Pete Richards
Site Admin
Joined: 29 Dec 2008
Posts: 683
Location: Northeast Ohio
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 Posted: Mar 29, 2020 14:13 Post subject: Re: Dark line in Brookite |
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Such dark zones seem to be common in brookite of this morphology. This is what is called sector zonation. The part of the crystal that was deposited on the very tiny face or faces at the very top of the crystal incorporated impurities of some sort ( possibly dissolved iron) more than the other faces, causing the color difference. Such sector zones are commonly seen in selenite ("hourglass figures"). In quartz, one rhombohedron can absorb ions differently than the other one, which can lead to smoky or amethyst zoned coloration.A couple of additional examples are attached.
| Mineral: | Brookite |
| Locality: | | Tête Noire, Le Trient Valley, Wallis (Valais), Switzerland |  |
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| Dimensions: | 3 mm crystal |
| Description: |
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| Viewed: |
4971 Time(s) |
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| Mineral: | Brookite |
| Locality: | | Dorénaz, Wallis (Valais), Switzerland | |
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| Dimensions: | 2 mm crystal |
| Description: |
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| Viewed: |
4970 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada | |
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| Dimensions: | 2 mm crystal |
| Description: |
| Sector zonation as a phantom under one rhombohedral face |
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| Viewed: |
4972 Time(s) |
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| Mineral: | Clinozoisite-Epidote Series |
| Locality: | | Lowell, Orleans County, Vermont, USA | |
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| Dimensions: | crystal is 13 mm tall |
| Description: |
| Composition is very close to the boundary between clinozoisite and epidote. The darker sector might be epidote and the lighter sectors clinozoisite - two minerals in one crystal! |
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| Viewed: |
4973 Time(s) |
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| Mineral: | Fluorapatite |
| Locality: | | Huron River, Ohio, USA | |
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| Dimensions: | 1 mm crystal |
| Description: |
| Unusual sector zonation revealed by crossed polarizers and a gypsum plate. Different colors represent different optical orientations within the crystal, aligned with different growth sectors. (This is not supposed to happen!) One half of a platy hexagonal crystal is shown. |
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| Viewed: |
4971 Time(s) |
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Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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