FMF - Friends of Minerals Forum, discussion and message board

[Ken]

This rock was found using a metal detector. I was curious as to why it was so magnetic so I broke it apart. I took one small piece and ground away the rock with a diamond rotary bit and was left with quartz and this black material that look as if they have been fused together. It has a hardness of 7.0 by my account.

[SteveB]

Hm, interesting. I think maybe your hardness test is picking up the quartz, did you use proper hardness scribes? I think you should also do more standard tests as decribed in the sticky thread. From the pictures i see similarities to tetrahedrites, plus galena specimens. I dont think you have a simple “pure” specimen with a simple straightforward name to put to it. If you can mention where you dug it up, someone with knowledge of the geology of the location may be able to give more assistance. Are there any crystals present in your specimen? These will be of pure form of a mineral present in the rock. Photos are not really enough, location is really needed.

[Pete Richards]

What kind of a magnet are you using? If it's one of the new powerful rare earth magnets, it could easily attract ilmenite, and maybe even strongly enough to hold the rock in the air, if there was enough ilmenite. If it's an old iron magnet, I doubt it if would hold up the specimen unless the susceptible mineral was magnetite. As far as I know, neither tetrahedrite nor galena are attracted even by strong magnets.

[Ken]

The hardness test set I used is the Deluxe test set by Minerallab. The magnet is just a typical magnet you buy at a auto parts stone, nothing special. This rock is littered with small crystals that I'm told are most likely garnets, none of the black material is in crystal form. This stone along with many others appears to have been washed out of the side of a mountain by rain. A lot of the other rocks would also set off a metal detector. This was found in the desert east of Los Angeles, California. I've had an edx, xrf and xrd analysis done. I can share the edx and xrf data but not the xrd. The first edx scan analysis was done on the quartz only and the second was done on the black material only. The xrf analysis was performed on the black material only. The xrd analysis was done on the black material only and states that the material is Magnetite Fe2.04, ilmenite Fe1.04, Ti03, quartz Si02 and albite Ca, rich

[Pete Richards]

It seems likely that your material contains several minerals mixed together at a very fine scale. The first spectrum looks more like a feldspar than quartz. Quartz rarely has much in the way of impurities, and this scan shows almost as much aluminum as silicon. The second spectrum also shows a number of elements that would not be expected in ilmenite or magnetite, especially silicon.

Several useful, one might say critical, pieces of information are not reported, and probably were not reported to you. A back-scatter image of the area analyzed would help to show whether it was homogeneous or not. Analyses of the same basic area using different spot sizes would help show how homogeneous or variable the area analyzed is. The chart does not indicate whether the results are % by weight or % of atoms. It's hard to interpret such a chart in terms of the minerals that might have produced the results without knowing this. For example, take ilmenite, FeTi03:

Fe Ti O
Atom % 20 20 60
Weight % 37 32 32

If you try to interpret these weight percents as atom percents, you would have to conclude that the formula is FeTiO, with some extra iron somehow. But FeTiO is not a mineral; in fact it is not a possible chemical compound.

[Rugie]

Have you rejected the possibility that this is of extraterrestrial origin? The rounding of edges suggests to me that this might be a stony aerolite which fell eons ago then was revealed as alluvium from the mountain. The elemental analysis is not inconsistent with high silica stony aerolite composition.

[Ken]