Weathering layers

Similar conditions can be produced when alkaline solutions saturated with silica are mixed with highly acid waters. Another genetic variant inevitably suggests acid thermal waters, in which the solubility of Si02 is a function of temperature (Khitarov, 1953), as the source of the silica. When thermal waters cool, mix with surface waters, and are partially neutralized, ionic-colloidal systems with anomalously high content of monomeric silica can arise. It is obvious that both these variants reflect specific conditions that are not typical of geochemical processes in the weathered layer. 

In examining the particulars of the distribution, migration, and deposition of iron and silica in natural waters it is advisable to distinguish normal sedimentary processes and volcanic sedimentary processes. By normal sedimentary processes we mean the cycle weathered layer-transport-deposition in a sedimentary basin. The main distinguishing feature of volcanogenicsedimentary processes is the endogenetic source of the material and ex-ogenetic method of its deposition. 

The appearance of acid solutions in the weathered layer cannot be substantiated physicochemically even if a high content of carbon dioxide in the atmosphere is assumed with =1 bar the pH of the saturated aqueous solution (pure water with no impurities of other anions and cations) is only 3.9—a value clearly insufficient for intensive removal of ferric iron in the form of stable Fe FeOH, or Fe(OH)2. Moreover, when pure (rain) carbonic acid waters react with rocks the pH value increases as the saturation with dissolved cations (mainly Mg, Na" ", K, and Ca ) increases. The constant presence of other volatiles of the acid fume group in the Precambrian atmosphere is not very likely, due to their high solubility... 

Ionic solutions of Fe, which are stable at very low pH values, could not have formed in the weathered layer. Thus no colloidal solutions of ferric iron could have appeared. 

To sum up, the main mode of extraction of dissolved silica coming into the seas and oceans from the weathered layer is biogenic precipitation—a process widespread both in recent and in ancient sedimentary basins. Biogenic precipitation of silica from undersaturated solutions could also have been of definite importance in the formation of the Precambrian iron cherts, but no direct evidence has been obtained as yet. 

Ginzburg, I.l. and Kabanova, Ye.S., I960. Silica content in natural waters and forms of its occurrence. In Kora vyvetrivaniya (The Weathered Layer). Izd. Nauka, Moscow, 3 313-342 (in Russian). 

After removal of weathered layer in most cases this approximates well to the original glass thickness, but in samples EC 1,2,6, and 7, this is an underestimate by about 2 mm. 

In accordance with the general conservation principle of minimum intervention, the main objective is to conserve the glass, and not to recover transparency, through removal of corrosion products and deposits. Only in exceptional circumstances, therefore, may weathering layers be removed to increase the transparency of the glass or to support its interpretation. In any case, damage to the hydrated layer must be avoided this layer is considered to be the skin of the glass, which protects it from further attack. 

The retention of this group increases, that is, mobility and plant availability decreases, rapidly at first and the rate slows with time. This behavior is similar to diffusion and suggests that the mechanism is the slow transfer of surface ions into the weathered layer on soil particles. The initial rate is rapid because the surface concentration is relatively high and die diffusion path length is short. As diffusion inward proceeds, the surface concentration decreases and the diffusion path length increases. The shallower concentration gradient slows the rate of further cadon diffusion. 

The Mount Weld carbonatite dates from the Proterozoic (approximately 2025 Ma ago) and is emplaced in the Yilgarn Craton (Hoatson et al. 2011). The carbonatite is covered by a thick lateritic soil, which in its turn is covered by lacustrine and alluvial sediments (Lottermoser 1990). The Mount Weld deposit occurs in the weathered layer. 

When a trial excavation is made, sampling and testing should, as far as possible, not be confined just to the surface of the limestone, but should extend down to at least below the top weathered layer of rock. In most cases this will require the aid of a heavy excavator or rock breaking hammers and a compressor. In young chalk limestones or coral limestones a ripper or even lighter equipment may suffice for the purpose. 

Except in cases where the outer layer is acrylic, for example, Acryhgard (Rohm Haas) or PVDF (Kynar ), vinyl siding is extmded in dual layers, mainly an inner layer, substrate, and the outer weatherable layer, capstock. (The terms come firom siding jargon that predates the appearance of PVC.) The bulk of the product is, of course, substrate, which uses reverse ester tin stabilizers ... 

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