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Goethite -not cacoxenite- inclusions in amethyst from Brazil
  
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Antonio Alcaide
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PostPosted: Sep 12, 2011 09:45    Post subject: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

We reproduce here in a new thread the posts between Xenolithos and John S. White from the thread of mineral inclusions:

http://www.mineral-forum.com/message-board/viewtopic.php?p=19404#19404

Duncan, as you can see, we already have a section for false identifications on FMF Forum. The information both of you provided is interesting enough as to open a new thread with it.

xenolithos wrote:
Maxilos

The 'rutile' in your specimen look a lot like the cacoxenite tufts in this amethyst. Admittedly it is tricky to identify such inclusions visually, but according to Webster's "Gems" cacoxenite inclusions are common in amethyst. (OK, I will stop posting cut stones now.)

Duncan


John S. White wrote:
Oh please, not "cacoxenite" again! What does one have to do to permanently bury this false identification. The inclusions in amethyst from Brazil are NOT cacoxenite, they are goethite, as has been stated more than once on FMF. In fact, on this very thread, just days ago, I posted a photo of amethyst with goethite inclusions. Here it is again. Duncan, did you not notice that? There are no phosphates associated with the geodes that contain the amethyst from Rio Grande do Sul. Never, in my experience, has any misidentification been perpetuated to the degree that this one has. Goethite, please, and it is quite clear that the piece Maxilos has is goethite.

Finally,just because our fearless leader Jordi does not like polished pieces, please continue to post photos of them when appropriate. I fully intend to.


xenolithos wrote:
Hi John

I am glad to have been corrected, thank you. Yes, I did notice your photo of the goethite, saw its similarity to the tufts in my amethyst, and tried to cover myself by saying that visual ID is dicey! (The edition of Webster's Gems I referred to was the 5th edition, reprinted 2003, edited by Peter Read.) As to what one has to do to bury such false identifications permanently, perhaps there is a need for a cumulative 'recurrent false IDs' thread with illustrated examples.

Duncan


John S. White wrote:
Hi Duncan:

Thank you for your response. It is interesting that the 2003 edition of Webster's Gems would make that mistake. I do not have that book, but my copy of Webster's Gemmologists' Compendium (6th edition 1979) carries the statement: "the name (cacoxenite) is often misleadingly applied to quartz (often amethyst) containing yellowish tufts of goethite."

It is amazing that Peter Read would change what was correct to something incorrect.


There are several pictures in the original thread.

Regards

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PostPosted: Sep 12, 2011 17:02    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

The pictures mentioned above.

Regards



quartz__cacoxenite_197.jpg
 Description:
Quartz with goethite inclusions
Brazil
12 mm square, 10.89 ct
Posted by Xenolithos
 Viewed:  22701 Time(s)

quartz__cacoxenite_197.jpg



goethiteinquartzovergrowth2_116.jpg
 Description:
Amethyst with goethite inclusions
Rio Grande do Sul, Brazil
Posted by John S. White
 Viewed:  22704 Time(s)

goethiteinquartzovergrowth2_116.jpg



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PostPosted: Oct 11, 2011 01:59    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

Antonio

Thanks for creating this thread. Now, does quartz EVER contain cacoxenite inclusions? If one searches the GemologyOnline forum (which has wonderful inclusion photos) for 'cacoxenite' one gets plenty of mentions of it in amethyst - and a 28 March 2011 posting pointing out that it is goethite. ( I don't have John Koivula's books so cannot check this myself.) Perhaps 'cacoxenite' is just a sexy sales tool. It appears that any word with an 'x' in it sells goods like hot . . . um . . .cakes.

Duncan
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PostPosted: Oct 11, 2011 02:28    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

You are welcome, Duncan. Perhaps Elise Skalwold, who owns Koivula's books, can clear it up. You could be right: cacoxenite sounds better than goethite when you want to sell.

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PostPosted: Oct 11, 2011 05:01    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

These amethysts in geodes can NEVER contain cacoxenite as there is no phosphorus in the environments in which they crystallized. None! This does not rule out the possibility that quartz crystals in pegmatites that do contain phosphorus can have cacoxenite inclusions, but as far as I know there have been no examples discovered.
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PostPosted: Oct 18, 2011 12:04    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

Here is a natural uncut crystal of amethyst containing goethite inclusions. This is the sort of rough from which the facetted stone illustrated previously was cut. I don't know what the little dot inclusions are. Some of them are black, but a couple are red, so they may be haematite. One face has quartz outgrowths covering the near-surface tips of the goethite spray, as in John White's example, just visible lower right, but too small for me to photograph separately.

Duncan



Amethyst with goethite inclusions.jpg
 Description:
Quartz with goethite inclusions
Brazil, presumably
33 mm long. 11.3 grams
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Amethyst with goethite inclusions.jpg


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PostPosted: Oct 18, 2011 13:19    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

The little dots are surely hematite. The black ones are thicker than the red ones which transmit enough light to reveal their red color. It is common to have the two together.
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PostPosted: Nov 29, 2011 01:18    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

Here's a quote from the Photoatlas vol.1 explaining a picture of a goethite inclusion in brazilian quartz.

"Much commoner, and truly characteristic of the endogenesis in quartz, are divergently radiating bundles of goethite needles like corn-sheaves. They used to be mistaken for cacoxenite, yet they cannot be cacoxenite, as in repeated analyses no phosphorous could be found. Another form of goethite is this broom-like yellow spray of individual acicular crystals, which nucleated growth at a single point and fanned out as the brazilian amethyst grew."
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PostPosted: Jan 08, 2012 17:37    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

cascaillou wrote:
Here's a quote from the Photoatlas vol.1 explaining a picture of a goethite inclusion in brazilian quartz.

"Much commoner, and truly characteristic of the endogenesis in quartz, are divergently radiating bundles of goethite needles like corn-sheaves. They used to be mistaken for cacoxenite, yet they cannot be cacoxenite, as in repeated analyses no phosphorous could be found. Another form of goethite is this broom-like yellow spray of individual acicular crystals, which nucleated growth at a single point and fanned out as the brazilian amethyst grew."


So Cacoxenite and Geothite are very similar other that cacoxenite contains phosphorous. What is in geothite contain if not phosphorous?
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PostPosted: Jan 09, 2012 09:01    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

Hi to everybody! I can confirm, I have some rough and cut crystals of amethyst with inclusions, and those inclusions were classified as goethite in my case too!
Greetings from Italy by Riccardo.

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PostPosted: Jan 22, 2014 10:45    Post subject: Re: Goethite -not cacoxenite- inclusions in amethyst from Brazil  

update:

Known misrepresentations (discredited by lab analysis, sources: Photoatlas vol.1 and vol.2, and the book Inclusions in Quartz):

-cacoxenite inclusions are actually goethite inclusions
-lepidocrocite inclusions are actually hematite inclusions
-hedenbergite inclusions in green prasem quartz are actually mostly actinolitic amphibole inclusions

another possible misrepresentation is celadonite in quartz from madagascar which, as far as I know, has never been confirmed by analysis (could be another mica group mineral, or even a chlorite).
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