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Dark line in Brookite
  
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tgleckler




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PostPosted: Mar 28, 2020 18:09    Post subject: Dark line in Brookite  

Does anyone know what the dark line is that runs up through the middle of a lot of brookite crystals? I've attached an example picture.


260px-Brookite-gem7-07a.jpg
 Mineral: Brookite
 Locality:
Pakistan
 Description:
 Viewed:  5131 Time(s)

260px-Brookite-gem7-07a.jpg


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Pete Richards
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PostPosted: Mar 29, 2020 14:13    Post subject: Re: Dark line in Brookite  

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.


Brookite 2b.JPG
 Mineral: Brookite
 Locality:
Tête Noire, Le Trient Valley, Wallis (Valais), Switzerland
 Dimensions: 3 mm crystal
 Description:
 Viewed:  5016 Time(s)

Brookite 2b.JPG



Brookite 6.jpg
 Mineral: Brookite
 Locality:
Dorénaz, Wallis (Valais), Switzerland
 Dimensions: 2 mm crystal
 Description:
 Viewed:  5013 Time(s)

Brookite 6.jpg



Quartz 6c.jpg
 Mineral: Quartz
 Locality:
Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada
 Dimensions: 2 mm crystal
 Description:
Sector zonation as a phantom under one rhombohedral face
 Viewed:  5016 Time(s)

Quartz 6c.jpg



Epidote-Clinozoisite.jpg
 Mineral: Clinozoisite-Epidote Series
 Locality:
Lowell, Orleans County, Vermont, USA
 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!
 Viewed:  5017 Time(s)

Epidote-Clinozoisite.jpg



Apatite 7b.jpg
 Mineral: Fluorapatite
 Locality:
Huron River, Ohio, USA
 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.
 Viewed:  5015 Time(s)

Apatite 7b.jpg



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tgleckler




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PostPosted: Mar 30, 2020 15:38    Post subject: Re: Dark line in Brookite  

Preferential impurity absorption. Makes great sense. Thanks, Pete. Loved the additional photos!
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