FMF - Friends of Minerals Forum, discussion and message board

[Antonio Alcaide]

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

https://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.

[Antonio Alcaide]

The pictures mentioned above.

Regards

[Duncan Miller]

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

[Antonio Alcaide]

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.

Regards

[John S. White]

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.

[Duncan Miller]

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

[John S. White]

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.

[cascaillou]

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."

[Rosanna]

[Riccardo Modanesi]

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.

[cascaillou]

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).