Assistance with a rock identification (found in the Southwestern US)

[C Mac]

Hello,
Thanks for providing a forum where people can share their education/passion and help others. Speaking of helping out, could I please get some feedback on this rock I came across in the southwestern part of the U.S.? I picked it up because it didn't match anything else in the area. It sat around my place and I eventually put a magnet to it and it was magnetic. I also did a scratch test on ceramic and it didn't leave a mark. In my not very scientific way, I thought it was magnetized sandstone. Maybe it's weathered magnetite? Attached are a few photos. Please let me know what you think when you have a moment.
Hopefully you can see the images.

 

[jedishrfu]

I don't have an answer however I found this site that has a list of common magnetic minerals:

http://www.galleries.com/minerals/property/magnetis.htm

Here's a picture of magnetite that looks somewhat similar to what you have:

http://www.galleries.com/scripts/item.exe?ENLARGE+Minerals+Oxides+Magnetite+MAG-30

 

[davenn]

Maybe it's weathered magnetite?

I don't think so ... magnetite will weather/corrode to a reddish colour aka rust ( as with most iron minerals)

Many rocks will stick to a magnet because of their iron content, even basaltic lava

1) is it really green in colour or is it the camera/lighting ?
2) the darker areas, do they really look like they are clumps of material in that green/white matrix ?
3) where in the SW be more specific ... state, area in the state ?
4) in that last photo with the white patches, it (well that area) looks much like dacite ( volcanic)
5) the overall appearance looks like a conglomerate of several different rock types


Variations of dacite ...

Dave

 

[C Mac]

Thank you for the replies. The rock came from southwestern New Mexico. I do historical research. While not looking for rocks, I'll pickup odd rocks that don't match the terrain. To answer additional questions, it's a dark gray/greenish. The photos, including the first one in this photo, were taken with direct sunlight since I was trying to pullout the various colors and features. The second photo in this posts shows the rock with no direct light.

Thanks again and please let me know if you have additional questions.

 

[Vanadium 50]

Could it be maghemite?

 

[davenn]

Thank you for the replies. The rock came from southwestern New Mexico. I do historical research. While not looking for rocks, I'll pickup odd rocks that don't match the terrain. To answer additional questions, it's a dark gray/greenish. The photos, including the first one in this photo, were taken with direct sunlight since I was trying to pullout the various colors and features. The second photo in this posts shows the rock with no direct light.

thanks for the additional info
Well, New Mexico - Arizona area has a lot of old volcanics so that could explain the dacite
the green stuff with streaks / almost banding of quartz ( white material) through it gives the appearance of gneiss - schist

one gneiss variation ...

The green is usually because of chlorite content.
Gneiss and schist are old metamorphic bedrock rocks. the presence of what looks like those darker lumps of dacite,
hints at the possibility that volcanics came up through the gneiss bedrock and tore bits off resulting in this mix of
two very different rocks.
Because the "gneiss" is more prominent than the darker patches, this hints that the sample was from the edge of the
magma duct that came to the surface through the bedrock... hence more bedrock than magma in the sample

This is my best educated guess without seeing the sample(s) up close for myself. I remember seeing a similar thing
(different rock types) but still volcanic and country rock mix when I was studying the "Dunedin Volcano" ( New Zealand)
back in my university days.



Dave

 

[davenn]

Could it be maghemite?

I had to google that one
not one I was familiar with

Maghemite - Wikipedia
https://en.wikipedia.org/wiki/Maghemite

Overview
Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same spinel ferrite structure as magnetite and is also ferrimagnetic.
Maghemite can be considered as an Fe(II)-deficient magnetite with formula where represents a vacancy, A indicates tetrahedral and B octahedral positioning.

I don't think so, I think it unlikely that you would see streaks of what looks like quartz through it as in the OP's sample


D

 

[davenn]

Here's a picture of magnetite that looks somewhat similar to what you have:

http://www.galleries.com/scripts/item.exe?ENLARGE+Minerals+Oxides+Magnetite+MAG-30

but you don't see the obvious magnetite crystals in the OP's sample

 

[C Mac]

Thank you for the feedback and great leads. (Yes, I had to google maghemite as well.) In addition, it really looks like gneiss (pictured above) with quartz banding. Of course, it also looks similar to the photo of magnetite, minus the crystals. Thanks for narrowing it down.

Hope the forum doesn't mind if I post a few more rocks that I've come across.

 

[davenn]

Hope the forum doesn't mind if I post a few more rocks that I've come across.

All good use a different thread for each sample just to keep comments/replies together for a particular sample