Polymorphism iron oxides

Ostwald s step rule holds that a thermodynamically unstable mineral reacts over time to form a sequence of progressively more stable minerals (e.g., Morse and Casey, 1988 Steefel and Van Cappellen, 1990 Nordeng and Sibley, 1994). The step rule is observed to operate, especially at low temperature, in a number of min-eralogic systems, including the carbonates, silica polymorphs, iron and manganese oxides, iron sulfides, phosphates, clay minerals, and zeolites. 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

Leigh, D.S. (1996) Soil chronosequence of Brasstown Creek, Blued Ridge Mountains, USA. Catena 26 99-114 Leland, J.K. Bard, A.J. (1987) Photochemistry of colloidal semiconducting iron oxide polymorphs. J. Phys. Chem. 91 5076-5083 Lengweiler, H. Buser.W. Feitknecht, W. 

Jackson, I. Ringwood, A. E. (1981) High pressure polymorphism of the iron oxides. 

Leland, J.K. and A.J. Bard (1987). Photochemistry of colloidal semiconducting iron oxide polymorphs. Journal of Physical Chemistry, 91, 5076-5083. 

Laberty and Navrotsky (1998) determined the enthalpies of formation of a number of iron oxide and oxyhydroxide polymorphs. Data are listed in Table 2 which also compares the enthalpy relations among aluminum, iron, and manganese. It is evident that the Fe oxyhydroxide phases are much less stable relative to the anhydrous ferric phase (hematite) than are the aluminum oxyhydroxides relative to corundum. This is consistent with the much more frequent observation of hematite than of corundum in the field. It is also evident that the iron phases are as rich in polymorphism as the aluminum phases. It is clear that the enthalpy differences for both anhydrous (AI2O3, Fe203, Mn02) and hydrous (AlOOH, FeOOH, MnOOH) polymorphs are small, setting the stage for nanoscale stability crossovers. 

I. Hydromolysite—42 °C endotherm, polymorphic transformation 124 °C endotherm, hydromolysite dehydrates 211 °C endotherm, FeClj decomposes to iron and chlorine and iron oxidizes to Fe203. 

History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

Iron and Oxygen—Ferrous Oxide and Hydroxide—Magnetic Oxide—Ferric Oxide—Polymorphism—Hydrated Ferric Oxide—Ferrous Acid—Colloidal Ferric Hydroxide—Ferrites—Ferrates—Perferrates. 

As seen earlier, the steps used to purify iron involves carbonaceous material to remove the oxide-based impurities, through exothermic formation of CO and CO2. Hence, carbon will be pervasive in a variety of concentrations throughout all phases of iron and steels, present as an interstitial dopant within these lattices. Experimental evidence shows that carbon-doped iron polymorphs are indeed interstitial solid solutions. For instance, the carbon atoms in bcc ferrite are located only on empty face-centered positions. However, very few of these positions are occupied throughout the lattice, as the maximum solubility of carbon in a-Fe is only between 0.01 and 0.02 wt%. From a metallic-bonding standpoint, the addition of carbon in the lattice acts as an electron sink, that is able to accept some of the delocalized electron... 

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