Electron microscope and microprobe analysis of chlorite–mica stacks in the wenlock turbidites, mid Wales, U.K.
By: Dimberline, Andrew J
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Material type: 
ArticleDescription: 299-306pp ; Illustration.Subject(s): Sedimentary petrology | Chlorite group | SEM data | Clastic rock | Petrology | Origin of chlorite - Mica stocks - Wales - United Kingdom
In:
Geological magazine : Vol. 123 Iss. 1-6 Year. 1986Summary: Chlorite–mica stacks in the Wenlock turbidites have been studied using backscattered electron microscopy and electron microprobe analysis, combined with thin-section work and bulk rock chemical analysis. The stacks occur in fine sandstones and silt–mud turbidites and range in length from < 30 μm to 1.5 mm. They consist of interlayered packets of Fe-rich chlorite and mica.
Combined textural and chemical data suggest that many of the stacks represent altered detrital biotite micas. A four-stage alteration sequence is proposed:
(1) Subaerial alteration of biotite, in the source area, to interlayered biotite–hydrobiotite/vermiculite.
(2) Post-depositional collapse of vermiculite to form a mica phase under conditions of high K+/H+ in the sediment pore waters.
(3) Decrease in K+/H+ ratio, possibly due to H+ build up in the fermentation zone, causing alteration of biotite layers to chlorite.
(4) Kinking of the stacks and pressure solution of chlorite early in the development of cleavage.
| Item type | Current location | Collection | Call number | Status | Date due | Barcode |
|---|---|---|---|---|---|---|
Article
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Library and Information Centre Periodical Section | Bound Journal Collection | Not for loan | 002533_52 | ||
Serials/Scientific Journal
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Library and Information Centre Periodical Section | Bound Journal Collection | 550 GEO (Browse shelf) | Available | 002533 |
Chlorite–mica stacks in the Wenlock turbidites have been studied using backscattered electron microscopy and electron microprobe analysis, combined with thin-section work and bulk rock chemical analysis. The stacks occur in fine sandstones and silt–mud turbidites and range in length from < 30 μm to 1.5 mm. They consist of interlayered packets of Fe-rich chlorite and mica.
Combined textural and chemical data suggest that many of the stacks represent altered detrital biotite micas. A four-stage alteration sequence is proposed:
(1) Subaerial alteration of biotite, in the source area, to interlayered biotite–hydrobiotite/vermiculite.
(2) Post-depositional collapse of vermiculite to form a mica phase under conditions of high K+/H+ in the sediment pore waters.
(3) Decrease in K+/H+ ratio, possibly due to H+ build up in the fermentation zone, causing alteration of biotite layers to chlorite.
(4) Kinking of the stacks and pressure solution of chlorite early in the development of cleavage.
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