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EducationRutile/ilmenite

21st Feb 2012 23:37 UTCDon Saathoff Expert

I've got a question concerning petrogenesis.....at the Beryllium Virgin claim in Paramount Canyon, Sierra Co, NM, most of the mineralization occurs in lithophysae in the rhyolite. We've found most of the published species and a couple not published (topaz). The ilmenite occurs as tiny euhedral xls and as anhedral blebs in the rhyolite gangue and in the lithophysae. In one instance we found small groups of pale yellow to colorless accicular xls grown on ilmenite grains (possibly epitaxially?). Since both ilmenite & rutile are listed for the location and both ilmenite & rutile are Ti-bearing phases, what are the chances that the micro (to .25mm) accicular xls are rutile? We have found nothing else that resembles "traditional" rutile and these xls are much too small to see a termination. Thoughts???

I'll post a pic later in the week in the identity section....

Don

22nd Feb 2012 03:31 UTCRonald John Gyllenhammer Expert

Hi Don,

It will be interesting to see those images if you have time. I only have a couple of thoughts on your question. Actually more questions than thoughts. Are there any other identified minerals associated with the "Ilmenite grains"? Are you confident that your Ilmenite grains are Ilmenite and not Hematite? (I guess the hematite is usually found specular.) If it's definitely Ilmenite, topotaxial or epitaxial Rutile I think could be a possibility. It's a known association elsewhere of course. I think Hematite + Rutile from Taylor Creek rhyolites would not be probable or at least far less likely to be found together there. The only other "yellow to colorless accicular xls" that come to mind from there might be Stellerite. However, if those "Ilmenite grains" are hematite, acicular Pseudobrookite could be a possible association, although Pseudobrookite there doesn't seem to typically form needles (it can and does elsewhere) and the color might not be colorless to pale yellow. Well, this just some speculation I suppose until images are posted. Good luck with it Don.

Ron

22nd Feb 2012 16:44 UTCDon Saathoff Expert

Ron, thank you very much for your reply! I'll have a window of opportunity on Thursday or Friday for photography and will post some pics in identity section. The hematite is not magnetic, is usually specular (or euhedral but slightly oxidized) while these grains are slightly magnetic so feel confident in ilmenite. The pseudobrookite, even in extremely thin xls, is black and opaque.

And thanks for the term "topotaxial" - I'd never heard the term before and it's not in my "Dictionary of Geological Terms" BUT, its meaning is obvious and I'll write it in!!!

Don

22nd Feb 2012 17:46 UTCReiner Mielke Expert

Ilmenite powders to black, hematite to red.

22nd Feb 2012 19:36 UTCDon Saathoff Expert

Thanks Reiner...forgot to mention that was done.....

Ron, now I'm confused!!! All references I can find regarding topotaxy concern themselves with intermetallic or bimetallic manufactured junctions. Is the term from a material science discipline?

Don

22nd Feb 2012 20:20 UTCRonald John Gyllenhammer Expert

Hi Don,

A discussion on this can get quite involved. Here's an excerpt of a relevant public discussion between J. White and P. Williams along with a link to the page. You may find the entire thread interesting as a whole.

"The terminology of epitaxy is somewhat messed up. Epitaxy proper refers to two-dimensional relationships. There is a term, monotaxy I think, for one dimensional relationships, but it's not obvious that anyone has come up with a good example. Syntaxy refers to three-dimensional similarities, but the term is also used in other geological contexts to mean quite different things, related to cementation of grains in sandstone, for example. Topotaxy also refers to three-dimensional similarities (usually, at least) but also carries the connotation of a phase change - for example conversion of ilmenite to oriented rutile mats by leaching of iron and recrystallization of rutile at an atomic scale such that the orientation between rutile and ilmenite is preserved even after the ilmenite is gone. Most folks use epitaxy to refer generally to all of these kinds of oriented relationships; I do, and I think John does too." - Pete Williams Source: http://www.mineral-forum.com/message-board/viewtopic.php?t=429&start=0&sid=f8e219aec622591c8cdcfc390606bfba 3/4 down the page. Hope this helps.

Ron

EDIT: It's worth noting that there are also other interpretations as well. One of my longtime favorite articles written by J. Rakovan can be found here. http://www.cas.muohio.edu/~rakovajf/WTTW%20Epitaxy.pdf

23rd Feb 2012 00:14 UTCDon Saathoff Expert

Ron, thanks for the links - good read!! Based on both discussions, I'm going to stick to epitaxy when describing oriented overgrowths of differing phases......right or wrong!! And I agree that the Rakovan's article is one of the better articles I've read!!

Don

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