| P number: | P524375 |
|---|---|
| Caption: | Photomicrograph of porphyritic basalt. Light: XPL, Magnification x 2. , Lanarkshire. |
| Description: | The image is a photomicrograph of a thin section of basalt. It is taken under crossed-polarized light in order to differentiate between the different mineral components in the rock. British Geological Survey Petrology Collection sample number S 19313. Light: XPL, Magnification x 2. Light: XPL, Magnification x 2. The large regular-shaped crystals which have variable colours of grey, white and black are phenocrysts of plagioclase feldspar. Plagioclase is characterized by the presence of lamellar twinning, seen well in the large black crystal in the image. Smaller more rounded crystals with dull greenish-yellow colours are olivine phenocrysts which have been altered and replaced by secondary minerals which mask the normal bright birefringence colours of that mineral. The crystals are 'floating' within a groundmass which is dominated by fine-grained randomly oriented laths of plagioclase. Olivine is a common rock-forming mineral of basic and ultrabasic rocks. It is however very prone to alteration and is frequently totally replaced. The most common replacement minerals are fibrous antigorite and/or green chrysotile. The alteration generally occurs first along fractures in the olivine, and then spreads through the body of the crystal. Less common alteration minerals of olivine are bowlingite and iddingsite. Some of the best-known alterations of olivine have been studied in Scotland, namely pseudomorphs after olivine which consist of about 66% penninitic chlorite, 32% hematite and 2% quartz. |
| Date taken: | Wed Jan 01 00:00:00 GMT 2003 |
| Photographer: | Hyslop, E.K. |
| Copyright statement: | NERC |
| Orientation: | Landscape |
| Size: | 359.64 KB; 1000 x 682 pixels; 85 x 58 mm (print at 300 DPI); 265 x 180 mm (screen at 96 DPI); |
| Average Rating: | Not yet rated |
| Categories: | Unsorted Images |
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