Showing papers on "Goethite published in 1985"

Spectral and other physicochemical properties of submicron powders of hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3), magnetite (Fe3O4), goethite (alpha-FeOOH), and lepidocrocite (gamma-FeOOH).

Abstract: The spectral properties (0.35-2.20 microns) of submicron powders of hematite, maghemite, magnetite, goethite, and lepidocrocite are determined. Other physicochemical data are obtained for the powders in order to determine if deviations from stoichiometry occur due to their small particle size, to determine their state of chemical and phase purity, and to determine the physical characteristics of the individual powders. The physicochemical data obtained include mean particle diameter, discrete particle shape, chemical composition, crystallographic phase, magnetic parameters, and Moessbauer parameters. The positions of the spectral features for the hematite, maghemite, and magnetite powders are independent of temperature over the interval between about +20 and -110 C. For the goethite and lepidocrocite powders, a small shift of about 0.02 micron to shorter wavelengths is observed for some of the features after cooling to about -110 C. The spectral properties of the iron oxides and oxyhydroxides are important not only for understanding the basic physics and chemistry of the compounds but also for applications such as the remote sensing of the earth and Mars.

Properties of Goethites of Varying Crystallinity

Abstract: Goethites were synthesized from ferrihydrite in 0.7 M KOH between 4° and 90°C. As temperatures increased, the goethite crystals became larger and of less domainic character, and surface areas decreased from 153 to 9 m2/g. Surface area, oxalate-soluble Fe to total Fe ratios, chemisorbed water, Mossbauer parameters, and dissolution rate in 6 M HCl at 25°C are particle-size controlled, whereas mean crystallite dimensions, a-dimension of the unit cell, differences between the two OH-bending modes, and dehydroxylation temperatures suggest the existence of a low-temperature (high-a-dimension) and a high-temperature (low-a-dimension) goethite, with a narrow transition range at a synthesis temperature of 40°–50°C. Hydrothermal treatment at 125°–180°C of a low-temperature goethite led to a healing of the multidomainic, microporous high-a-dimension goethite into a monodomainic low-a-dimension goethite of similar overall crystal size with the properties of a low-a-dimension goethite.

Properties of goethite and hematite in kaolinitic soils of southern and central Brazil

Abstract: Iron oxides in clay fractions of Inceptisols, Ultisols, and Oxisols from Rio Grande do Sul and Goias State in southern and central Brazil, respectively, were essentially goethite and hematite. Their Al substitution was around 10 mole % for hematites and between 15 to 25 mole % for goethites in southern Brazil and around 15 and 33 mole %, respectively, for those in central Brazil. The ratio of the Al substitution of hematite and goethite in the same sample is concentrating around 0.5:1. The size of the hematite crystals as measured by XRD-line broadening was between 10 to 15 nm thick (along the z axis) and 20 to 30 nm wide (along the x axis). The goethite crystals generally appeared to be smaller, as judged from the speClfic surface area obtained from a correlation between line broadening and surface area of 42 synthetic goethites. Under the electron microscope goethite and hematite crystals look very much alike: they appear mainly as subrounded, isodimensional particles. Hexagonal (hematite) and aClcular (goethite) particles are rare. The Fe-oxide crystals appeared not to be closely assoClated with kaolinite crystals, but were aggregated among themselves.

Chemical modeling of anion competition on goethite using the constant capacitance model

Abstract: The constant capacitance model was used to describe phosphate, selenite and silicate adsorption on goethite (a-FeOOH) in single anion systems. The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. It is therefore appropriate for describing the adsorption behavior of these specifically adsorbed anions. The computer program FITEQL, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on goethite. The model represented adsorption of these anions well over the entire pH range studied (3-12). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describing anion competition in mixed systems using the anion surface complexation constants from single anion systems was qualitatively successful. In mixed anion systems the model predictions reproduced the shape of the adsorption isotherms well over the pH range 3 to 12. However, phosphate adsorption was overestimated while adsorption of the competing anion was underestimated. Additional Index Words: phosphate adsorption, selenite adsorption, silicate adsorption, ligand exchange, FITEQL, MICROQL. Goldberg, S. 1985. Chemical modeling of anion competition on goethite using the constant capacitance model. Soil Sci. Soc. Am. J. 49:851-856. T CONSTANT CAPACITANCE model of the Oxidewater interface was developed by Stumm, Schindler and their co-workers (Stumm et al., 1970; Schindler and Gamsjager, 1972; Stumm et al., 1976; Stumm et al., 1980). It is a chemical model which differs from the Langmuir and Freundlich adsorption equations in that it explicitly defines surface species and chemical reactions. The adjustable model parameters are the conditional equilibrium constants for the surface reactions as well as the capacitance—an empirical parameter. The effect of pH on the extent of adsorption and on the magnitude of surface charge is included in this model. The model successfully describes the amphoteric behavior of inorganic surface hydroxyl groups as well as the adsorption of specifically adsorbed anions. Anions become specifically adsorbed by undergoing ligand exchange with reactive surface hydroxyl groups. Specific adsorption of anions produces a shift in the zero point of charge (ZPC), of the oxide mineral to a more acid value. A shift in ZPC was observed for adsorption of selenite on goethite (Kingston et al., 1968) and on hydrous alumina (Rajan, 1979) and for silicate adsorption on goethite (Kingston et al., 1972; Sigg and Stumm, 1981) indicating that these ions are adsorbed specifically. For this reason, the constant capacitance model is appropriate for describing 1 Contribution from the U. S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 24 Sept. 1984. Approved 22 Jan. 1985. 2 Soil Scientist. the adsorption behavior of these anions. The adsorption behavior of various other anions has been described with the model, including adsorption of the aromatic compounds catechol, salicylic acid, benzoic acid, and phthalic acid on 7-Al2O3 (Kummert and Stumm, 1980), and F~, SO^, acetate, H2SiO^-, and PO]" adsorption on goethite (Sigg and Stumm, 1981). The ability of the model to describe phosphate adsorption on various x-ray amorphous as well as crystalline aluminum and iron oxides was recently demonstrated by Goldberg and Sposito (1984). Selenium is an essential element in animal nutrition being required in small amounts and producing toxicity symptoms at higher concentrations. Since the concentration range between deficiency and toxicity is narrow, reactions affecting selenium availability have been investigated. Wells (1967) postulated that selenium was retained by the clay size particles, especially gibbsite and iron oxides, since marked selenium accumulation occurred in soil horizons enriched in these colloids. Geering et al. (1968) and Gary and Allaway (1969) found evidence for the retention of selenite by adsorption onto soil iron oxides. Gary and Allaway (1969) indicated that the adsorption complex was a finely divided sesquioxide having a solubility equivalent to or less than freshly precipitated Fe(OH)3. Hamdy and Gissel-Nielsen (1977) found that selenite was fixed very rapidly by Fe2O3 and adsorbed strongly, indicating a major role of iron oxides in selenite retention in soils. Rajan (1979) found that selenite adsorption on hydrous alumina occurs specifically via a ligand exchange mechanism with surface hydroxyl ions. By determining the ratio of hydroxyl ions released to selenite ions adsorbed, Rajan (1979) proposed that selenite forms both monodentate and bridging complexes at low surface coverage but forms almost exclusively monodentate complexes near the experimental adsorption maximum. Dissolved silica in soil solution plays an important role in soil development and a beneficial one in plant growth. Silicate was chemisorbed on crystalline aluminum hydroxide (Kingston and Raupach, 1967) and was adsorbed with high affinity on iron and aluminum hydrous oxides (McPhail et al., 1972). McKeague and Cline (1963) found that freshly precipitated iron and aluminum hydroxides exhibited a much higher capacity for removing silicate from solution than did crystalline oxide minerals. They found that silicate adsorption on a bauxite sample containing mostly gibbsite increased with increasing pH over the pH range 4.8 to 9.7. Similar behavior was found for the soils studied by McKeague and Cline (1963) in that silicate adsorption increased over the pH range 4 to 9. Decreases in soil silicate sorption were found above pH 10. Beckwith and Reeve (1963) also found that silicate adsorption behavior on aluminum and iron oxides was very similar to that on soil samples in its pH dependence in the pH range 3 to 9. Obihara and Russell (1972) obtained silicate adsorption curves of soils

Effect of solution conditions on the proportion and morphology of goethite formed from ferrihydrite

Abstract: In alkaline media and at 70°C dilute suspensions of ferrihydrite transformed to goethite between pH 112 and 14 and to a mixture of goethite and hematite above and below this pH range Increasing the temperature of the transformation or the concentration of the suspension reduced the pH range in which goethite alone formed The morphology of goethite was chiefly a function of the pH of the system Acicular crystals formed at all pHs and exclusively above pH 122 Epitaxial twinned crystals predominated at pHs below 11, and twins free from hematite formed at higher pHs Increasing the suspension concentration, ionic strength, or temperature extended the pH range over which twinned crystals formed Electron micrographs showed that twins formed mainly during the initial stage of the transformation, whereas acicular crystals formed over a longer period Thus, the twins appeared to nucleate in the ferrihydrite; nucleation of acicular particles took place in solution

Influence of aggregation on the uptake kinetics of phosphate by goethite

Abstract: A coagulation model is postulated in which phosphate bridges to primary goethite particles and causes increased aggregate order, phosphate burial, and oscillatory uptake kinetics. This hypothesis is supported by electron diffraction patterns of single aggregates, decreases in exchangeable phosphate, phosphate-induced reduction in BET surface area, and long-term kinetic results showing slow release of phosphate.

Cluster formation versus isolated-site adsorption. A study of Mn(II) and Mg(II) adsorption on boehmite and goethite

Abstract: The experiments measured the extent of adsorption of Mn(II) and Mg(II) on goethite [α-FeOOH] and boehmite [γ-AlOOH] from pH ≅ 4 to pH ≅ 11 and the resulting changes in surface charge. The electron spin resonance spectra for the Mn(II)/AIOOH system to monitor changes in the local environment of Mn(II), were also measured as adsorption occurred. The results indicate that Mg(II) adsorbs at relatively few, isolated sites on the goethite surface. When Mn(II) adsorbs on boehmite it forms magnetically ordered arrays, which are referred to as surface “clusters.” On the basis of the negligible effect of adsorption on surface charge, it was concluded that there is a high probability of “cluster” formation when Mn(II) adsorbs on goethite and Mg(II) adsorbs on boehmite. The mechanism of adsorption and the tendency to either form “clusters” or adsorb at isolated sites is strongly dependent on the nature of the oxide surface.

Effect of Simple Sugars on the Alkaline Transformation of Ferrihydrite into Goethite and Hematite

Abstract: The transformation of ferrihydrite to goethite and hematite in the pH range 9–13 is retarded by the presence of simple sugars (>- 10-4 M concentration). The retarding effect is related to the extent of adsorption of the sugar on ferrihydrite. Maltose and glucose adsorb strongly and inhibit the transformation by preventing both aggregation and dissolution of the ferrihydrite. Sucrose adsorbs to a much lesser extent than the other sugars and appears to hinder the nucleation and growth of goethite in solution. Hematite formation relative to that of goethite is favored by the sugars in the order: maltose >glucose ≫ sucrose. Maltose and glucose cause hematite to grow as prismatic crystals rather than as hexagonal plates and also lead to a new type of twinned goethite; one with epitaxial outgrowths of goethite on a prismatic crystal of hematite. In alkaline media glucose and maltose are partly transformed into a mixture of different sugars and hydroxycarboxylic acids, and it is probable that modification of the hematite crystal shape is due to the presence of the degradation products rather than to the nature of the original sugar. The results of this work suggest that cyclic molecules influence the transformation of ferrihydrite to a lesser extent than do acyclic molecules.

Occurrence and Properties of Lepidocrocite in some Soils of New Zealand, South Africa and Australia

Abstract: Fifty-one samples, collected from 26 sites in New Zealand, South Africa and Australia, were tested for the presence of lepidocrocite (a-FeOOH) and goethite (a-FeOOH). The samples were predominantly orange-coloured mottles, bands, crusts and pipestems from hydromorphic soils, but also included a placic horizon, iron-rich precipitates from water courses, altered pyrite cubes, and geode-like features in weathered saprolites. The iron oxides were identified and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Lepidocrocite was identified in 49 of the samples, and in 44 was present in concentrations exceeding I%, with the highest being approximately 70%. Crystallinities of the lepidocrocite were assessed from the widths and positions of XRD peaks, and, in some cases, from TEM. Goethite was also present in most samples, and predominated over lepidocrocite in some. In all three countries, the most common occurrence of lepidocrocite was associated with the gleyed soil materials commonly found in wet, poorly drained pseudogley soils (mostly 'humic slope gley') of humid temperate climate. Under these climatic conditions the high degree of water supply, relatively lower soil temperatures, lower evaporation and slower water movement caused reductomorphic conditions. At these sites soil iron oxides are reduced and, on reoxidation, lepidocrocite and goethite are generally formed. The colour generally associated with lepidocrocite in iron-rich segregations in hydromorphic soils is 7.5YR6-7/4-8 Lepidocrocite was also identified in all of the other samples mentioned above. A study of the relative proportions of lepidocrocite and goethite, and the crystallinity of these associated minerals in the various morphological concentrations (profile trends), suggests that their formation is strongly governed by soil microenvironmental factors (pH, Eh, and ionic environment). However, climatic and pedogenic factors such as podzolization and high organic matter contents may modify these mineral phases or induce further transformation. These findings indicate that lepidocrocite occurs in a remarkably wide range of materials and weathering environments. Moreover, the morphology, crystallinity, differential XRD line broadening and line shift of the lepidocrocite, and the commonly associated goethite also vary markedly in the respective materials. This suggests that other factors associated with particular weathering environments, such as those involved in pseudomorphous alteration of pyrite and the influence of chloride ions, may be more important in the formation of lepidocrocite than the conditons associated with hydromorphy. The morphology and characteristics of lepidocrocite and goethite, together with field associations, appear to be useful indicators of soil genesis.

Crystallization of Iron Oxides on Calcite Surfaces in Static Systems

Abstract: Iron salts react readily with calcite in oxidizing calcareous environments to produce solid phase Fe-oxides. These reactions represent important processes in aqueous, geologic, and pedogenic environments. In the present investigation, Fe-oxides were precipitated from Fe(ClO4)2 solutions on undisturbed calcite grains in aqueous suspension. In this way it was possible to investigate the sequence of events in the crystallization process. Following an initial precipitation on the calcite grains, a period of slow growth of lepidocrocite was noted wherein 4-µm euhedral platelets formed with uniform orientation perpendicular to the calcite surface. The slow growth and highly crystalline nature of the Fe-oxide product may be partially due to the diffusion barrier formed by the growing oxide crystal mass which influenced rate of movement of HCO3− to the dissolved Fe phase and Fe ions and H+ towards the calcite surface. Upon continued aging, the supernate became noticeably opaque. As the suspended nuclei settled, new surfaces for crystal growth were provided which resulted in somewhat less crystalline lepidocrocite and goethite.

The aqueous photolysis of ethylene glycol adsorbed on goethite

Abstract: — Suspensions of goethite (α-FeOOH) were photolyzed in aerated ethylene glycol-water solutions at pH 6.5, with ultraviolet light in the wavelength range300–400 nm. Under these conditions, formaldehyde and glycolaldehyde were detected as photoproducts. Quantum yields of formaldehyde production ranged from 1.9 7times; 10-5 to 2.9 × 10-4 over the ethylene glycol concentration range of 0.002-2.0 mol/l, and gave evidence that the reaction occurred at the goethite surface. Quantum yields of glycolaldehyde were 20% less than those of formaldehyde, and displayed a concentration-dependent relationship with ethylene glycol similar to that of formaldehyde. Immediately after photolysis, Fe2+ was measured to be 4.6 × 10-7 mol/l in an aerated suspension containing 1.3 mol/l ethylene glycol, and 8.5 × 10-6 mol/l in the corresponding deoxygenated suspension. Glycolaldehyde was not generated in the deoxygenated suspensions. These results are consistent with a mechanism involving the transfer of an electron from an adsorbed ethylene glycol molecule to an excited state of Fe3+ (Iron[III]) in the goethite lattice, to produce Fe2+ and an organic cation. In a series of reactions involving O2, FeOOH, and Fe2+, the organic cation decomposes to form formaldehyde and the intermediate radicals “OH and” CH2OH. OH reacts further with ethylene glycol in the presence of O2 to yield glycolaldehyde. Aqueous photolysis of ethylene glycol sorbed onto goethite is typical of reactions that can occur in the aquatic environment.

Estimation of iron oxides in soil clays by profile refinement combined with differential X-ray diffraction

Abstract: A B S T R A C T: A procedure is described for the quantitative estimation of hematite, goethite and lepidoerocite in soil clays by differential X-ray diffraction combined with profile refinement. The method has been applied to clays from 12 palaeosol horizons from south-east England. Samples containing as little as 1% dithionite-extractable iron can be analysed in this way. The hematite :goethite ratio is determined to within +3% for soil clays having ~2.5% of each oxide and to within 10% in less favourable cases. The concentration of oxides in the soil determined solely from X-ray diffraction and from a combined chemical analysis/X-ray method are in good agreement. In addition to improving the accuracy and extending the range of iron-oxide concentrations over which analysis can be carried out, the method may, in principle, be used to provide information about the cell parameters and line profiles of each component present.

Mn (II) Oxidation in the Presence of Metal Oxides

Abstract: The oxidation of Mn(II) by oxygen in the presence of goethite (α-FeOOH), lepidocrocite (γ-FeOOH), silica and alumina was studied. All the solids, except perhaps alumina,enhanced the rate of Mn(II) oxidation. The degree of enhancement was as follows: lepidocrocite > goethite > silica > alumina. At constant pO2 Mn(II) oxidation on goethite, lepidocrocite and silica can be described by the following equation [Equation; see abstract in scanned thesis for details.] where is the concentration of the surface hydroxyl group and a is the solids concentration. Mn(II) oxidation in the presence of goethite or lepidocrocite is first order in pO2. Both these reactions are strongly temperature dependent (apparent activation energy ~100 kJ/mol). Normal laboratory lighting has no effect on the rate of these reactions. The rate of Mn(II) oxidation in the presence of lepidocrocite is about 4 times slower in 0.7M NaClO4, than in 0.1M NaClO4. This reaction is inhibited by the following ions; Mg2+, Ca2+, silicate, salicylate, phosphate, chloride, and sulfate. Phthalate has little or no effect on the rate of this reaction. The adsorptive behaviour of Mn(II) on the metal oxides studied could be described using a surface complexation model. Using this model it was shown that the rate of Mn(II) oxidation on the metal oxides studied is described by the equation [Equation; see abstract in scanned thesis for details.] where (≡SOH)2Mn is a bidentate surface complex. It is possible that a hydrolyzed surface complex (≡SOMnOH) rather than the bidentate complex is involved in the reaction. The results of the laboratory studies indicate that in natural waters the important factors which influence Mn(II) on metal oxides are pH, iron oxide concentration, temperature, [Mg2+], [Cl-], and ionic strength. These studies predict that at pH

Iron oxides in two Oxisols from the Brazilian coastal plain

Abstract: SUMMARY Two Oxisols from the B2 horizon at similar locations 2 km apart on the Brazilian coastal plain with a udic, isohyperthermic regime were practically identical in chemical composition but differed markedly in colour; one was a dark reddish brown, the other a pale ochre. Mossbauer spectra at 295 K, 80 K and 4.2 K indicated a clear difference in iron oxide mineralogy. Iron was present as aluminous hematite and goethite, with Al substitutions of approximately 20% in the two phases. However, the relative proportion of iron present in hematite and goethite was 45:55 in the first sample and 12:88 in the second. Possible reasons for these differences are discussed.

The Solubility Product of GOETHITE1

Abstract: Iron (III) oxyhydroxides were prepared by hydrolyzing and aging Fe(CIO4)3 solutions at room temperature for periods ranging from 9 to 16 years. Pure goethite was found in most samples, but lepidocrocite was found as an admixture in some specimens. Only those specimens containing pure goethite were analyzed for their (Fe3+) (OH-)3 activity products, which were taken to be the Ksp (solubility product) of goethite. The values determined were found to vary with ionic strength. They were 39.80, 40.32, and 40.83 for ionic strength. They were 39.80, 40.32, and 40.83 for ionic strengths in the neighborhood of 0.005, 0.04, and 0.2 M, respectively, when calculated using Siddall and Vosburgh's hydrolysis constants. Because much of the contribution to Ksp was due to the OH- activity, the effect of the variation in the hydrolysis constants on calculation was small.

Production of acicular goethite

Abstract: PURPOSE: To produce acicular goethite having excellent magnetic properties and suitable for the production of magnetic powder, by adding an alkaline aqueous solution to a solution of a ferrous salt, suspending and oxidizing the precipitated hydroxide, and heat-treating the suspension. CONSTITUTION: An aqueous solution of a ferrous salt (e.g. ferrous sulfate) is added with an aqueous solution of an alkali (e.g. sodium hydroxide) until the concentration of the alkali reaches ≥2N, and a hydroxide is precipitated and suspended at 5W30°C. An oxygen-containing gas (e.g. air) is introduced into the suspension to effect the oxidization of the hydroxide to acicular goethite particle having an average major axis of ≤0.2μm and an average minor axis of ≤0.03μm. The suspension is heat-treated at 70W100°C to effect the growth of crystals having uniform particle size and having an average major axis of 0.06W0.2μm and average minor axis of 0.01W0.03μm and obtain the objective acicular goethite. COPYRIGHT: (C)1986,JPO&Japio

Formation of Secondary Iron Oxides in Various Environments

Abstract: Iron oxides are among the most common minerals formed during rock weathering. They vary in mineral species (goethite, hematite, lepidocrocite, maghemite, ferrihydrite) and, additionally for any mineral, in crystallinity and Al- for Fe-substitution in the structure. All three parameters may reflect the weathering environment. For example, the ratio of goethite to hematite, which is the most important pair of Fe oxides, varies widely with climatic conditions (as caused by latitude and altitude), with topographic position in a landscape, and with profile depth. This can be partly explained by such factors as soil temperature, moisture, organic matter and pH. A kinetic model based on synthesis experiments is proposed, according to which goethite may be formed from various Fe sources via a solution-nucleation-crystallization process, whereas hematite is formed from ferrihydrite via a dehydration-rearrangement process within the defect hematite-like structure of ferrihydrite. These two processes are competitive. Their relative rate depends on the environmental conditions and determines the final goethite/hematite ratio. This kinetic concept is preferred over a thermodynamic stability concept with which, because of kinetic hindrance, experimental results are often in disagreement.

Manufacture of goethite

Abstract: PURPOSE:To manufacture needlelike goethite for a magnetic recording medium such as a magnetic tape in a short time by subjecting an alkaline Fe(OH)2 suspension to oxidation treatment with a gas contg oxygen in two steps under specified conditions CONSTITUTION:An aqueous soln of a ferrous salt such as FeSO4 or FeCl2 is mixed with an aqueous soln of an alkali such as NaOH to prepare an alkaline Fe(OH)2 suspension, and a gas contg oxygen such as air is blown into the suspension in 2-3 ratio of alkali:Fe at 10-30 degC to produce seed crystals of goethite An aqueous alkali soln is added to the suspension and heated, and a gas contg oxygen is blown again into the suspension in >=3 ratio of alkali:Fe at 40-60 degC to grow the seed crystals of goethite by oxidation Fine and uniform needlelike goethite by oxidation Fine and uniform needlelike goethite of 002- 005mum particle size can be manufactured in a short time

Manufacture of needlelike alpha-iron oxyhydroxide crystal

Abstract: PURPOSE:To form uniform needlelike goethite crystals having a narrow grain size distribution when an aqueous suspension contg. seed crystals and a ferrous salt is oxidized to manufacture the titled crystals, by keeping the suspension at a specified pH so as to grow uniformly the seed crystals. CONSTITUTION:An aqueous suspension contg. a ferrous salt such as ferrous sulfate and alpha-iron oxyhydroxide (goethite) microcrystals of <= about 0.15mum major axis size as seed crystls is kept at 5.5-7.5pH by adding an alkali such as sodium hydroxide by an amount equal to or less than the amount of dissolved ferrous ions to form green rust. The total concn. of the iron components in the suspension is about 0.05-1mol/l (expressed in terms of Fe). Oxygen is then blown into the suspension in the presence of the green rust, and an oxidation reaction is caused at about 20-80 deg.C to grow goethite on the seed crystals. It is preferable that the alkali is added in about 2-10 steps.

Iron oxides of Mars: Evidence for contemporary weathering

Abstract: Reflectance spectra of Mars were analyzed using a multiple high order derivative spectroscopy technique. Among the results of the analysis was the presence of suites of bands in each of the spectra that can be attributed to Fe(3e) phases. Several of the spectra contained bands that are very close to the band positions in the laboratory spectra of goethite, an hexagonal hydrated ferric oxide. Spectra of other areas showed absorption bands that were within 3% of the positions for hematite, and hexagonal close packed unhydrated Fe203. Remaining areas showed bands that are intermediate in position to the goethite and hematite bands, suggesting that there may be mixtures of goethite and hematite, and/or intermediate (partially dehydrated goethite) phases present in those areas. Both bright areas and dark areas showed suites of goethite bands and hematite bands, and there does not therefore appear to be a correlation with albedo. The areas that showed the goethite bands are, however, within zones of ongoing or historically frequent dust cloud activity, and the areas with the hematite bands were outside of the zones of frequent dust cloud activity. This suggests the possiblility that the more hydrated phase may occur within a mobile dust component.