FMF - Friends of Minerals Forum, discussion and message board

[GabiG]

Hi all, Could you help me identify the metal in these rocks? The samples contain tiny specks of shiny white metal with a somewhat splattered look. The samples respond to a neodymium magnet, i.e. the magnet wants to attach to the rocks at the point of the white metal specks and the smallest rock can actually be pulled by the magnet. My Garret Pointer is very active towards them. The quantity of metal is unfortunately very small, so a streak test would be very difficult. Most asked so far suggest it's chalcopyrite, but I'm confused about the strong magnetic response. I collected the samples at a disused lead/zinc mine. Thanks.

[Matt_Zukowski]

Can you find the locality in mindat?

[GabiG]

Yes, the locality on Mindat is Isle of Man - 72

[Bob Carnein]

It looks to me like pyrite. Try the streak (black, possibly with a greenish tint, if it's pyrite); try the hardness (close to that of a knife blade; it will scratch steel if the test is done carefully).

[GabiG]

Isle of Man near the Snaefell/Laxey Mine

[Bob Carnein]

Sorry, I didn't read the original post carefully; pyrite isn't magnetic (nor is chalcopyrite). Pyrrhotite is somewhat magnetic; color and lower hardness also help to distinguish it from pyrite (pyrrhotite usually has a bronze tint, which I don't see in the photos, and it's softer than steel). Can you try scratching your sample with a needle? You can try the streak of even a small sample by rubbing it against the white ring on the bottom of a porcelain teacup or saucer. Are you certain that there isn't another mineral present that accounts for the magnetism?

[GabiG]

I'll have a go at what you suggest. Will try later as going to work now! :-( I will also try breaking one of the rocks further to see if any other metal is present. Thanks very much!!

[Pete Richards]

Mindat's list of minerals for the Laxey mine mostly contains sulfides, including pyrrhotite. Magnetite seems unlikely in this setting, I vote for pyrrhotite (tentatively). My reasoning, of course, assumes that the Laxey mine geology is similar to the place you collected.

Magnetite should be much more strongly attracted by a magnet than pyrrhotite - you might try an old-fashioned "ordinary" iron magnet for perspective.

If you break up one of your lumps, you may well release some isolated fragments of your mystery mineral, which should be easier to work with.

[Joseph DOliveira]

Hexagonal pyrrhotite is not magnetic but monoclinic pyrrhotite can be quite magnetic.

[GabiG]

Oh! :-D That sounds confusing! Not sure how to tell the difference with the small sample size I have :-/ But I'll continue to investigate to still my curiosity :-D Thank you :-)

[GabiG]

I tried the magnet test, and you are right. The Ferrite magnet doesn't have any effect on the small rock. The Neodymium of course pulls it straight away. I can't test the other rocks because they are too heavy. I need to break them up first. The sample comes from near the North Laxey Mine. It's a lead mine but apparently a little galena was discovered there also and pyrites and zinc of course. I will try and isolate some of the metal :-) Thank you :-)

[GabiG]

I tried scratching some of the specks with a needle. Difficult because they are so small. They seem hard and a tiny fragment of quartz came off with a metal speck left on it. It literally flew towards the magnet as soon as I put it near it. Actually, it was drawn by both magnets - ferrite and neodymium. I haven't been successful with the streak test yet, but I keep trying. Will also try to break some rock a bit more to see if other metals are present. The mine did have a small amount of galena apparently, but isn't that black?

[alfredo]

Galena is silvery grey in color, not black.
The magnetic material sounds like pyrrhotite, as others have pointed out. And both pyrrhotite and galena will react to electronic metal detectors. Pyrrhotite, galena, covellite, and graphite are the four minerals that frequently give "false alarms" (or "false excitement"?) to a friend of mine who detects professionally for gold.

[GabiG]

Well, taking all the information into consideration, I think it must either be pyrrhotite or galena :-) I will try and collect sufficient of the material to do a streak test. But I wonder if they would both have similar streak colour? Anyway, it's worth a try and all good for learning! :-D

[Bob Carnein]

Once again, you need to try scratching the mineral with a pin or needle, or try scratching a steel knife blade with it. (Be careful you aren't simply crushing the mineral--try both the needle on the mineral and the mineral on the knife blade.) It it's harder than steel, it isn't galena, pyrrhotite, or chalcopyrite. It appears to have a conchoidal fracture, which would rule out galena. Streak isn't going to help you much to distinguish between these and pyrite. You also should crush up a sample and see whether you can separate out the mineral you're trying to ID; then try the tests on it (including magnetism, using an "ordinary" magnet).