralization in weathered volcanic ash deposit and formation of biogenic minerals by experimental incubation

Microbial biomineralization in a weathered volcanic ash deposit from the 1914 to 1915 A.D. eruption of Sakurajima volcano was investigated by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). The solution chemistry of pore water was also analyzed to elucidate saturation conditions. In addition, experimental incubations of bacteria collected from the volcanic ash were performed to confirm bacterial mineralization. TEM revealed that the weathered volcanic ash contains significant amounts of spherical to rod-shaped bacteria ranging from 1.3 × 108 to 2.6 × 108 cell/g, most of which have cell wall surfaces that are completely covered or decorated by either massive aggregates of allophane-like granular materials or irregular aggregates of smectitelike fibers and/or flakes. EDX confirmed that the granular minerals have chemical compositions similar to proto-imogolite allophane, whereas the smectite-like fibers and/or flakes show a wide range of chemical compositions corresponding to the compositional field between allophane and nontronite. The volcanic ash contains about 22 wt% of pore water, which is slightly acidic, relatively low redox potential, and enriched in Si, Na, Cl-, and SO 4 2- ions. The saturation indices (SI) calculated by the PHREEQC geochemical code indicate that the pore water is almost saturated with respect to amorphous Al(OH) 3 , ferrihydrite, amorphous silica, and cristobalite, and significantly oversaturated with respect to silicate minerals in the order: halloysite < kaolinite < montmorillonite < allophane < nontronite. The allophane-like granular minerals seems to be preferentially precipitated by bacterial interaction with Al and Si ions in the pore water as a metastable phase. The poorly ordered smectite-like fibers and/or flakes may be transformed from the allophane-like materials as a intermediate phase between allophane and nontronite by the driving force originated from the greatest SI value of nontronite. The experimental incubation confirmed that amorphous silica containing a small amount of Fe is formed on the bacterial cell surfaces in liquid media with both Fe and Si ions. Likewise, beidellite-like smectite associated with the bacterial surfaces is produced in liquid media containing both Al and Si ions. However, no minerals are produced in the same media containing no metal ions or no bacteria. These results imply that bacteria play an important role in the accumulation of metal ions and in the formation of silicate minerals during weathering of volcanic ash.

Conversion of silica gel and silica glass mixed with various metal oxides into quartz

Mixtures of silica gel and various metal oxides were heated at temperatures within the range of 500 to 1200°C for 1 h. When mixtures of silica gel and intentional additives such as Li 2 CO 3 , MgO, CaCO 3 , SrO, Ba(CH 3 COO) 2 , and ZnO were heated at temperatures from 700 to 1050°C, quartz phases formed. Similarly, heating a mixture of powdered silica glass and Li 2 CO 3 yielded quartz. In all cases the formation of quartz phases always accompanied the formation of silicate minerals. When a mixture of silica gel and 1 wt% Li 2 CO 3 was heated at 900°C, quartz phases and a very small amount of Li 2 Si 2 O 5 (hereafter denoted as Q-S mixture) were formed. Silica gel mixed with 10 wt% of Q-S mixture was transformed almost completely into quartz phases at 1000°C. The Q-S mixture consisted of only quartz phases after washing with hot H 2 SO 4 . Silica gel mixed with 10 wt% of the washed Q-S mixture was not transformed into quartz, but rather into cristobalite above 1200°C. This fact also indicates that the formation of the silicate mineral is a necessary precursor to the conversion of silica gel and silica glass into quartz.