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EducationQuartz variety

22nd Jul 2017 05:52 UTCCostas Constantinides

Could these quartz specimens be considered as Tessin habit..They come from Mount Isa /Cloncurry-Qld-Australia and come from the well known Crystal Mt..I also notice that both crystals have a small pyrite inclusion where the steps start.

22nd Jul 2017 16:16 UTCAmir C. Akhavan Expert

The first one certainly not. The tapering is caused by alternating prism and (normal) rhombohedral faces.

The second crystal looks a bit as if the shape developed because the prism broke off obliquely and later healed. So the tapering does only occur on one side, and I wouldn't call it Tessin either.

A Tessin/Rauris/Penninic/Binntal habit is caused by steep rhombohedral faces dominating the shape. These faces may be striated like ordinary prism faces. At the "type locality" of this variety, the Penninic unit in Switzerland, the crystals are all macromosaic quartz, so in a strict sense, Tessin habit means "macromosaic quartz tapered by steep rhombohedral faces", whereas "steep rhombohedral habit" would be a more general term used for crystals tapered by steep rhombohedral faces including those that are not macromosaic. However, the term Tessin habit is often used very loosely by collectors and dealers.

At places where Tessin habit crystals in the strict sense occur, the habit is seen more or less clearly on all crystals, the habit does not occur as a single "oddity" between normal habits.

22nd Jul 2017 19:04 UTCLarry Maltby Expert

Amir,

I thought that you may be interested in this study that I did in 2008. I found a loose piece of basalt with calcite filling the vesicles. It was probably a glacial deposit from the Portage Lake Volcanics. I leached away the calcite with acid and was surprised that crystals of this shape survived the acid bath. After a conversation with George Robinson he analyzed the specimen and found it to be quartz. I then thought that it may have been a pseudomorph so I did the following comparison. All of this was before I read your information on the Tessin habit of quartz crystals.

23rd Jul 2017 06:59 UTCCostas Constantinides

Thank you Amir and Larry. So what you are saying is (maybe ) ,all the quartz crystals in a pocket will be Tessin(macromosaic) not just one ,and all the faces will be tapered with striations and stepped.

23rd Jul 2017 11:01 UTCAmir C. Akhavan Expert

At a Tessin habit locality, the habit can be seen on all crystals more or less clearly.

So the crystals tend to be tapered on all sides.

They may show striations on steep rhombohedral faces.

They may be stepped, but not in the way your first crystal is, that one has steps made of normal rhombohedral faces and prism faces, which does not count as "Tessin".

About the crystals with tapered habit in the volcanic rock cavities: beta quartz may show a tapered habit if growing freely into a cavity. Check if the cross sections of the crystals are roughly hexagonal and if the matrix matches a high-temperature growth.

Interestingly, because of the hexagonal symmetry of beta quartz, the tapered faces are hexagonal dipyramids during growth which would transform into positive and negative rhombohedra at the transition to alpha quartz with trigonal symmetry - all without any noticable change in the appearance of the crystals.

My guess would however be that these were formed as alpha quartz - just a gut feeling.

23rd Jul 2017 14:37 UTCRolf Luetcke Expert

Amir,

This is one I found on a specimen I purchased of fluorite but it had these quartz crystals on it also. I had not seen any like this. Any idea about this habit?

23rd Jul 2017 14:47 UTCDoug Schonewald

Those look very similar to the ones I found on a specimen of chalcedony Rolf. I was told they were quartz but I haven't had them analyzed. Weird growth habit for sure. Hopefully Amir can shed some light on their formation process. It is interesting that the striations follow the axis rather than across it. They pretty much look identical to yours except the color.

23rd Jul 2017 15:13 UTCAmir C. Akhavan Expert

In both cases that's split growth (not a habit) like in sprouting and artichoke quartz, but I can't give you a specific reason why it formed.

23rd Jul 2017 17:04 UTCRolf Luetcke Expert

Amir,

Thank you for the "split growth" for the name, will put it on the piece.

Douglas,

Sure looks like the one I posted. Mine was not listed on the specimen but I did some testing and it turned out to be quartz.

Thanks

23rd Jul 2017 20:08 UTCUwe Kolitsch Manager

The quartz photos shown by Rolf and Douglas are typically indicative of quartz that was formed at relatively low temperature. Such quartz crystals occur in various rock types (another example: the Aris phonolite).

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