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Green unknown from Colorado

Posted by James Christopher  
avatar Green unknown from Colorado
May 06, 2012 06:27AM
    

Today I was at a mine in this district, [www.mindat.org] possibly the Annamosa but not sure as there are many mines with only a few marked. I found this green crystal on one piece I picked up. It's about 4mm on a side, and looks almost black just looking at it. Under the microscope, it is a green color however. It appears frosted/etched, but looks vitreous on a tiny chip. If it is the same a another green area, it has a hardness of at least 6.5 - 7, seems to have cleavage in 3 planes, better in 2 and not as good in the other. One cleavage plane isn't at a right angle to the other two, the others are hard to tell, but they may be. Whatever it is, I don't think it's in the mineral list for the district. the host rock itself is mostly some unknown black mineral, along with quartz, chalcopyrite, possibly minor siderite. The photos are opposite sides of the crystal. Ideas?
avatar Re: Green unknown from Colorado
May 06, 2012 02:26PM
    
I believe this is the same mineral. I used different lighting and the color balance isn't right, but it is here to show the cleavage more than anything else, if it helps.
avatar Re: Green unknown from Colorado
May 06, 2012 04:46PM
    
Hi Jim,

You're constrained to a few choices by the observed hardness, color and cleavage. Of the possibilities, I think Andesine (a variety of Albite) may be a good guess. The last image shows a somewhat wavy cleavage surface that would fit well with Andesine. The hardness, color, cleavage directions and angles, the overall rectangular shape of the subject also seem to be a match for Andesine. Your second image seems to show vertical linear striae features that look like reentrants, to me indicate a twin plane that seems to repeat. Could be contact twinning like Albite twin law. This is my best guess, might be something else. Good luck with it Jim.

Ron
avatar Re: Green unknown from Colorado
May 06, 2012 05:44PM
    
Jim,
Based on its' prismatic nature, dark green color and iron-rich environment, I would guess diopside, possibly twinned. But I would also expect that you would have found more diopsides.
-Dean Allum
avatar Re: Green unknown from Colorado
May 06, 2012 05:54PM
    
Looking from above, it almost seems to have a hexagonal shape, but the twinning may be giving it that appearance. What I had been looking at that seemed the best match, to my limited mineral knowledge, were feldspar type minerals such as anorthite, oligoclase. So your best guess may very well be what it is. Looking at the gallery, I take it crystals are rare? Thanks for the suggestion!
avatar Re: Green unknown from Colorado
May 06, 2012 06:01PM
Ron,

I'm fairly new here and am still feeling my way into the ins and outs of nomenclature so, please, bear with me.

You describe Andesine as a variety of Albite. My present understanding is as follows:

- The Feldspar group divided into two series, Orthoclase and Plagioclase.
- The Plagioclase series has six members with the end-members being Anorthite and Albite. The classifications of Anorthite and Albite and also the four intermediary members of the series (Bytownite, Labradorite, Andesine and Oligoglase) are strictly (and arbitrarily) differentiated by the %age Ca/Na substitution present in the specimen. I.e. there are six varieties of Plagioclase, no one being a variety of any other.

I have been unable to trace the primary authority for the 'received wisdom' of this arbitrary division of the Plagioclase series (though it seems copied everywhere in respectable secondary sources). This arbitrary scheme of division ignores the natural sub-division points for the series, where the optic sign switches from positive to negative, back and forth. I therefore wonder if you are following some other method of subdividing the Plagioclase series. If so I would be very glad to learn of it and the primary authority for it.
Re: Green unknown from Colorado
May 06, 2012 09:44PM
    
The mineral in the first two photos is almost unmistakeably Sphalerite (var. Cleiophane). The modified tetrahedral (this is not prismatic!) xl shape showing twinning, the translucent green color that becomes obvious only when backlit, and the association with other species typical of hydrothermal veins, are diagnostic.

Lefteris.
avatar Re: Green unknown from Colorado
May 06, 2012 09:45PM
    
Lefteris is right.
avatar Re: Green unknown from Colorado
May 06, 2012 10:16PM
    
Hi Owen,

You've asked quite a few questions and since you've asked me to do so, I'll do my level best to address them all. Since it's a little off the topic I will try to limit my response.

> "The Feldspar group divided into two series, Orthoclase and Plagioclase."

Let's start here. The feldspar group is very large and the nomenclature is somewhat complex. They are best classified not only chemically but also structurally. The feldspar group is probably best seen as; alkali feldspars (K,Na)[AlSi3O8], plagioclase feldspars Na[AlSi3O8] - Ca[Al2Si2O8] and barium feldspars Ba[Al2Si2O8]. Orthoclase is generally considered to be an intermediately ordered K (potassium) rich feldspar. It's a mineral species and not a Series.

Structurally, you have to consider the temperature at which the feldspar crystallized as well as the time it took and even later thermal exposure. So you can have low temp, intermediate temp and high temp formation of feldspars, each possessing it's own structural characteristics. So generally, you can say that there are higher temp feldspars and lower temp feldspars as well intermediate. We like things to be simple and easily catagorized but this is the wrong group for simplicity.

Most of the feldspars in the "Group" could be plotted on ternary diagrams (individually as either structurally ordered or disordered diagrams) as products of the components: Anorthite, Albite and Orthoclase and not just Anorthite and Albite. Although for just "Plagioclase series" (rather should be called Albite-Anorthite series) these are the mole %s:

Albite (Na 100%, Ca 0%) AlSi3O8 90-100% Ab; 0-10% An
Oligoclase (Na 90%,Ca 10%) Al1-2Si3-2O8 70-90% Ab; 10-30% An
Andesine (Na 70%, Ca 30%) Al1-2Si3-2O8 50-70% Ab; 30-50% An
Labradorite (Na 30%, Ca 70%) Al1-2Si3-2O8 30-50% Ab; 70-50% An
Bytownite (Na 10%, Ca 90%) Al1-2Si3-2O8 10-30% Ab; 70-90% An
Anorthite (Na 0%, Ca 100%) Al2Si2O8 0-10% Ab; 90-100% An

> " ...there are six varieties of Plagioclase, no one being a variety of any other."

So Andesine as you can see, generally has a majority Albite component (50-70%). This is essentially why I would call it a variety of Albite, ie Albite (var: Andesine) and of course it is a "plagioclase" as it is in the Ab-An series as you rightly say but a not a "variety". This is true just as Bytownite which is 70-90% Anorthite component, is really a variety of Anorthite. ie Anorthite (var: Bytownite) and Labradorite which is 70-50% Anorthite component is really a variety of Anothite, ie Anorthite (var: Labradorite).

> "...I therefore wonder if you are following some other method of subdividing the Plagioclase series. If so I would be very glad to learn of it and the primary authority for it."

Here's a good reference for you concerning The Feldspar Group. It seems a broad and complex group but feldspars are very interesting and quite important, worth the effort to understand. All the best.

Ron

Click on this link, go to the top of the document and start on page 1.
[books.google.com]
avatar Re: Green unknown from Colorado
May 06, 2012 10:28PM
    
Hi Lefteris,

> "The mineral in the first two photos is almost unmistakeably Sphalerite (var. Cleiophane)."

I agree. This occured to me also but I thought Jim said it had a hardness over 6. I suppose Jim was refering to the last image which I think may be different material altogether.

Ron
Re: Green unknown from Colorado
May 06, 2012 10:43PM
Ronald John Gyllenhammer Wrote:
-------------------------------------------------------
> Hi Owen,
>
> You've asked quite a few questions and since
> you've asked me to do so, I'll do my level best to
> address them all. Since it's a little off the
> topic I will try to limit my response.
>
> > "The Feldspar group divided into two series,
> Orthoclase and Plagioclase."
>
> Let's start here. The feldspar group is very large
> and the nomenclature is somewhat complex. They are
> best classified not only chemically but also As we sa here calm down calm down ! that is something that left me after the second line, yet my inexperience in putting the things i have descovered and now know more about than any man on earth ie the ground beneath my own feet, into words, leaves me at a slight disadvantage here, ime too old to learn how to word my intelligence like some of you lads n lasses can, and to be honest i don't whant to ime interested in learning about geology not how to spell long words!!!
> structurally. The feldspar group is probably best
> seen as; alkali feldspars (K,Na)3O8], plagioclase
> feldspars Na3O8] - Ca2Si2O8] and barium feldspars
> Ba2Si2O8]. Orthoclase is generally considered to
> be an intermediately ordered K (potassium) rich
> feldspar. It's a mineral species and not a
> Series.
>
> Structurally, you have to consider the temperature
> at which the feldspar crystallized as well as the
> time it took and even later thermal exposure. So
> you can have low temp, intermediate temp and high
> temp formation of feldspars, each possessing it's
> own structural characteristics. So generally, you
> can say that there are higher temp feldspars and
> lower temp feldspars as well intermediate. We like
> things to be simple and easily catagorized but
> this is the wrong group for simplicity.
>
> Most of the feldspars in the "Group" could be
> plotted on ternary diagrams (individually as
> either structurally ordered or disordered
> diagrams) as products of the components:
> Anorthite, Albite and Orthoclase and not just
> Anorthite and Albite. Although for just
> "Plagioclase series" (rather should be called
> Albite-Anorthite series) these are the mole %s:
>
> Albite (Na 100%, Ca 0%) AlSi3O8 90-100% Ab; 0-10%
> An
> Oligoclase (Na 90%,Ca 10%) Al1-2Si3-2O8 70-90% Ab;
> 10-30% An
> Andesine (Na 70%, Ca 30%) Al1-2Si3-2O8 50-70% Ab;
> 30-50% An
> Labradorite (Na 30%, Ca 70%) Al1-2Si3-2O8 30-50%
> Ab; 70-50% An
> Bytownite (Na 10%, Ca 90%) Al1-2Si3-2O8 10-30% Ab;
> 70-90% An
> Anorthite (Na 0%, Ca 100%) Al2Si2O8 0-10% Ab;
> 90-100% An
>
> > " ...there are six varieties of Plagioclase, no
> one being a variety of any other."
>
> So Andesine as you can see, generally has a
> majority Albite component (50-70%). This is
> essentially why I would call it a variety of
> Albite, ie Albite (var: Andesine) and of course it
> is a "plagioclase" as it is in the Ab-An series as
> you rightly say but a not a "variety". This is
> true just as Bytownite which is 70-90% Anorthite
> component, is really a variety of Anorthite. ie
> Anorthite (var: Bytownite) and Labradorite which
> is 70-50% Anorthite component is really a variety
> of Anothite, ie Anorthite (var: Labradorite).
>
> > "...I therefore wonder if you are following some
> other method of subdividing the Plagioclase
> series. If so I would be very glad to learn of it
> and the primary authority for it."
>
> Here's a good reference for you concerning The
> Feldspar Group. It seems a broad and complex group
> but feldspars are very interesting and quite
> important, worth the effort to understand. All the
> best.
>
> Ron
>
> Click on this link, go to the top of the document
> and start on page 1.
> [books.google.com]
> A111&lpg=PA111&dq=ribbe+nomenclature&source=bl&ots
> =eTSz-u84Ma&sig=HXggihfVwuBvBSvuZVXaees8vRc&hl=en&
> sa=X&ei=iPOmT6qUNqLY0QGi2sDCBQ&ved=0CE0Q6AEwAw#v=o
> nepage&q=ribbe%20nomenclature&f=false
Re: Green unknown from Colorado
May 06, 2012 11:09PM
At the risk of going even farther off topic, although Heath is exactly correct, I agree with Owen that there are good reasons to retain the "old" classification, in which 6 arbitrarily defined "varieties" of plagioclase are recognized. First, calling andesine or oligoclase a variety of albite will confuse many who learned the old system, and for what purpose? It will lead to misunderstanding of the usage in many thousands of older publications, in which the terms "albite" and "anorthite", as well as the intermediate "varieties", had strict and well understood meanings.

Second, it seems to me that the new classification is just as arbitrary as the old, with a boundary between albite and anorthite at the 50-mole-percent Ab/An composition. As far as I know, nothing of any structural or morphological significance occurs at the 50:50 composition. To me, this simply takes one clearly understood but faulty system and replaces it with a poorly understood and equally faulty one. There are also good arguments for other systems, but most of those are faulty, too.

This discussion reminds me of other recent debates in mineralogy, in which clarity has been sacrificed for "innovation" or "modernity" (or, perhaps, for additions to some workers' publications lists). As for the plagioclases, one still must do more than a simple sight identification to be certain which mineral is present in a sample.



Edited 1 time(s). Last edit at 05/06/2012 11:10PM by Carl (Bob) Carnein.
Re: Green unknown from Colorado
May 06, 2012 11:33PM
I feel obliged to defend the division of the plagioclase series into 2 species rather than the old 6. There is nothing arbitrary about the "new" division - either Na dominates over Ca, or Ca dominates over Na - what's arbitrary about that? The old divisions were ridiculously arbitrary. The site dominance rule is what is currently applied to all the other 4,000+ species in the mineral kingdom (well, almost all; we won't talk about actinolite or augite... yet), so why treat the plagioclase series differently?
avatar Re: Green unknown from Colorado
May 07, 2012 12:41AM
The old division of plagioclase was splittery and the new one is lumpery. I celebrate this change, the IMA could continue with micas too!

Josele
avatar Re: Green unknown from Colorado
May 07, 2012 12:54AM
    
Yes, I tested the hardness of the last photo as it looked similar color wise, so I was assuming it was probably the same. Since the crystal in the first 2 photos wasn't really damaged, I didn't want a big scratch across as it's fairly small.
Ronald John Gyllenhammer Wrote:
-------------------------------------------------------
> Hi Lefteris,
>
> > "The mineral in the first two photos is almost
> unmistakeably Sphalerite (var. Cleiophane)."
>
> I agree. This occured to me also but I thought Jim
> said it had a hardness over 6. I suppose Jim was
> refering to the last image which I think may be
> different material altogether.
>
> Ron
avatar Re: Green unknown from Colorado
May 07, 2012 01:00AM
    
Thanks James.
Re: Green unknown from Colorado
May 07, 2012 02:47PM
James
Nice xl. I find sphalerite in Illinois, but none green, unless I haven't looked closely enough.
And I deeply appreciate all the feldspar comments; It has been confusing, which one is a species, which one is a variety. Thanks for clarifying...
avatar Re: Green unknown from Colorado
May 07, 2012 07:09PM
Thanks to all. I now comprehend the arguments better than I did two days ago.
Re: Green unknown from Colorado
May 07, 2012 09:46PM
Please forgive me for beating a (perhaps) dead horse, but I think the subject of plagioclase classification is an interesting one. It involves history, philosophy, and a tremendously useful and interesting group of minerals.

Also, please forgive me for confusing Heath's post with Ron's--it was Ron's expostulation that (unfortunately) first caught my interest.

I can't argue against the logic of Alfredo's position, though I disagree with it. The literature about, and including, references to the plagioclase series is so enormous that mineralogists will be forced either to abandon it (I hope not!) or teach the old subdivisions in their original meanings so that undergrads can read it intelligently (my bailiwick was the world of undergraduate teaching for over 35 years). I guess the "new" definitions of albite and anorthite just add a little to what is already a confusing topic for many. As for the 50:50 split, I still think this is arbitrary, though consistent with usage in other, less important, mineral series. (Yes, Alfredo, I'm a stubborn guy.)

Although I do generally agree with Jose's comment about "splittery" vs. "lumpery" , I'd suggest taking this one step farther and just calling all of this stuff either plagioclase, albite, or anorthite (whichever has priority). The geology students out there would love this smileys with beersmileys with beer

As a final note, I agree with others that the green crystals from Colorado look, to me, like twinned sphalerite.
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