Aluminium-rich minerals

The preparation of silicon-rich zeolites, such as zeolite Y, can be achieved by varying the composition of the starting materials but can also be done by subsequent removal of aluminium from a synthesized aluminosilicate framework using a chemical treatment. Several different methods are available, including extraction of the aluminium by mineral acid, and extraction using complexing agents. 

Some of the ions shown in Table 3.1.6 have been proved to enter the structure of fluorapatite, either as synthetic products or as naturally occurring minerals. For example, S and Si replace P in about equal amounts in ellestadite (McConnell, 1938), and Al replaces both Ca and P in heated morinite, when converted to apatite (Fisher and McConnell, 1969). Presumably, these same situations can obtain for biologic apatites also that is, small amounts (traces or more) of the constituents shown in Table 3.1.6 are permissible theoretically. Whether they actually do substitute depends, among other factors, upon their relative availabilities within the biologic environment. Thus, the formation of an aluminium-rich apatite within the organic milieu seems highly improbable one would expect instead... 

When the clay mineral kaolinite is heated to about 980°C, it transforms to an intimate mixture of amorphous silica and an aluminium-rich spinel phase, the latter having potentially useful catalytic properties if the silica can be removed. Selective leaching... 

Boudot J P, Bel Hadj, B.A. and Chone, T (1986) Carbon mineralization in Andosols and aluminium-rich highlands soils. Soil Biology Biochemistry 18(4) 457-461... 

Doyleite is a triclinic aluminium hydroxide mineral with composition Al(OH)3. It occurs as masses of soft white tabular crystals with pearly lustre which form from the weathering of aluminium-rich rocks in tropical climates. Hence, doyleite is often found in bauxites (. v.). Doyleite is closely related to another triclinic form of A1(0H)3, nordstrandite, and the monoclinic polymorphs, gibbsite and bayerite qq.v.), with which it may be found (Chao et al., 1985) it is also related to the orthorhombic dehydrated forms of AIO(OH), boehmite and diaspore qq.v.). Synthetic alu-minimn hydroxide Al(OH)3 is commonly encountered as a substrate for lake pigments. 

Fluoride is a natural component of most types of soil, in which it is mainly bound in complexes and not readily leached. The major source of free fluoride ion in soil is the weathering and dissolution of fluoride rich rock that depends on the natural solubility of the fluoride compound in question, pH, and the presence of other minerals and compounds and of water. The major parameters that control fluoride fixation in soil through adsorption, anion exchange, precipitation, formation of mixed solids and complexes are aluminium, calcium, iron, pH, organic matter and clay [19,20]. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

Initially forest damage was believed to be another effect of acid precipitation toxic aluminium ions released from the soil minerals were poisoning the root system of the trees. But the forest damage also occurred in areas with soils rich in carbonates, and in valleys it often appeared at certain height levels where fog was frequently observed. This pointed to air pollution as a direct main cause for the damage. 

A comprehensive review of the chemical composition of the lunar surface, as ascertained by analysis of samples obtained on the Surveyor and subsequent Apollo and Luna missions, has been collated by Turkevich. The lunar surface is made up of silicate rocks, the principal minerals being calcium-rich feldspars and pyroxenes. In many respects the chemical composition of the maria analysed are similar to those of the terrestrial basalts. The terra regions analysed are distinctly different from the maria in having considerably smaller amounts of iron and titanium and larger amounts of calcium and aluminium. Apollo missions have shown that the lunar mare material is very dry and was produced under relatively reducing conditions. 

Mineral components—the coarse fraction is rich in aluminium, silicon, iron and calcium. 

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