Sphene or Rutile?

Here's a neat little (~1.5mm) crystal in microcline from a beryl pegmatite in the Thackaringa district near Broken Hill (just north of the big Triple Chance mine). I found it while hunting for mosandrite, of which I didn't find any unfortunately. It is flattened and wedge shaped like a sphene, but the striations look more like rutile. Reports on the area say that both minerals occur in these pegmatites. It's not particularly metallic, though my camera makes it seem so from the reflections. Too small to do any quick tests on, any ideas?





Edited 1 time(s). Last edit at 11/10/2009 05:34AM by Ryan Eagle.

I dont know that it is either of those. but who knows... any idea on hardness streak etc that could help

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A buena hambre no hay pan duro

Ryan,
I have seen striated titanites, and if the mineral is red/brown, glassy and not semi-metallic, titanite could be a good guess.
Knut



Edited 1 time(s). Last edit at 11/10/2009 06:40PM by Knut Eldjarn.

Hi Matt. That raises some interesting possibilities if it proves not to be either. There are strange REE silicates in this peg anyway, with the occurence of mosandrite. I don't think I could get a decent streak or anything off it, and I'm a long way from a microprobe at the moment. Not fluorescent in LW UV, possibly radioactive. I had to pull the cover off my geiger counter to get it close enough to the tube for anything. There were a few tightly grouped clicks which at least with my counter usually happens on radioactive material, less so for background. Still not enough to definitively say. Davidite is known from about 5km up the road, which in this case has a little brannerite associated with it. If not sphene or rutile, maybe brannerite or samarskite?

Ryan,

Based on your description of the crystal shape and the photos I'd vote for titanite.

Olav

I found a couple more...


This group doesn't look like the first mineral, I suspect it could be maybe monazite or zircon? Not radioactive (at least compared to some saleeite of the same size). I think that it might be just iron staining in the surrounding feldspar, not radiation damage. Some of the faces have a pretty significant 'stepped' habit.


Another wedge


edit: now that i have more than one, i could bring myself to give one of the wedges a hardness test. A needle will scratch it rather easily, didn't have to try at all. Probably a bit soft for titanite?

just tried the only other titanite I have from Queensland, it scraches with the needle, but not as easily.



Edited 2 time(s). Last edit at 11/11/2009 06:06AM by Ryan Eagle.

that last photo I'd give a vote for titanite. I think a needle is the same as a nail for hardness maybe im wrong. I think it could depend on how cheap your needles are. I put my vot e for titanite now.. the previous could be the same. they look twinned.

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A buena hambre no hay pan duro

Ryan
Titanite seems a good bet for both.
I would be a bit dubious about mosandrite in these pegmatites, it usually occurs in alkaline complexes and the Thankaringa pegmatites are rather granitic as I recall (you may know better?). There are often various phosphates in them, but your specimens do not look like any I have seen.

Ralph

Hi Ryan,

My thoughts on this may or may not help, since my first idea here has a hardness greater than Titanite. I do like Titanite somewhat here but these last two images remind me of Cassiterite. What about cassiterite? It is isostructural with rutile, one of your first suspects but has a hardness of 6-7. It is also known from the Thankaringa District and referenced as such in, Records of the Geological Survey of New South Wales, Volumes 1-2, By Geological Survey of New South Wales an old (1889) but still a somewhat relevant and revealing work. [books.google.com] Appropriate testing will tell the truth, all else is speculation. Good luck and all the best.

Ron

Haha that Google Books link for the old report is such a tease... I want to see what context those words are in. It wouldn't suprise me if there was some cassiterite kicking about, but these dykes do appear to be significantly different to the main tin bearing ones up in the Euriowie area. There is a little tantalite kicking about in the Lady Beryl mine.


I suppose it would help if I paraphrase my main reference, Non-Metallic and Tin Deposits of the Broken Hill District by S.R. Lishmund, published in 1982:

Quote

PEGMATITE C

Situated immediately south of pegmatite B (), this pegmatite is notable for the presence of the rare mineral johnstrupite, which is a dark greenish-brown, resinous, radioactive cerium fluoro-silicate containing sodium, calcium and titanium. The mineral occurs at the margins of an irregular quartz core as small scattered lumps up to 0.1m in diameter. The pegmatite appears to be zoned, and also contains beryl, black tourmaline, and arsenopyrite (weathered to scorodite). Strengite and possible frondelite were reported from this locality by Tonkin (1969).

The Tonkin reference is an Honours thesis which is not readily available unfortunately. "Pegmatite B" turned out to be a beryl locality our university likes to visit, even though this report doesnt mention it occurring, so it was an easy walk over to "Pegmatite C". The quartz core is fairly obvious, and there was plenty of tourmaline. Interestingly there appear to be a lot of miarolitic cavities, or at least numerous small segregations that consist of tiny books of dark green mica. The existence of this reference for mosandrite probably explains why I didn't see any, being so easily accessible, the locality has probably been picked clean.

My earlier claim that sphene and rutile are found in these dykes turns out to be incorrect, that was only in referral to the Euriowie tin bearing dykes. Considering the separation and different mineralogy between the two areas, it might be a long stretch to say that they definitely occur at the Thackaringa district, unless someone has seen them. Even though its fun, I should probably stop speculating and get it under the probe.



Edited 1 time(s). Last edit at 11/11/2009 11:19PM by Ryan Eagle.

Ryan
I was always dubious about "johnstrupite" - this mineral resembles some of the Fe-Mn phosphates you get in these pegmatites (wolfeite, triplite, graftonite, etc), and I dont know that any was ever analysed. Lets know what the probe says.

Ralph