Ok, you rock and mineral experts....I need some help

[Jim in Idaho]

I finally had a decent weather day to head out to the desert. Found a somewhat strange rock. Extremely light, which isn't strange on the local desert, there's lots of porous basalt. But, this one has a silvery, metallic-looking coating on one side, as show in the pics. When placed in water, it initially floated, and then slowly sank as the surface voids filled with water. I scraped a bit of the silver coating off onto a piece of yellow paper. The particles are the size of dust. You can easily see the metallic-looking material. It's slightly magnetic, even the silver stuff. On the rock, it feels somewhat slick, and looks like little spheres. What is it?
EDIT: When viewing this in the sunlight, it had a little prismatic effect....3 or 4 colors.
Jim

 

[DeepseekerADS]

Jim, is that a scraping of the surface of the rock?

 

[Machkin]

Something in the Silica family with other minerals absorbed? May be able to explain the floating then sinking.

 

[Jim in Idaho]

Jim, is that a scraping of the surface of the rock?

That's some of the shiny stuff scraped off the rock with my pocket knife. A magnet, if held close enough, will pick up those tiny particles.
Jim

 

[Jim in Idaho]

Something in the Silica family with other minerals absorbed? May be able to explain the floating then sinking.

Naah....the floating and sinking is just voids in the rock slowly filling with water, and displacing the air. The light weight isn't a big deal...it's the shiny material that makes this rock special, and which has me puzzled.
Jim

 

[Machkin]

okay, then how about Obsidian. Obsidian can include trace amounts of Magnetite (slightly magnetic), also Obsidian can produce the prismatic effect that you are getting.

 

[Jim in Idaho]

I've never seen silver obsidian. It's definitely possible the black material that the silver is attached to is magnetite, but while it is attracted to a magnet, the force is weaker than any other magnetite I've seen. The rock itself doesn't show any magnetic attraction. Also, the silver material doen't show any fluorescence, at least under LW UV.
I just tested the rock with the GM-2, and is gave a strong non-iron signal. The TDI SL doesn't see it. So, definitely a hot rock of some kind. The coating shows very low resistance....about .1 to .7 ohms, depending on where the probes are placed.
Jim

 

[Jim in Idaho]

Ya know, Machkin, it may be magnetite. Looking at the coating under a weaker magnifier, it looks like various sizes of very dark, shiny spheres. Could be hematite, too.
Just did a scratch test.....seems very soft, and leaves a dark gray, or black scratch. Not hematite.
Jim

 

[Jim in Idaho]

I'm starting to think it's ilmenite. Dark streak, weakly magnetic....fits. There's quite a bit of titanium dioxide in the lavas at Craters of the Moon, and the surrounding lava fields. Also, this rock looks like part of a cavity, and ilmenite can form in cavities, similar to the way quartz grows.
Jim

 

[Kray Gelder]

Hey Jim, interesting post. Here's my deduction. The rock itself is volcanic tuff, the layer in question probably formed on that side, when it was sharing a crack with another layer of tuff, through which a mineral-rich solution flowed. The fully formed crystal growth suggest a mineral solution environment.

 

[Machkin]

I thought the silver color in the last picture was a light source reflection. I would agree with Ilmenite based on your description and pictures and also referencing, Information Circular No. 1 Idaho Bureau of Mines and Geology, Black Sands report by Lewis S. Prater dtd 1957; Table 1, it would appear Ilmenite does fit. Good call.

 

[Jim in Idaho]

Well, it's a nice specimen, in any case. If it is ilmenite, it's probably also got magnetite in there to some extent. That's probably what's making it set the detector off. I'd bet, if I sawed off a slice that included the shiny stuff, the remainder would still set off the detector. Lots of basalt out there does. I appreciate all the input. I think I'll try the DFX on it, and see what VDI it shows.
jim

 

[Jim in Idaho]

The DFX was interesting. Using "prospecting" mode it shows a VDI of 2 or 3, and no other bumps on the chart. A pretty "pure" VDI. Good, strong signal, too. Easily 10" in air. If it were showing as iron, I would expect the VDI to be ringing in at the high minus numbers like -85, or -75, or even higher. I have no idea where Titanium would come in. It's definitely seeing a strong conductor, and something specific. I'm surprised the TDI doesn't see it. May try that again with a smaller coil.
Jim

 

[Jim in Idaho]

Nope. TDI shows nothing, even with the little Sadie coil on there. So, the other detectors are probably seeing a conductive mineral rather than a metal.
Jim

 

[Eu_citzen]

I'm starting to think it's ilmenite. Dark streak, weakly magnetic....fits. There's quite a bit of titanium dioxide in the lavas at Craters of the Moon, and the surrounding lava fields. Also, this rock looks like part of a cavity, and ilmenite can form in cavities, similar to the way quartz grows.
Jim

Another option might be tarnished pyrrhotite?

 

[Jim in Idaho]

I'm still trying to find pics of spherical pyrrhotite, EU. Color doesn't look right...this stuff is dark gray. Here's a couple more pics. The coppery color is not true. It's something the camera did. The color is a uniform silvery gray.
Jim

 

[Jim in Idaho]

It's interesting that the GMT, and GM-2, even though they easily detect this, don't show it is as iron. Every hot rock I've ever found on the desert indicates as iron on the GMT's bar graph.
jim

 

[Eu_citzen]

I'm still trying to find pics of spherical pyrrhotite, EU. Color doesn't look right...this stuff is dark gray. Here's a couple more pics. The coppery color is not true. It's something the camera did. The color is a uniform silvery gray.
Jim

Try searching for botryoidal pyrrhotite. Just wanted to throw an alternative out there.
I was thinking it might be tarnished; if it isn't, then likely not pyrrhotite.

From your detector reactions: I've seen the DFX do that on sulphides.
Are there known sulphides in the area?

 

[Jim in Idaho]

None that I'm aware of, though with the amount of volcanism on the Snake River Plain, anything is possible. As Kray Gelder said, it's volcanic tuff. That coating is something unusual for the Plain, for sure. I've been walking this desert for 40 years, and it's the first thing I've seen like that. I did try scrubbing the coating with soap and water, but it didn't change anything.
Jim

 

[Jim in Idaho]

After checking the pics on botryoidal pyrrhotite, that's defintely a possibility, except for the color. Looks like there are lost of mineral in that
botryoidal form. here's another tidbit....this was found pretty close to the old stage road that runs from the mines at Salmon, Idaho. It's possible it was being transported from there, and ended up where it did. No way to know.
Jim