| Autor: |
Allan Mackenzie, Graham J. Borradaile, Eleanor Jensen |
| Rok vydání: |
1991 |
| Předmět: |
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| Zdroj: |
Tectonophysics. 200:157-172 |
| ISSN: |
0040-1951 |
| Popis: |
The purple Cambrian slates of North Wales show colour changes to green in bedding-parallel patches and ellipsoidal spots. These are referred to as reduction spots but the process is more complex; the colour change is not simply associated with the reduction of iron. The green discoloration is accompanied by diffusion of Fe 3+ away from, and possibly by Mg and Ca towards the green material. The actual colour change is brought about by the replacement of the fine metamorphic hematite pigment with magnetite. However, coarser clastic hematite persists in the green slate. The green slate has twice as much chlorite and traces of magnetite, despite having less total Fe. The additional chlorite is chiefly responsible for the doubling of the magnetic susceptibility of the green slate with respect to the purple slate. Chlorite and epidote have the same compositions in both green and purple slates despite the difference in whole rock Fe content. Both purple and green slates behave predominantly as paramagnets between −194°C and 700°C. The anisotropy of low-field susceptibility (“magnetic fabric”) is increased by heating and therefore made easier to measure and the principal directions of anisotropy are preserved. Heating to 700°C increases the saturation isothermal remanence dramatically and increases its anisotropy due to the growth of single-domain magnetite. However, heating is not universally recommended as a means of enhancing otherwise feeble magnetic fabrics. Some other slates and schists show regressive changes in magnetic fabric on heating with erratic changes in principal directions. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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