Bob Carnein
Joined: 22 Aug 2013
Posts: 353
Location: Florissant, CO
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 Posted: May 05, 2025 12:41 Post subject: Quartz Twins |
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Considering the great recent "Rocks & Minerals" articles by Carl Francis and Pete Richards on quartz twins, I will post a few specimens from my collection.
| Mineral: | Quartz |
| Locality: | | Crystal Peak area, Teller County, Colorado, USA |  |
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| Dimensions: | 9.6x3x2.5 cm |
| Description: |
| Smoky quartz, Left-handed Dauphine twin |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Switzerland | |
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| Dimensions: | 7.2x3.5x2.8 |
| Description: |
| Right--handed Dauphine twin from an unknown Swiss locality. This is a bit odd, in that the crystal exhibits only 2 trapezohedral x faces--one at the "upper right" corner of the front-facing prism face and the other in the "middle" of that prism face, at a suture line. |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Switzerland | |
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| Dimensions: | 7.2x3.5x2.8 |
| Description: |
| Closer view of x faces on a quartz crystal from an unknown Swiss locality |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Gamsberg area, Windhoek District, Khomas Region, Namibia | |
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| Dimensions: | 4.6x2.1x1.3 cm |
| Description: |
| Left-handed Dauphine twin exhibiting both repeating x faces and repeating s faces. Striations on the s faces all point downward toward adjacent x faces (see close-ups below). |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Gamsberg area, Windhoek District, Khomas Region, Namibia | |
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| Dimensions: | 4.6x2.1x1.3 cm |
| Description: |
| Close-up showing s face exhibiting striations pointing downward toward adjacent x face. |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Gamsberg area, Windhoek District, Khomas Region, Namibia | |
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| Dimensions: | 4.6x2.1x1.3 cm |
| Description: |
| Close-up of another s face with striations pointing downward toward adjacent x face. |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Mount Malosa, Zomba District, Malawi | |
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| Dimensions: | 4.3x1.6x1 cm |
| Description: |
| Smoky quartz crystal with unknown inclusions. This crystal exhibits Brazil twinning, as shown by symmetric repitition of x faces on both sides of front prism face. See close-up below. |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Mount Malosa, Zomba District, Malawi | |
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| Dimensions: | 4.3x1.6x1 cm |
| Description: |
| Close-up of previous photo, showing symmetrical repitition of x faces. It appears that the x faces are each actually 2 faces; perhaps a more adept crystallographer can tell me what the "extra" faces are? |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Hunter Canyon, Huachuca Mountains, Hartford District (Huachuca Mountains District), Cochise County, Arizona, USA | |
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| Dimensions: | 3.9x2.3x1.1 cm. |
| Description: |
| According to the person I bought this from, this odd little crystal is from Hunter Canyon, Huachuca Mts., Arizona. Although I suspect it's a contact Dauphine twin, I am not certain and would appreciate comments. Close-up shows very odd distribution of faces around a "notch" at the contact between the two twin halves. |
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| Viewed: |
110 Time(s) |
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| Mineral: | Quartz |
| Locality: | | Hunter Canyon, Huachuca Mountains, Hartford District (Huachuca Mountains District), Cochise County, Arizona, USA | |
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| Dimensions: | 3.9x2.3x1.1 cm. |
| Description: |
| Close-up of previous photo, showing "notch" separating two twin halves and odd distritution of adjoining faces. |
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| Viewed: |
111 Time(s) |
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Bob Morgan
Joined: 18 Jan 2018
Posts: 250
Location: Savannah, Georgia
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| Posted: May 05, 2025 22:27 Post subject: Re: Quartz Twins |
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'Some really nice crystals! - particularly the first from Colorado and the Namibian one. It's nice to see you have an interest in crystal morphologies.
Your crystal with two 'x' faces on the same prism face that are oriented the same way does not show Dauphene twinning. I'm guessing the suture is not a twin boundary but the beginning of a block split. It's a 'cool' feature.
I have a few crystals from Malawi that have the same appearance of Brazil Law twinning as yours. The shapes are that of 'x' faces on both sides of the prism face, but try as I may to make them show Brazil twinning they are just too different. One will have a smooth face with sharp features. The other always has surface pitting. I think this is what Rykart would consider 'x' and '-x' faces.
Your Arizona crystal is really interesting. It could be a vertical twin which would be really odd - like a 180 degree rotation around a c-axis, thus a Dauphene twin. Or it could be just an odd notch at the crystal tip. Is there any indication of twin boundaries extending down from the notch corners? If there are rounded triangular face features on the rhombohedral faces, it may be possible to read whether the faces are 'r' or 'z' which could be a way of discerning whether there is Dauphene twinning or not.
Carl and Pete's article in Rocks and MInerals really covers the subject well and will be a reliable guide for all. May more find the pleasure you've had looking again at your well chosen specimens.
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