Botryoidal Goethite: a Transmission Electron Microscope Study

Self Cite

Abstract--The formation of iddingsite by the oxidative weathering of FoB0 olivine begins by solution of Mg from planar fissures, 20 ~ wide and spaced 200 ~ apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks <50 ~ in diameter allow Na, AI, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20-50 & wide and 1-7 layers thick, form bridges 50-100 ~. long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 • 100 • 200 A (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Section: Transmission Electron Microscopysupporting

confidence: 63%

Weathering of Basalt: Formation of Iddingsite

Smith¹,

Milnes

Eggleton

1987

Self Cite

“…Given such contrasting values, a simple explanation for the relatively similar P-sorption capacity of goethites is that only one type of crystal face dominates in most synthetic and naturally occurring crystals. The fact that, as for synthetic samples, TEM observations show that natural goethites are also bound by (110) faces (Smith and Eggleton, 1983;Amouric et al, 1986) supports this unifying principle. An alternative hypothesis to explain the constancy of P-sorption capacity in natural crystals (Ainsworth et aL, 1989) is that, when the SA is above a threshold value, the goethite crystals can be described as spheres or cubes rather than needles of various morphologies.…”

Section: A ( P Mol P/m 2 )mentioning

confidence: 55%

“…Consequently, the relatively significant amounts of Sid --SiKo, can be allocated to silicate intimately associated to goethite (and, to a lesser extent, to the small amounts of accompanying ferrihydrite). This silicate probably fills the narrow spaces between crystals, and also bonds the individual crystals in goethite aggregates, as observed in other natural goethites (Smith and Eggleton, 1983). Alternatively, Si 4+ may replace Fe 3+ in the goethite structure but, so far, little quantitative importance has been ascribed to this type of substitution (Schwertmann and Taylor, 1972).…”

Section: Silicate Release Versus Phosphate Sorptionmentioning

confidence: 89%

Fast and Slow Phosphate Sorption by Goethite-Rich Natural Materials

Torrent

Schwertmann

Barrón

1992

170

100

Abstract--Although phosphate sorption by goethite and other less-abundant Fe oxides strongly influences the concentration of this anion in the soil solution and aquatic environments, relatively little is known on the P-sorption characteristics of natural goethites. For this reason, we examined the P-sorption capacity and time course ofP sorption of 10 goethite-rich soil, ferricrete and lake ore samples, in which the content and nature of mineral impurities were unlikely to affect P sorption significantly. Phosphate sorption could be adequately described by a modified Freundlich equation including a time term. The amount of P sorbed after i day of equilibration at a concentration of 1 mg P/liter ranged widely (0.36-2.04 #mol P/m2). The total P sorbed after 75 days of equilibration varied less, in relative terms (1.62-3.18 #tool p/m2), i.e., a higher slow sorption tended to compensate for a lower initial (fast) sorption. Total sorbed P (X = 2.62, SD = 0.52 ~mol P/m 2) was similar to the sorption capacity of synthetic goethites, suggesting a common sorption mechanism and the predominance of one type of crystal face, which, according to previous transmission electron microscope observations, might be the (110).The extent of the slow reaction correlated to the ratio between micropore surface area and total surface area, as well as to oxalate-extractable Fe, which is an estimation of the ferrihydrite content. Ferrihydrite impurities might affect the slow reaction by contributing to the microporosity of some samples. Silicate adsorbed on the surface of the goethites was readily desorbed during phosphate sorption and did not significantly affect the extent of the slow sorption process.

“…In some channels the saponite conforms in a general way to the early orientation relation; in others it is quite irregular. The wider channels also contain remnant diamond-shaped crystallites which are similar in appearance to microcrystals of fibrous goethite (Smith and Eggleton, 1983), and on this basis are interpreted as goethite. These crystallites, however, also resemble the initial diamond-shaped domains of phase M, and their identification as goethite must be regarded as tentative (Figures 11 and 12).…”

Section: [1001 Sectionsmentioning

confidence: 94%

Formation of Iddingsite Rims on Olivine: A Transmission Electron Microscope Study

Eggleton

1984

Abstract--lddingsite rimming olivine in a basanite from the Limberg, Germany, is composed ofsaponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-A diameter {110} bounded needleshaped domains which change to a metastable hexagonal phase having a = 3.1 ~ and c = 4.6 A, probably of close-packed, metal-oxy.gen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-A diameter and are parallel to the olivine y-axis. Laths of smectite, one or two layers thick, 20 7k wide, and as much as 100/~, long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic { 110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.