Nepheline, formation

Diffractograms for different thermal treatments show that the nepheline formation is accompanied by an increase in the annealing temperature, at least for fdtns containing 40% of beta-alumina and a large quantity of glass. 

All of the Type A and B inclusions studied are surrounded by a layered rim sequence of complex mineralogy [21] which clearly defines the inclusion-matrix boundary. Secondary alteration phases (grossular and nepheline, especially) are also a common feature of these inclusions, suggesting that vapor phase reactions with a relatively cool nebula occurred after formation of inclusions. Anorthite, in particular, is usually one of the most heavily altered phases the relationship between Mg isotopic composition and alteration is discussed below. (See [12] for striking cathodoluminesce photographs of typical Allende alteration mineralogy.) Inclusion Al 3510 does not fit the normal pattern as it has no Wark-rim and does not contain the usual array of secondary minerals. 

Examples of Formation of Intermediate Compounds with Fixed Stoichiometry Nepheline-Silica and Kalsilite-Silica Joins... 

Green River Formation of Colorado and in Pleistocene ash beds at Olduvai Gorge Tanzania was formed by reactions of aqueous carbonate species with nepheline in the sediment (Smith and Milton 1966). 

The geochemistry of angrites is characterized by strong silica undersaturation, by which we mean that there is not enough SiC>2 to combine with various cations to form common silicate minerals. The result is the formation of silica-poor minerals like kirschsteinite and nepheline. These meteorites also show strong depletions in moderately volatile elements. They are thought to have formed as partial melts of a chondritic source under oxidizing conditions. 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

Also common is the mechanical destruction of refractories resulting from reactions with melts or vapours and gases producing new crystalline phases with substantially different density or thermal expansion. One example is the oxidation-reduction reactions of materials containing higher amounts of iron oxides, or the formation of nepheline by the effect of alkali bn mullite, sillimanite and other materials. These reactions are responsible for spalling of the affected layers. 

In the second phase of his study, Yamaguchi [28] investigated the corrosion of certain burned refractories by sodium carbonate vapor. He suspended the test piece with a platinum wire from the bottom of an alumina crucible placed upside down. The entire assembly was heated at 1200°C for various times. Included in this part of the study was a fireclay refractory composed of mullite and silica minerals. Mullite reacted with NazO to form nepheline and alumina. The nepheline increased in amount as the remaining soda vapor reacted with the newly formed alumina and the preexistent silica. For refractories composed of mullite and corundum, carnegieite solid solution was the major reaction product. The formation Yamaguchi described resulted when Na2C03 vapor reacted with the alumina liberated from mullite and preexistent as corundum, forming... 

The alkali-bearing minerals found in the blast furnace include kalio-philite-nepheline, leucite, plagioclase, and alkali carbonates [40-44], Kaliophilite was the dominant reaction product on the inside surface of the lining and in the joints between brick. The formation of this compound is accompanied by about a 45% volume increase. No kaliophilite, however, formed in the interior of the brick except in the carbon-disintegration zones. 

Van Vlack [42] observed that the bosh region contained alkalies in excess of the requirements of kaliophilite and nepheline. This excess did not result in the formation of alumino-silicate or silicate minerals of higher alkali content but was present as alkali carbonates. The carbonates formed toward the end of the furnace campaign from the free alkali deposited in the bosh lining. For the particular furnace examined, Van Vlack determined that alkali-bearing minerals were present only where porosity and proximity of the surface permitted expansion. Therefore, the presence of alkalies did not prove seriously detrimental in the bosh and... 

In systems G and H, sodium sulfate was used as a sodium source mixed with kaolinite and calcium acetate. Sodium sulfate melts at approximately 884 C. Therefore, at 750 C no interaction between kaolinite and sodium sulfate was seen (systems G and H). In system G nepheline was formed at 1000 C, with none of the sulfur released from the sodium sulfate involved in the formation of any new high temperature minerals. However, in system H additional feldspathoids (i.e., gehlenite and hauyne) other than nepheline were formed. In this case, sulfur was involved in forming new high temperature minerals. 

Most zeolites in sedimentary rocks formed during diagenesis by the reaction of aluminosilicate materials with the pore water. Silicic volcanic glass is the aluminosilicate material that most commonly served as a precursor for the zeolites, although materials such as clay minerals, plagioclase, leucite, and nepheline also have reacted locally to form zeolites (48). Solution of silicic glass by the pore water provided the constituents necessary for the formation of the zeolites. Deffeyes... 

Let s suppose that a measurement of quartz solubility has been used to obtain the free energy of formation (standard or apparent) of H4Si04 in the ideal one molal standard state. This number can then be used (with A/G° terms for the minerals) to calculate the equilibrium constant of the albite-nepheline reaction (equation (13.11)), giving the equilibrium silica concentration in a solution that may never have been experimentally determined, or perhaps never existed, and in which quartz is not stable. Thus knowing the solubility of quartz, one could in a similar way calculate the silica concentration in fluids in contact with a variety of mineral assemblages. 

Kubo, Y., Yamaguchi, G., and Kasahara, K. (1966) Inverted phase relation in the formation of nepheline and camegieite from the system kaolinite-sodium carbonate. Am. Mineral, 51, 516-521. 

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