Odd Beach Find?

[Kevin Walenciak]

I posted this over in the meteorite forum, but after some research I wonder if the mystery rock I found is Limonite.  I found this at a Fort Lauderdale beach yesterday, pretty far away from the water at a volleyball court.  It was about 10 inches down in the sand, and my PI metal detector locked on to this pretty easily.  I have a Pro Find 35 pinpointer, which makes a different sound for ferrous items, which it does on this rock.  It weighs about 76 grams, seems unusually heavy for its size, and a magnet does stick to it.  Before I send it anywhere for analysis, I was wondering if anyone here might have any thoughts?  Possible Limonite?

 

 

[oldmancoyote1]

It could be a hunk of deep rust off a large iron object.  Perhaps it has an iron core.  You could crack it open to see.

[Jeff McClendon]

Hi,

Limonite is a mineral that forms when an iron rich mineral like magnetite, hematite, marcasite, pyrite and even some iron rich garnets begin to decompose through exposure to water and air. There are several more iron rich minerals that can decompose that are more obscure. Generally there is still some of the original iron mineral left towards the inner parts of the specimen.  The only one of the five above that is strongly magnetic and will easily set off your detector and pinpointer is magnetite. So, if that is an actual mineral specimen it is probably limonite and magnetite. It would be pretty odd to find a magnetite specimen in your area since magnetite is usually associated with higher temperature igneous/metamorphic rock and mineral formation and not low temperature sedimentation. Also, it does not have the same outer appearance as the meteorites I am familiar with. They are usually much more compact and hard in appearance without so many uneven and eroded surfaces as the specimen you found. That makes oldmancoyote1's guess a very good one. Limonite looks a lot like rust on man-made iron and the two have similar characteristics in other ways too.

 

Jeff

[Steve Herschbach]

It is near impossible to tell from a photo but it looks like decomposed sulphides, typical of gossans.

[Jim Hemmingway]

Hi Kevin… I think that oldmancoyote and Jeff have probably nailed down the more likely alternatives for you, given the location. Its moderate depth in Florida sand  leads me to suspect that it is probably a manmade ferrous object that may have reached that depth over some number of years with the ensuing obvious corrosion. 

As Steve mentions, it is difficult to assess just what types of corrosion products exist on your sample. Iron metal corrodes to rust and manmade rust is a form of iron oxide called maghemite, which frequently does react to metal detectors depending on its physical characteristics, the ground conditions, the type of metal detector, and how it is operated. I very much doubt that a chunk of magnetite would naturally occur in your area.

As mentioned above, limonite is a generic term describing a number of hydrated (containing water) iron oxides that can present themselves in different ways and in vastly different environments. 

A good example is to recall driving along a highway that has been blasted / cut through pre-existing igneous rock that could be comprised of any one of several different types of rocks. Iron is a very abundant element and in all probability does exist within such rocks. Upon exposure to weathering, even trace amounts of ferrous material will oxidize to form a natural rust on the exposed surface of those rocks. 

This is a good example of why limonite is described in mineralogical texts as “Nature’s Rust”. It is typically the goethite constituent that produces generally brown soils in temperate regions around the globe.

My limonite samples produce no effect on a PI metal detector and very little on a VLF detector. It exhibits relatively little magnetic susceptibility and incidentally can be characterized as having a fairly low magnetic loss angle, thus permitting it to occupy a somewhat similar area to magnetite on VLF ground phase readouts. But it simply has no ferromagnetic strength that is comparable to magnetite or maghemite. Hence, I rather do think something metallic within your sample has produced the signal.

The photo below depicts another commonplace example of what is described as amorphous (without shape) limonite. The F75 GB45 compensation point should ensure that all iron oxides produce only a negative threshold signal and that is the case. This sample does not react to my TDI Pro at the sensitive 10 usec pulse delay, small mono coil, full gain, and with ground balance turned OFF.

 

[oldmancoyote1]

Jeff's comment reminds me of my youth when I hiked around and into Arizona's Meteor Crater.  Back then you could find hand-sized pieces of hard compact meteoritic rust called "shale balls".  I really don't understand how they were created, but they were fascinating.  You could actually find small blebs of melted meteorite within them.  A guy I knew was finding diamonds by sawing them with a diamond saw.  When the saw stopped cutting, he knew that he had found one.  All of which is irrelevant to this discussion, but interesting.

[BeachHunter]

A diamond saw blade in trade for a diamond. Awesome trade!