Hydrothermal investigation of a portion of the system Fe2O3‐H2O has served to locate approximately the hematite‐goethite phase boundary. Calculations derived from the experimental data provide the following estimates of the thermodynamic properties of goethite at 25°C: ΔHƒ = −132 Kcal/mol; S° = +18.1 cal/mol‐degree; ΔSƒ = −51.0 cal/mol‐degree; ΔGƒ = −117 Kcal/mol. Goethite is shown to be the stable ferric oxide phase under sub‐aerial weathering conditions where relative humidity exceeds 60 percent. Goethite is also shown to be stable in all normal marine environments, though hematite may become stable in contact with saline liquors of an evaporite basin.
Bermuda marine waters are not in equilibrium with the bulk carbonate sediments with which they are in contact, and they are supersaturated with respect to pure calcite. This apparent supersaturation seems to result from a metastable equilibrium between sea water and the most soluble solid phase available locally in excess.
X-ray diffraction studies, chemical analyses, infrared-absorption studies, and nuclear-magnetic-resonance spectrum analysis demonstrate the existence of the mineral brucite, Mg(OH)(2), in the skeletal carbonate secreted by the red marine alga Goniolithon sp. Electron microprobe examination of the carbonate shows that the brucite is concentrated in certain areas of the skeletal structure. These results explain the anomalously high solubility and large cell size of the Goniolithon "carbonate" observed by earlier investigators.
The alteration of black or dark-coloured flint artifacts to produce a white surface is familiar to archaeologists. This patination is attributed to weathering, and was first explained by Judd in 1887. Judd suggested that flint is composed of a felted aggregate of euhedral quartz needles with colloidal (opaline) silica occupying the interstices between the crystalline grains. He postulated that carbonate-bearing ground-water leached the more soluble amorphous silica, leaving minute interstitial cavities in the altered margin of the flint. The multitudes of surfaces thus produced reflect and refract light falling upon or passing through the flint, and the resulting scattering causes the white appearance of the altered portion. Judd cited the white aspect of powdered black obsidian as an example of such scattering effects.
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