FMF - Friends of Minerals Forum, discussion and message board

[Linus]

Hi All! This is one I probably should know, but its not registering.
pyramidal, Black, sparkly metallic on broken surfaces, strong cleavage, non-magnetic, larger broken surfaces that cut across the crystal seem to be oxidizing to yellow crusts... not sure if it is radioactive or not, location unknown. Ive seen a couple of these that were 5 - 8cm tall. Any thoughts?

[Bob Morgan]

Could it be sulphur in a limonite pseudomorph of pyrite?

[SteveB]

Magnetite? Yellow oxide it often zinc related and here it may have nothing to do with the crystal itself but from the host matrix. Could be residue from “YellowTak” similar to “BluTak” that rubbery gum stuff you use to stick posters to your wall. Its commonly used to “stick down” mineral samples into small cases. If it is it could be crumbly or maybe still sticky depending on how old it is.

Did you yourself collect this in the field? Where exactly? Or was it given to you or did you buy it? The yellow I’m certain is related to previous adhesive material and nothing to do with mineral. More testing needed like mentioned in the sticky forum thread on what to do before posting a photo and asking what is this. It wastes everyones time and discourages relevant answers. So read that thread and host back with more data.

My guess for now is magnetite. Just a guess.

[Matt_Zukowski]

Another wild guess - could it be franklinite? Strong cleavage argues against franklinite. What is the hardness, streak, and SG?

[Linus]

I stated it was non magnetic, very strong cleavage too, as shown, almost looks like carbonate type cleavage...

[Linus]

Matt, I think Franklinite is an excellent guess... I'll play with a bit more.

[Linus]

Hardness is between 4 and 5 (done with hardness points) and it seems to have at least three pronounced cleavage planes... so probably not franklinite. It is not fluorescent.

[Matt_Zukowski]

It would be really good to get a streak and a SG. Are you able to do these things? If you don't have a balance beam setup to do SGs, i can tell you a way to do it with just a cheap jeweler's scale and a measuring cup if you want.

Also, is it cubic, tetragonal, or orthorhombic? I thought it was cubic but pictures can deceive.

Another wild ass guess - cuprite like maybe from Rubtsovsk. Perfect cleavage again argues against this guess but at least curpite has some cleavage.

[Linus]

Sort of cubic, but there is a flattening at the pyramid base that is offset versus the opposite side, so maybe not cubic? (I haven't had mineralogy in about 40 yrs). It is noticeably heavy. I may be able to do a spg in the next day or so. Streak is a sub-metallic red brown. This piece, along with several similar others came to me from an estate - we have absolutely no location info available. thank you very much for the ongoing feedback.

[Linus]

oh, and the whitish spots in the picture look to me slightly aplitic, they fluoresce a weak tan-yellow.

[Bob Carnein]

How about sphalerite? It should have a pale tan to brown streak and 6 directions of cleavage (seldom easily counted). The "other" mineral could be a break-down/alteration product of sphalerite (maybe smithsonite or hemimorphite). A streak would definitely help--try it on the bottom ridge of one of your wife's teacups (but don't tell her I said to do that!?)/

[Pete Richards]

the first one suggests brookite (orthorhombic) altering to earthy anatase. The second one does not....

I don't know how this fits with the apparent cleavages.

[Linus]

Hi Bob, I think youre right about the orthorhombic, but the streak and pronounced cleavages suggest other than brookite... I think we're closing in on it!

[Tobi]

The second photo made me think of scheelite, the shape would fit, also the hardness between 4 and 5 and the fact that is is "noticeably heavy". But the streak and the fact that it is not fluorescent would not fit ... :-/

I hope someone can solve that riddle!

Regards
Tobi

[Linus]

well, we are closing in on it. XRF (Niton XL5) shows strong tungsten and iron... see the pic.