Found a magnetic rock, Solved - Goethite

[turtlefoot13]

Hi all,

I am posting this here plus in the What is it area.

I found something that has me stumped. I found a rock that absolutely makes my metal detector scream. I can turn the disc all the way up and the metal detector still acts like it is in all metal mode. Here's the funny thing, it sticks to a magnet.

Here is what I am sure that it is NOT:

meteorite - found in Missouri and also does not pass most of the "tests"

slag (unless I don't understand slag) - one can see metal on the outside of the rock but you can also tell that the metal is inside of the rock. This was found in the middle of the woods where there hasn't been any metal working place of any kind that I know of.

-------------------------------------------------------------------------------------------------------------------

I am guessing that it is some sort of iron ore, although it really doesn't look like any of the images of iron ore or load stone that I have found on the net. Any information is appreciated.

Doug

 

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[Eu_citzen]

Re: Found a magnetic rock, Solved - Hematite

Emmm.. Hematite is NOT magnetic!!!

I'm thinking goethite, possibly with a addition of Magnetite to add magnetism.
http://www.galleries.com/Minerals/OXIDES/goethite/goethite.htm

* Color is yellow, brown, brownish red to black. Sometimes tarnished with irridescent colors.
* Luster is earthy to dull for massive forms and metallic for crystals.
* Transparency crystals are opaque.
* Crystal System is orthorhombic; 2/m 2/m 2/m
* Crystal Habits include prismatic and platy crystals, massive and aggregate forms include internally radiating or fibrous;
reniform, botryoidal and stalactitic.
* Cleavage is perfect in one direction.
* Fracture is uneven to splintery.
* Hardness is 5-5.5
* Specific Gravity is variable at 3.3 to 4.3 (light for metallic minerals)
* Streak is brown, brownish yellow to orange yellow.
* Associated Minerals include a large list of minerals (particularly secondary deposit minerals).
* Notable Occurences include many iron mines around the world; especially nice specimens come from England, Mexico, and
the Lake Superior region in Michigan, USA.
* Best Field Indicators are crystal habit, streak and tarnish.

// Eu

 

[turtlefoot13]

Re: Found a magnetic rock, Solved - Hematite

I believe that you are right. I am going to change this to goethite.

Doug

 

[PINKICE]

Hematite may or may not be magnetic. It is never always magnetic. You need to understand the physical properties about the Earth, rocks, and how the magnetic poles work. I am taking Oceanography class right now, and we did an experiment on rocks. Hematite mineral rock that we observed, is magnetic. But not all will be.

 

[Eu_citzen]

Umm.. Say what?

 

[oneeye]

"Hematite is an antiferromagnetic material below the Morin transition at 250 K, and a canted antiferromagnet or weakly ferromagnetic above the Morin transition and below its Néel temperature at 948 K, above which it is paramagnetic."

http://en.wikipedia.org/wiki/Hematite

 

[woof!]

The one remaining photo looks like botyroidal (sp?) hematite. The other photos (no longer linked) might have suggested some other identification. I'm pretty sure it's not goethite.

Hematite itself is never more than weakly magnetic, and with respect to metal detectors it's fairly benign stuff. Same is true of goethite.

There are two minerals that do as you describe-- magnetite and maghemite. Neither would normally be strongly associated with botyroidal hematite, but I don't suppose it's impossible.

Magnetite is black, sometimes bluish black or grayish black. It is hard and gives a black streak.

Maghemite although normally red, can cover a wide range of color (as can hematite, with which it is most commonly confused). It is soft and earthy, although may be disseminated in harder rock (for example weathered basalt or burned pyroxenite). The streak is brownish red to red, as with hematite. Unlike hematite, it is never found as visible crystals, although it's sometimes a pseudomorph for example after pyrite.

If your metal detector has all metals mode ground balancing capability, magnetite balances pretty close to ferrite (in the high 80's to mid-90's on most machines). Maghemite however sounds like metal if you're balanced to ferrite, and balances at numbers typically in the 60's to high 70's.

Breaking the rock open with a sledgehammer (wear protective goggles!) will help resolve the mystery, but if you really like this rock you probably don't want to smash it up just to satisfy your curiosity.

--Dave J.

 

[Eu_citzen]

Dave, that is very possible. Maghemite has a brownish streak according to webmineral and other sites.

 

[woof!]

Red tropical and subtropical (and occasional temperate zone) soils are usually that color because of maghemite. Red soil horizons of cold climates are usually low in maghemite but contain some combination of hematite and/or amorphous ferric oxide. Agricultural soil analysis usually lumps all ferric together as "hematite" since to agriculturalists it doesn't make much difference as long as they've described separately the physical-textural properties of the soil profile.

Of course when it comes to magnetite, you've got Kiruna. I've occasionally wondered what was written on the runes there.

And then..... there's Kraja. I tell my co-workers they're my Swedish girlfriends, but they don't believe it. I don't suppose that Freya the goddess, Lisa the smooth one, Eva the sexy one, or Linnea the scientist believe it either.... long distance romances are so difficult!



"Lars Persson" was my favorite but copyright restrictions got it yanked.

Their stuff is disappearing from YouTube, so look quick. Here's another one that's survived the axe so far.



--Dave J.

 

[Eu_citzen]

Kiruna, yes. I'm pretty well aware of that area. Used to live not to far from there.
I even think I have a sample of their magnetite here, massive one. Not a crystal, though.

Personally I like Triakel, Ranarim and Euskefeurat. Try them if you enjoy Swedish music.
Gällivarevisor is pretty good to:


But in order to not Hijack this thread to much, if you enjoy talking shoot me a PM.