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Quartz and family
  
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arturo shaw




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PostPosted: Jan 21, 2010 10:48    Post subject: Quartz and family  

Hi all,

Dealing with quartz "siblings" I have prepared this scheme in order to clarify my ideas and help new people to fix some points. I would appreciate any comments on the correctness and completeness of it.

I have already published it in the "Spanish side" where I've been asked to add Eisenkiesel as variety of quartz and the varieties of opal as well.

In blue the names of varieties.

Thanks in advance

Arturo Shaw



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Elise




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PostPosted: Jan 21, 2010 12:06    Post subject: Re: Quartz and family  

Dear Arturo and all,

I would be interested in how "rose" is qualified in the upper part of your diagram. My background in mineralogy is not as strong as it should be, but I have been sporadically discussing this over the last year or so with Amir (the Quartz Page, of which I am a great fan), Dr. George Rossman and my very own "mineralogist in residence," Dr. William Bassett (aka, that's my co-worker). Dr. Rossman makes a distinction between "rose" and "pink" quartz, the former being massive and the latter as single crystal, each colored by a different mechanism. My interest is in how asterism in a cut cabochon of massive quartz can be so sharp, in particular with one astounding example which I showed to Dr. Rossman last year. His and other's opinion is that there are areas of crystallinity which can be relatively large which account for the strong and unbroken star effect. This can be seen in this image www(dot)nordskip(dot)com(slash)rose(dot)html with a link lower down to a picture which Jeff Scovil did at my request showing the two types of quartz. The explanation as far as I have taken it, is also on that page.

The idea of "massive" in such a crystal clear (that is a pun) material is still illusive to me and though I understand asterism, I am looking for a clearer understanding of "massive." If I scheduled some time on a Laue real-time, would it make this any clearer, or would that be a wasted effort ; I would use a lesser example as I don't know if x ray would affect the color of the nanofibers. My only experience with that instrument is with finding the cubic faces of diamond; the confusion of dots was hard to understand at first but then became clear. In a massive material, might it be impossible to discern what is happening? I would like to find some way to actually visualize what is going on and be able to explain that to others who ask me.

Thank you for sharing your diagram; I look forward to comments on it.

Elise

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PostPosted: Jan 21, 2010 12:33    Post subject: Re: Quartz and family  

I believe in this case the distinction is whether the material shows crystal faces or not. Massive rose quartz shows no crystal faces, and in outcrop may form large masses. The large masses very likely contain many individual grains of quartz, each of which is a single crystal, but they show no crystal form because they've grown together to form the mass. Some of the individual crystals could very well be large, certainly large enough to cut a stone from.

Pink quartz (to adopt Dr. Rossman's terminology) that I am familiar with grows in voids in pegmatites associated with cleavelandite albite, and forms reasonably well-formed small crystals. Several localities in Maine have produced it, as has at least one locality in Brazil, but generally it's not very common.

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PostPosted: Jan 21, 2010 13:13    Post subject: Re: Quartz and family  

Arturo,
A nice effort, to try to compile such a diagram; I'm sure you will receive many comments from the Forum about it. And, because the definitions of some of these variety terms as commonly used can be a bit "fuzzy", I suspect that you'll never be able to produce one diagram that quite satisfies everyone. I'll just offer a few of my own comments:

I think many would advise that a primary division should first be made between quartz and the other SiO2 minerals--tridymite, cristobalite, coesite, stishovite--rather than placing this further down in your "tree".

Here in the U.S., I would say, few people distinguish morion from smoky quartz, though we know what the term is taken to mean (very dark quartz).

Chrysoprase is missing from your list of chalcedony varieties? And of course, in English, we properly say cryptocrystalline, rather than microcrystalline, though I guess the two would mean the same thing.

Does your diagram leave room for a place where microcrystalline quartz is just chalcedony, but not banded or patterned so as to be called agate?

I think that the exact definitions (and placement in your diagram) of "onyx" and "sardonyx" are always rather confusing, and I won't attempt to say any more about them!

I think heliotrope (= bloodstone) belongs with the "cherts", not with the chalcedonies.

Likewise to above, how about a place for chert, that is not "jasper" or "flint"?

To most of us, the terms "lidite" and "basanite"--well, without trying to look them up and research the meanings, they are ambiguous or unfamiliar terms. "Basanite" has been used to refer to a large variety of different things, including (to petrologists) a type of basaltic igneous rock that it is not silica at all.

My two cents about rose/pink quartz, I have always felt that this is a very nit-picking distinction, not that I can deny the work that says the mechanisms of color are different in the two. (We don't have two different names for emerald that is colored by Cr versus V--they are still both "emerald", though of course, there's not some obvious physical characterisctic that allows one to distinguish them, without chemical analysis.) I have also often wondered, as an aside, in the massive, coarsely crystalline, traditional "rose quartz" that occurs in pegmatites, how large the grain size really is--when one holds a fist-sized or football-sized chunk of rose quartz in one's hand, is it likely to be a broken chunk of one single crystal, or composed of multiple (still large) crystal grains, intergrown? I've never been sure if there's an easy way one could tell this.

Those are enough comments for one message! I'm sure others will have more. Good luck with this,

Pete
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Amir Akhavan




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PostPosted: Jan 21, 2010 19:50    Post subject: Re: Quartz and family  

I've been thinking of sorting things myself, and I think that I got some consistent system, but it is not really in line with common nomenclature. One has to take into account the practical benefit of such a scheme, too.
I still don't sort things on my website this way, because 1. I'm not done with it, and 2. people would not find what they are looking for.

I have fairly strong opinions on some matters (like "rose and pink quartz are very very very different" and "a burnt amethyst that turned yellow is a ferruginous quartz, not a citrine" and "a citrine is not a single variety"), but I usually close with a "you are free to call it whatever you want, don't worry, you won't upset me, as long as I know what you are talking about".

The names serve a purpose and as long as the wording is not confusing I don't care. I get upset if you say that "rose quartz" and "pink quartz" are synonyms, because that's confusing (the statement contains an error).

The idea behind such a "tree" is that you are referring to what something *is* and not only what it appears to be (or looks like).
Unfortunately quartz variety names have a long history and do not necessarily reflect what something *is*.
So if you want to find a consistent system for varieties, you have to give up old schemes and traditions and prepare to fight with those who want to stick with their names.

It's late here, so I will write something about it the next days.
In fact I was thinking of writing a little mindat article on this and a few related matters (like why it's o.k. to call a V-beryl an emerald).

My main point is this:
"Mineral species" are intended to be mutually exclusive terms.
Ideally, a mineral can only be one thing or another.

(((
I say ideally, because (excuse me for my strong words) the IMA is messing everything up by introducing mineral species like oldstuffite-Fe, oldstuffite-Mg, oldstuffite-Mn, Oldstuffite-Ca, etc. and this undermines the idea that a mineral can be treated as a single phase in a system: you end up with crystals that lack phase boundaries and are still composed of different minerals.
)))

Anyway, so mineral definitions are mutually exclusive.
But variety definitions are *not*.

You can come up with a system in which varieties are sorted in some way and draw a nice tree, but you cannot define varieties in the same reference frame (using the same properties and parameters) as minerals.

In that sense, a variety is not a subspecies of a mineral.
And a mineral is not the common denominator of the properties specific to the varieties (that's because some properties that are defining for a mineral, like the symmetry, are missing).

So an amethyst can be a smoky quartz, even though you can clearly define what comprises an amethyst or a smoky quartz.

And one also has to be careful not to lump rocks and minerals together.
If I was really pedantic about it, I would not call jasper, chert, agate, etc a variety of quartz, because in some sense these are rocks.
Instead, I'd have things like "length-fast chalcedony" and "quartzine" in that tree.

I'm going to plot my -current- understanding of the silica "system" in a few days.

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PostPosted: Jan 22, 2010 11:45    Post subject: Re: Quartz and family  

It is another thread (Rose Quartz - Single crystal or not?) related with this thread -> https://www.mineral-forum.com/message-board/viewtopic.php?t=903
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PostPosted: Jan 25, 2010 15:38    Post subject: Re: Quartz and family  

Come on Amir!!, I cannot sleep waiting for your "silica system", seriously. :-)

You don't realize how much I agree with your points.

Pete, I have comments for you also (heliotrope is a chalcedonie for Mindat) but Amir keeps the subject in he air... :-)

Thanks

Arturo
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Amir Akhavan




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PostPosted: Jan 25, 2010 17:04    Post subject: Re: Quartz and family  

O.k. I start with the easy one.
Just silicon dioxide, no varieties yet.
Not everybody will completely agree on this,
but nobody will start a fight over it.

"Opal" does not exist.
Opal-C and opal-CT is absent.
I was agonizing over whether I should include opal-AG or not.
In dubio pro reo, so to say.
Depending on the amount of my innocence of the topic
it might be kicked out again later.

There are 3 groups of closely related polymorphs:
1. moganite, low-quartz, high-quartz
2. low-tridymite, high-tridymite
3. low-cristobalite, high-cristobalite


Varieties are more difficult, because there is no system.
It will be a real mess of overlapping clouds.
That's why I'm not done yet.

It will still be interesting, because I will devide quartz into
something like 4 to 6 basic structural types, and try to
diverge textural varieties from rocks.
Still, some things won't fit into that scheme, like opal-CT,
which is not a textural variety of a mineral (thus also not a mineral),
not a mineraloid and (perhaps) not a rock.
Oh, I know .... opal-CT should be discredited ;-)
(the IMA has not "credited" it yet, of course)



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PostPosted: Jan 25, 2010 18:36    Post subject: Re: Quartz and family  

Why is stishovite an oxide and the other forms of silica are silicates? Silicates of which cation? I think they should all be classed as oxides.
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PostPosted: Jan 25, 2010 19:26    Post subject: Re: Quartz and family  

Ahh, first response ;-)

I never understood why quartz is called a silicate myself :-)
But I was referring to the structure, not the composition.
American mineralogical literature lists quartz as a tectosilicate/network silicate.
A feldspar with Al and Alkaline ions replaced by silicon, so to say ;-)
In German literature they would all be oxides.

I did it the Anglo-American way because I wanted to stress what all silicates, silica and the corresponding silicic acids have in common: a tetrahedral SiO4 group.

Stishovite is different and structurally more related to rutile, if I remember that correctly. It has no structural resemblance with silicates/silica/silicic acid, because each silicon is surrounded by 6 oxygen atoms (coordination number 6)
The character of the Si-O bond changes, too.
But if you asked me to give an impromptu talk on stishovite structure, I'd probably hide under the desk.

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PostPosted: Jan 31, 2010 19:22    Post subject: Re: Quartz and family  

O.k. a first attempt, based on what I know and what I do not know.
That was difficult.
Still without varieties (varieties will be much easier).

I'm surprised that it is possible to come up with something hierarchical.
Some things look very confusing, like "how does a gwindel contain a twisted quartz?" (in fact, each gwindel does contain a twisted quartz).

If you don't like it, don't worry, I don't like it either ;-)

Explanations will follow, it's too late now.
Cheers,
Amir



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PostPosted: Feb 01, 2010 07:37    Post subject: Re: Quartz and family  

Which group to put quartz in can provoke a bit of disagreement. The people who like oxides would say it is SiO2. The silicate people would say Si(SiO4). One mineral + two slots= controversy.
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PostPosted: Feb 01, 2010 19:45    Post subject: Re: Quartz and family  

Hi,

some sort of an extended legend for the Chart of Quartz.
I already would like to fix some things, but it's not so easy to move all the arrows in this figure, so I save this for the next version.

It is crazy to put this in a forum. Not that I'm afraid of discussing this, but it's more a topic you'd discuss in a well structured article.



Rationale
========

The chart is based on the structure of individual crystals and crystal aggregates.

It is not an abstract classification system in which members are sorted by predefined properties, like mineral classifications.

It is not an evolutionary tree, a quartzite does not "evolve" from megaquartz.

There are 3 "rules":

(1) The system is hierarchical: each term inherits all properties of one or more terms to the left of it. So one *cannot* say that one term describes a subset of another.

(2) The more fundamental properties of the crystals are on the left side.
These properties reflect the microscopic and submicroscopic structure of the crystals.

(3) Each column describes just one property.
Sometimes a field in a column is skipped because there is nothing that fits the description (there is no "Macrocrystalline Textural Variety", for example).



Nanostructures I & II
----------------------------

Rule (3) is the reason for having 2 "Nanostructure" columns.

I could have done it like this:
- Ideal Quartz that is Optically Uniaxial
- Quartz that is Optically Biaxial
- Twisted Quartz that is Optically Uniaxial

Instead, I considered the optical properties more fundamental, and made two columns. Maybe I change that in the next version.

But one can memorize this:
There are probably 3 fundamental species of quartz:
1. Ideal Quartz - this one behaves as it is supposed to.
2. Twisted Quartz - *Smoothly* twists around the a axis. This is no gwindel yet.
3. Biaxial Quartz - Behaves badly and shows two optical axes.

If a twisted quartz is inherently uniaxial, then these 3 species are mutually exclusive, just as mineral species.

One would need to do experiments to support such a statement.
If, for example, twisted quartz is composed of stacks of macroscopic crystallites, then there would be just 2 species.
It should also be noted that biaxial quartz is likely never pure quartz and contains considerable amounts of trace elements.

(There are no experiments, so if you think this is all nonsense, that's fine.)


Crystallite Aggregates
------------------------------

The structural composition of individual crystals, not counting twinning.

- Normal quartz is an ideal crystal with textbook properties.
This is what a quartz manufacturer tries to get.

- Macromosaic quartz is composed of wedge-shaped macroscopic crystallites.

- A gwindel is composed of a central core of twisted quartz surrounded by macroscopic crystallites of normal quartz.

- Lamellar quartz is composed of onion-like layers of biaxial and uniaxial quartz (that has nothing to do with twinning)

- Amethyst has been occasionally reported to be biaxial. I do not know if this is related to lamellar quartz.


Crystal Aggregates
--------------------------

The mode of intergrowth of crystals

Grainy
---------

Non-fibrous, but not necessarily random intergrowth

- Megaquartz, one or more visible crystals

- Microquartz, randomly intergrown microscopically small quartz crystals


Fibrous
----------

Ordered intergrowth leading to a fibrous appearence

- Quartzine (Length-Slow Chalcedony) - Microscopic crystals stacked along the c-axis

- Pseudochalcedony - Microscopic crystals stacked obliquely, along {20-21}. Not mentioned in all nomenclatures

- Length-Fast Chalcedony - Microscopic crystals stacked along the a-axis, often twisted



Textural Varieties
------------------------

Textural varieties may contain considerable amounts of other minerals (agates or flint may contain moganite, for example).
However, I distinguish them from rocks.
In textural varieties the mode of growth of the aggregates is determined by inherent properties of the minerals, and reflected in their structure and shape. The formation of banding in agate is driven by something inherent to the agate, not by environmental factors. Likewise, due to the way they grow, chalcedony is typically botryoidal and flint nodules often zoned.
Because growth interference between crystals are necessarily localized, textural varieties are always relatively small and occur as nodules, geodes, vein fillings etc.


Rocks
--------

The structure is determined by external factors, not by the growth of individual grains. Hence chert and radiolarite, with a structure that reflects sedimentation, are distinguished from flint.
Jasper is a difficult case, the grains seem to be randomly interlocked, but I have never seen images of thin sections. Silica Sinter is also on the border line to textural varieties.



"Amir, this is really boring. Where are the varieties?"
"Err....boring... varieties... yes..."

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PostPosted: Feb 01, 2010 20:54    Post subject: Re: Quartz and family  

Amir Akhavan wrote:
...It is crazy to put this in a forum. Not that I'm afraid of discussing this, but it's more a topic you'd discuss in a well structured article.

Is not crazy Amir, it is lovely! ;-)

Thanks a lot to make the effort to publish your fine work here. I'm sure that a lot of silent fans are clapping...
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PostPosted: Feb 02, 2010 05:40    Post subject: Re: Quartz and family  

And not only the silent fans :-)

It takes time to deal with all that information and everytime I have prepared some questions/ideas Amir is there giving us another piece of information!!

I am posting a link in the Spanish side to this one because I have already prepared a translation of the first structure to raise reactions there but I think it is of no use now.

There will be an answer (as soon as I can manage all that :-) )

Thanks

Arturo
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PostPosted: Feb 06, 2010 17:41    Post subject: Re: Quartz and family  

Just a few corrections and simplifications.

(meanwhile edited, replacing Version 2 with Version 3)
The number of question marks is increasing.
Still only 9 out of 50 connections are doubtful ;-)



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