| Nontronite | |
|---|---|
 Nontronite fromSlovakia | |
| General | |
| Category | Phyllosilicates Smectite group |
| Formula | (CaO0.5,Na)0.3Fe3+2(Si,Al)4O10(OH)2·nH2O |
| IMA symbol | Non[1] |
| Strunz classification | 9.EC.40 |
| Crystal system | Monoclinic |
| Space group | C2/m (no. 12) |
| Identification | |
| Color | Yellow, olive-green, green, orange, brown |
| Crystal habit | Earthy masses |
| Cleavage | Perfect basal |
| Fracture | uneven |
| Mohs scale hardness | 1.5 to 2 |
| Luster | Earthy to dull |
| Streak | Colorless |
| Specific gravity | 2.3 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.530–1.580; nβ = 1.555–1.612; nγ = 1.560–1.615 |
| Birefringence | δ = 0.030–0.035 |
| References | [2][3][4][5] |
Nontronite is the iron(III) rich member of thesmectite group ofclay minerals. Nontronites typically have a chemical composition consisting of more than ~30%Fe2O3 and less than ~12%Al2O3 (ignited basis). Nontronite has very few economic deposits likemontmorillonite.[6][7] Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.
A typical structural formula for nontronite isCa.5(Si7Al.8Fe.2)(Fe3.5Al.4Mg.1)O20(OH)4.[8] The dioctahedral sheet of nontronite is composed mainly oftrivalent iron (Fe3+) cations, although some substitution by trivalent aluminium (Al3+) anddivalent magnesium (Mg2+) does occur. Thetetrahedral sheet is composed mainly of silicon (Si4+), but can have substantial (about 1 in 8) substitution of either Fe3+ or Al3+, or combinations of these two cations. Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet. The layer charge is typically balanced by divalent calcium (Ca2+) or magnesium (Mg2+).
Nontronite forms from theweathering ofbiotite andbasalts, precipitation of iron and silicon richhydrothermal fluids and in deep sea hydrothermal vents.[9][10] Some evidence suggests that microorganisms may play an important role in their formation.[11] Microorganisms are also involved in reduction of structural iron in nontronite when soils undergo anoxia, and the reduced form of the clay appears to be highly reactive towards certain pollutants, perhaps contributing to the destruction of these compounds in the environment.[12][13]
The only known commercially viable and operational nontronite mine is located inCanterbury, New Zealand. The mine is operated by Palmer Resources and the finished products are used internationally in industrial applications (pulp & paper, surface coating) and in cosmetics marketed as New Zealand Glacial Clay.[14]
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