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Peter Seroka
Joined: 13 Oct 2012
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Location: L'Escala
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 Posted: May 26, 2016 18:08 Post subject: Rare quartz crystals |
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Hi
could anyone guess or tell me, what kind of crystal (and/or habit) those two xtls could be ?
Thank you
Peter
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Matt_Zukowski
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Joined: 10 Apr 2009
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Location: Alaska
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| Posted: May 26, 2016 19:16 Post subject: Re: Rare quartz crystals |
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| The second picture suggests Tessin habit to me.
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Pete Richards
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| Posted: May 27, 2016 14:48 Post subject: Re: Rare quartz crystals |
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I'm not sure there is any name for the habit of your first crystal. The prism faces are alternately wide and narrow, and some of them taper inwards toward the top. The termination consists of faces of a single rhombohedron rather than the usual alternating faces two rhombohedra. This combination of traits gives the crystal its interesting and unusual habit. It would be good to determine what specific rhombohedron is present - it may not be either of the common ones - but this cannot be determined from the perspective of this image. Can you provide an estimate of the angle that the faces on the termination make with the c-axis?
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Peter Seroka
Joined: 13 Oct 2012
Posts: 17
Location: L'Escala
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| Posted: May 27, 2016 17:01 Post subject: Re: Rare quartz crystals |
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Hi Pete
thanks for your answer. Sorry, I really do not know, how to make an estimate of the angle that the faces on the termination make with the c-axis.
I am completely unexperienced in measuring angles, nor do I have a goniometer.
If it helps, I enclose another view of the same crystal. Its size is 5,5 cm.
And, as an additional question: Thats another very similar xtl from a different locality; I guess, it's the same unknown "habit" (?)
Any idea ?
BTW: Would you agree, that the second specimen of my first post could be a Tessin-Habit crystal ? (I am just trying to get a second opinion)
Peter
| Mineral: | Quartz |
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14146 Time(s) |
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Ivan Blanco (PDM)
Joined: 14 Apr 2012
Posts: 33
Location: Antwerp, Belgium
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| Posted: May 29, 2016 03:29 Post subject: Re: Rare quartz crystals |
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Hi, the second picture looks like a Muzo-habit quartz to me.
I would say that the first and last pictures (T_Quarz_1.jpg and T_quarz_3.jpg) are trigonal-habit quartzs.
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Pete Richards
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Joined: 29 Dec 2008
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| Posted: May 30, 2016 09:19 Post subject: Re: Rare quartz crystals |
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| Peter Seroka wrote: | ...I really do not know how to make an estimate of the angle that the faces on the termination make with the c-axis.
...
BTW: Would you agree, that the second specimen of my first post could be a Tessin-Habit crystal ? |
In this case, the angle I asked about can be estimated from a photo not too different from your second one, but with one of the rhombohedral faces perpendicular to the "page", i.e. parallel to the viewing direction, so that it shows up just as a line, the face itself neither visible nor hidden by being slightly on the back side of the crystal.
The discussion about habit illustrates the difficulty, and perhaps the futility, of giving names to habits. According to Rudolf Rykart in Quartz-Monographie (in German), the Muzo habit is characterized by three prism faces alternating with three steep rhombohedral faces (or, every other "prism" face tilts inwards) with the result that the true prism faces get narrower and narrower, and ultimately disappear at the top - the cross section just below the termination is a triangle.
The Tessin habit is characterized by the prism faces being replaced by steep rhombohedral faces, all of which tilt inwards toward the top. The cross section remains more or less hexagonal, but gets smaller as the top is approached.
In reality, these two habits grade into each other. Nor are they unique to specific places - both can be found in the same locality and even on the same specimen. On your specimen, if I were required to make a decision, I would say that the top end of the major crystal is of Muzo habit but the bottom end of the same crystal is of Tessin habit!
Sometimes a picture is much more informative than many words. This is especially true when the words represent idealizations which may not be matched by real-life crystals.
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Harjo
Joined: 04 Nov 2009
Posts: 32
Location: Vessem
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| Posted: Sep 19, 2016 04:28 Post subject: Re: Rare quartz crystals |
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Ivan is right. The first and last are trigonal habits. They lack rhombohedral z-faces.
The second photo is a group with Muzo habit and Tessin (or alsa called Penninic habit or Rauriser habit) habit quartzes. A Muzo habit crystal shows a six sided prism at the bottom that tapers to three sides at the top. If I'm right (?) a Muzo crystal can be distinguished from a Tessin crystal in that in a Muzo crystals the m prism faces (that terminate at the z-rhombohedron) are replaced by alternating z- and m-faces whereas in a Tessin crystal the m-faces alternate with steep rhombohedral faces.
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