Abstract

Iowaite, a new hydrous magnesium-ferric oxychloride, was discovered in diamond drill core at a depth of 1000 to 1500 feet in Precambrian serpentinite from Sioux County, Iowa. It is associated with chrysotile, dolomite, brucite, calcite, magnesite, and pyrite, all of which post-date the serpentinization of the rock.

Iowaite occurs as bluish-green crystals up to 2-3 mm in length. On exposure to the atmosphere, it undergoes color changes from bluish-green to light green, to whitish-green, to whitish-green with a rusty red overtone, due to loss of zeolitic water. The mineral is insoluble in water and readily soluble in mineral acids. Its hardness is about 1.5; density (measured) 2.11 ± 0.02 g/cm3; greasy luster; greasy or soapy feel; white streak; perfect basal (0001) cleavage. It is uniaxial negative, with ω = 1.543 ± 0.005 and ∈ = 1.533 ± 0.005; B = 0.010.

X-ray and electron diffraction indicates a hexagonal cell with a = 3.119 ± 0.005, c = 24.25 ± 0.02 Å, C = 7.775 ± 0.02; V = 204.3 ± 1.5 Å3; Z = 1 [4Mg(OH)2·FeOCl·xH2O] in the hexagonal unit. The amount of zeolitic water (x) may vary up to 4. The strongest X-ray lines are: 8.109 (100), 4.047 (40), 2.363 (27), 2.019 (23), 2.639 (17), 1.530 Å (13).

Iowaite contains zeolitic water which is continuously released on heating to 280°C, increasing the surface area from 2.15 to 12.7 m2/g. The structure begins to collapse at 315°C. with the evolution of more water and the formation of periclase. At 400°C magnetite forms. Both phases become better developed on heating to 750°C.

Chemical analysis of the purified mineral gave: MgO 39.00 Fe2O3 21.16, Cl 8.50, NiO 0.10, SiO2 2.00; ignition loss (due to water, hydroxyl, chlorine, iron, etc.) 39.82 percent.

Iowaite is named for its occurrence in the state of Iowa.

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