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PhotosMinehillite?

1st May 2012 04:10 UTCPeter Chin Expert

http://www.mindat.org/photo-462933.html is not minehillite. The photo apparently shows a matrix of axinite(?), andradite and hendricksite. It is also noted that willemite and hancockite is present but not shown in the photo. To the extent of my knowledge, this is not the assemblage for minehillite, however, it is for johannsenite, which the white mineral in the photo greatly resembles.

1st May 2012 20:47 UTCVandall Thomas King Manager

This is a Modris Baum photo and he is very knowledgeable. Are you saying the silvery mica is probably hendricksite?

2nd May 2012 00:16 UTCPeter Chin Expert

Van,

The white platy mineral in question, based on the photo image and reported lack of fluorescent response, is likely johannsenite, although non-fluorescent prehnite could also be a possibility. Both Prehnite and Johannsenite from Franklin can occur as platy white aggregates, specimens of both can be found in the Franklin Mineral Museum. Prehnite is generally but not always fluorescent. The assemblage shown in the photo is axinite/garnet(andradite)/hancockite/hendricksite(brown plate)/willemite, is a known assemblage for johannsenite and prehnite. Modris, A rare species for Franklin and Nice specimen!

2nd May 2012 02:25 UTCModris Baum 🌟 Expert

Hi Peter,


Thanks for the input. Johannsennite is probably the last thing I expected. - not that I have ever seen any. Despite Van's accolade - I'm really NOT very knowledgeable about Franklin and Sterling at all. (In fact, I'm posting some of this stuff in the sneaky hope that somebody will tell me what it really is. Seems to be working :-) )


For the time being, I think I'll leave it as is - but with a caveat - just to see if anybody else chimes in. After I have thought about it- or heard any other opinions - it I will either label it as johannsenite or put it in my private gallery as unidentified.


Regarding the matrix, there is indeed some hendricksite (not very much), andradite, hancockite (not much), axinite (maybe - it doesn't fluoresce much at all) and willemite (only a couple of small spots). The major UV response - to my eyes - seems to be dingy pale blue white with some red areas (as you might expect for margarosanite except much weaker and washed out - no saturation). And - as far as I can tell, this fluorescence is not coming from the stuff I was calling minehillite.


Maybe I'll make a macro shot of the specimen.

2nd May 2012 02:45 UTCModris Baum 🌟 Expert

Peter,


Actually - it seems I have some johannsenite from the Iron Cap mine. In fact I even posted a photo


It sure doesn't look anything like the platy stuff In my "minehillite" photo. But if you say that the Franklin Museum has johannsenite that looks like my "minehillite" - well what do I know? I'm going to have to go down and take a look. Still - it seems rather surprising.


Modris

2nd May 2012 03:30 UTCChester S. Lemanski, Jr.

The minehillite occurs in a restrictive assemblage only, which is NOT represented in the photo. For once in a long while I have to plead ignorance on a Franklin species. I am familiar with johanannsennite, including from Franklin, but not the habit which Peter is talking about. My guess would have been prehnite, but definately not minehillite.


Chet

2nd May 2012 04:20 UTCModris Baum 🌟 Expert

Hi Chet,


Thanks for the input. I have now added a macro shot of the entire specimen . Perphaps that will provide a more definitive clue.


I'm going to add another photo later of a different specimen which has platy areas similar to (but not quite the same as) the first one that I posted.

This stuff too was sold as margarosanite but seems to fluoresce more violet than blue (and not all that strong). It is actually this stuff that led me to suspect minehillite.


But the matrix of this second specimen seems even more "politically incorrect" for minehillite. It's just ordinary calcite with a band of willemite and garnet grains in which there is a hendricksite looking mica that has apparently altered to the pearly colorless stuff that exhibits the blue/violet SW response.. (In some cases the pearly stuff seems to be a rim around the mica.)


Modris

2nd May 2012 05:07 UTCSteven Kuitems Expert

Hi Modris, have you looked at the white areas under higher power? Most of the johannsenite in the altered garnet/axinite matrix ( from what I have seen very minor hancockite not profuse as your specimen exhibits) will have distinct radiating sprays with a very pale green or gray look. The other possible replacing mineral that may be fluorescing that pale violet is xonotlite. In low concentration it will have a very weak fluorescent response. It will be as white, very fine fibers radially or parallel oriented, often depends on what and where deposited. In open voids it forms nice radiating groups, when replacing relict feldspar it can be oriented as parallel bundles.

Let me know what you observe.

Steve.

2nd May 2012 05:50 UTCModris Baum 🌟 Expert

Hi Steve,


Thanks for responding.


As far as I can tell, neither the stuff in my original "minehillite" post nor on the other specimen has any radial or fibrous structure. They both look micaceous/platy. (The stuff in the original post looks a bit "leathery". Hard to describe. Maybe I need another photo that emphasises the texture rather than the luster/reflectivity.)


In any case I have now posted (as a private gallery UK) a sample of the second mineral


The stuff is hard to photograph so I don't know if you will be able to tell very much. But this one does fluoresce.


Modris

2nd May 2012 15:43 UTCPeter Chin Expert

Modris,

Thanks for the image of the entire specimen. The minehillite assemblage is very distinctive,The matrix is microcline and accessory minerals, present but necessarily in every specimen, are margarosanite, wollastonite (for many years misidentified as "pectolite" until Fred Parker proved otherwise), grossular, allanite, native lead, vesuvianite, diopside etc. A complex mess! From what can be ascertained from the image, your specimen appears to be typical hydrothermally altered "Parker Shaft" GOOP - hancockite, axinite, garnet, hendricksite. Most of these minerals have a partially "bleached" look to them. I don't see in your image of the specimen, the type of matrix I would expect to see for Minehillite. I don't think I have seen hancockite in the minehillite assemblage; perhaps, Steve or Van may know of one? The "minehillite" plates of your specimen are beginning to look more like prehnite.


In the 1960's minehillite specimens were offered by miners and dealers as a cheap ("lousy")margarosanite, or for its "pectolite" content and for those few "lucky" collectors in need, as "barylite" in light of it's weak violet fl. For a while it was relatively abundant and inexpensive. I guess good things don't last.


Steve,


Thanks for your input and I would like to mention that there are specimens of Johannsenite in which the mineral appears as plates without a radial morphology. It also appears in an entirely different assemblage, epitaxially on rhodonite crystals.


Peter

2nd May 2012 16:23 UTCModris Baum 🌟 Expert

Hi Peter,


Thank you very much for the information. It's quite difficult to make sense of the - as you put it - "GOOP". You can read Dunn and Palache all you want but it is really input from experienced collectors like you, Steve and Chet that helps.


I'm going to stew on this for a few more days before I decide whether to call it prehnite or just leave it as UK.


Any thoughts on the platy stuff in the second - very different - specimen?


Modris

2nd May 2012 17:18 UTCPeter Chin Expert

-- moved topic --

2nd May 2012 17:23 UTCSteven Kuitems Expert

Modris, your second sample "looks" like plates of feldspar with some relict hendricksite inbetween the plates. Some hyalophane or microcline can have a very week fluorescent response but if there is some replacement going on you may not see it in white light. Is there a short wave fluorescence?

Steve.

2nd May 2012 18:33 UTCModris Baum 🌟 Expert

Hi Steve,


Yes, there is a blue/violet response SW that appears to be confined to just the hendricksite that has turned colorless. The UV response is not very strong but it's clearly noticeable (even with the very strong competing willemite and calcite). In fact, that's how I find the altered hendricksite areas too look at under the scope.


Thanks again - Modris

2nd May 2012 19:26 UTCPeter Chin Expert

There is another possibility especially if the white mineral appears to replace hendricksite and that would be grossular. Grossular can occur in a wide variety of colors including white and is quite common for it to replace mica in the hydrothermally altered areas.

2nd May 2012 19:54 UTCModris Baum 🌟 Expert

Hi Peter,


Yes - I had though of "caswellite". But the specimen of "caswellite" that I have doesn't really look similar under the scope. Also, it doesn't fluoresce at all (if I recall - perhaps worth another look).


Modris

2nd May 2012 23:46 UTCJeff Weissman Expert

To confuse things further, take a look at what I was told is platy vesuvianite, http://www.mindat.org/photo-151704.html

3rd May 2012 00:02 UTCModris Baum 🌟 Expert

Jeff,


That's remarkable - whatever it is.


Modris

3rd May 2012 00:07 UTCJohn Magnasco Manager

Not versed on all the permutations of assemblages at Franklin, but on appearance only, it certainly resembles some mooreites I've seen.

3rd May 2012 00:28 UTCModris Baum 🌟 Expert

John,


I don't think mooreite has been found at Franklin - only at Sterling Hill. Both of the specimens I posted are from Franklin. In any case, they don't look like mooreite under the scope.


Jeff,


I just remembered that Dunn states that vesuvianite is sometimes a component of "caswellite". So maybe you have an unusual form of the latter. The caswellite (grossular) I have doesn't look like your photo but (as we're being reminded) photos can be deceiving.


Modris

3rd May 2012 00:58 UTCChester S. Lemanski, Jr.

Mooreite only occurs at the Sterling Mine in a hydrothremally and Mg-enriched enviornment.

3rd May 2012 02:29 UTCModris Baum 🌟 Expert

Hi all,


I think I'm about to give up on both of these specimens. The "minehillite" might well be prehnite but I'm not comfortable labeling it as such.


Thanks for all the input. I learned a lot of useful stuff - but not enough to make a definitive call for either specimen.


I'll take care of the changes tomorrow.


Modris

3rd May 2012 12:29 UTCPeter Chin Expert

Jeff,

It could be vesuvianite but I think it is more likely grossular pseudomorph after hendricksite or phlogopite. Grossular is quite common and pervasive in the type of assemblage ("Parker Shaft") shown in Modris' photo whereas the vesuvianite pseudomorph in the photo in your message is quite rare and may have been a one time find (perhaps, Chet can confirm this).

3rd May 2012 15:52 UTCJeff Weissman Expert

Peter, I don't disagree - most caswellite is grossular, but some is, according to Dunn, indeed vesuvianite, and in reality a varying complex mixture containing both of these plus andradite, chlorite, etc... I think the color, which in person has a non-grossular-like appearance, suggested the likelihood that this particular example of caswellite is richer in vesuvianite led me likelihood this one as stated.


Analysis, of course, is needed


regards, Jeff

3rd May 2012 17:27 UTCModris Baum 🌟 Expert

After ruminating on all of the input (plus Dr. Dunn), I took another look at both the "minehillite" specimen and the other one that I posted.


What I see is that the platy stuff on both specimens is remarkably similar - despite one being "Parker GOOP" and the other being really very ordinary calcite with a band (1-2 cm) of garnet/willemite/mica running through it. In both cases there is relict hendricksite (or phlogopite). (The sample shown in the "minehillite" photo is actually atypical in that almost no relict mica is visible.) The main difference between these two replacements is that one fluoresces while the other does not.


So the bottom line seems to be that both of the specimens are "caswellite like" - i.e. NOT minehillite, johannsenite or prehnite.


But they don't look like "genuine" caswellite either. According to Dun (p. 424), "Mica (presumably phlogopite) is replaced by blue vesuvianite in some specimens, and some caswellite is in part vesuvianite."


Interestingly, some of the mica on the second of my specimens has turned green (another vesuvianite color) instead of colorless.


To determine what the stuff really is - grossular, vesuvianite or something else entirely - would, I think, require some pretty rigorous anaylsis - clearly not worth the effort.


So what to do with the "minehillite" photo? I think this is one of those case where a photo showing what a mineral doesn't look like can be almost as useful as one that does show what it looks like. Specifically my "minehillite" photo shows what minehillite, johannsenite and prehnite do not look like at Franklin.


But I don't think I can post it as "caswellite" (i.e. grossular). The only thing I can think of is to post it as hendricksite. I think it's fairly clear that there is hendricksite on the specimen and some of the colorless platy areas do show relict hendricksite.


Or is this "much ado about nothing"? Opinions? Chet gets three votes :-)


Modris

6th May 2012 01:49 UTCPaul Shizume

Another indication of minehillite is a stronger response under mid-wave UV, the color is similar to a weak margarosonite (blue white). The specimens I've seen including one at the Franklin Mineral Museum have microcline, daylight white. The microcline fluoresces a red.
 
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