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Minehillite?

Posted by Peter Chin  
Minehillite?
May 01, 2012 03:10AM
[www.mindat.org] 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.
avatar Re: Minehillite?
May 01, 2012 07:47PM
us    
This is a Modris Baum photo and he is very knowledgeable. Are you saying the silvery mica is probably hendricksite?

Best Wishes, Van King
Re: Minehillite?
May 01, 2012 11:16PM
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!
Re: Minehillite?
May 02, 2012 01:25AM
us    
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 smiling smiley )

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.
Re: Minehillite?
May 02, 2012 01:45AM
us    
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
Re: Minehillite?
May 02, 2012 02:30AM
us    
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
Re: Minehillite?
May 02, 2012 03:20AM
us    
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



Edited 1 time(s). Last edit at 05/02/2012 03:27AM by Modris Baum.
Re: Minehillite?
May 02, 2012 04:07AM
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.
Re: Minehillite?
May 02, 2012 04:50AM
us    
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
Re: Minehillite?
May 02, 2012 02:43PM
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
Re: Minehillite?
May 02, 2012 03:23PM
us    
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
Re: Minehillite?
May 02, 2012 04:23PM
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.
Re: Minehillite?
May 02, 2012 05:33PM
us    
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
Re: Minehillite?
May 02, 2012 06:26PM
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.
Re: Minehillite?
May 02, 2012 06:54PM
us    
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



Edited 1 time(s). Last edit at 05/02/2012 08:56PM by Modris Baum.
avatar Re: Minehillite?
May 02, 2012 10:46PM
To confuse things further, take a look at what I was told is platy vesuvianite, [www.mindat.org]
Re: Minehillite?
May 02, 2012 11:02PM
us    
Jeff,

That's remarkable - whatever it is.

Modris
avatar Re: Minehillite?
May 02, 2012 11:07PM
us    
Not versed on all the permutations of assemblages at Franklin, but on appearance only, it certainly resembles some mooreites I've seen.

Cheers from NorCal, John

"God gave me the stubbornness of a mule and a fairly keen scent." - Einstein
Re: Minehillite?
May 02, 2012 11:28PM
us    
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
Re: Minehillite?
May 02, 2012 11:58PM
us    
Mooreite only occurs at the Sterling Mine in a hydrothremally and Mg-enriched enviornment.
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