Black Sand

[mn90403]

I encounter two different areas of black sand.  One is on the California beaches.  The second would be washes and gold areas of the desert and mountains.

My last trip out in a wash there was lots and lots of black sand.  It latched on to a magnet that I have on my pick.  When I cleaned it off I put it in a bucket.  It is quite heavy.  The wash has gold in it but I didn't find any that day.  What is it made of?

Is it the same chemical composition as the black sand at the beach?  That sand is in salt water.

Neither location has 'rusty' black sand.  I know it is stabilized but I haven't asked AI.

What should I know about black sand and detectors?

[Norvic]

In the country I mainly detect "black sand" (iron mineralization) and gold are together, if I`m detecting on the slopes below the reefs/veins ie. eluvial detecting, the black sand is in the soil (eroded out of the reefs with the gold) at relatively shallow depths along with the gold, I cannot detect this ground in modes other than difficult with both the 6K and the Z.

In the creeks/gullies where sometimes the gold has washed to ie. alluvial, I can often detect in normal if there is depth as the black sand has washed out of the sand and is on the creeks bottom with the gold (because of its density compared to sand). Thus at times I get surprised at the depth the detector picks up gold in these creeks compared to the depth it achieves below the reefs. 

 Sea beach "black sand" I do not know if it`s the same stuff as I`m a "20 minute" beach detectorist.

 

[phrunt]

Our black sands are mostly magnetite, and near impossible to detect.  Some detectors do better than others.  There is gold mixed in with the black sands too and some do beach mining getting it out with large sluices.  Very fine gold though.  It's basically the nastiest ground I've ever detected, the Aussies thinking they've got terrible ground with their red soil have no idea what they're on about 🙂

In New Zealand the iron in the ironsands is mostly present in the mineral titanomagnetite (chemical formula Fe3 (Ti)O4), which is basically magnetite (an iron oxide) with a bit of titanium (Ti) thrown in.  The mineral, on its own has about 80% iron and 8-9% titanium.  However, the ironsands also contain a few other dark, iron-bearing minerals (hornblendes that give the sands their greenish hue, and pyroxenes), plus lighter coloured quartz and feldspar.  Overall, the sands contain from a few percent to 70% titanomagnetite grains.  The sands occur in modern and old (up to 2 million years) sand dunes and coastal deposits.  Transport and deposition of the sands by sea currents and winds produced large sand bars, or barriers that created the remarkably straight to slightly arcuate coastal profile along the west side of the North Island.

These ‘iron sands’ were created over millions of years, as our mountains slowly eroded, washing volcanic material into rivers and out to sea. Ocean currents then spread the sand north and south along the coastline forming deposits in nearly 20,000 km2 of the adjacent seabed.

While the black sand we see on land is striking, the deposits beneath the seafloor are far larger.  They lie in layers up to 30 metres deep, in varying concentrations along the West coast.

[mn90403]

Here is what Copilot says:

Desert black sand is magnetic because it contains magnetite, an iron oxide mineral, but it doesn't rust because the iron is chemically bound within the mineral structure and not exposed as free iron.
Here’s a deeper breakdown of what’s going on:
🧲 Why It’s Magnetic
•     Magnetite (Fe₃O₄) is the primary mineral responsible for the magnetic properties of black sand in deserts and beaches.
•     It’s a naturally occurring iron oxide with a strong magnetic charge, often enhanced by environmental factors like lightning strikes.
•     Magnetite is denser than typical sand grains, so it tends to concentrate in streaks or patches due to wind or water sorting.
🧪 Why It Doesn’t Rust
•     Rusting requires free iron (like in steel or iron filings) that can react with oxygen and moisture to form iron oxides like hematite or goethite—what we commonly call rust.
•     In magnetite and other iron-bearing minerals like olivine or pyroxene, iron atoms are locked into the crystal lattice of the mineral. This makes them chemically stable and not readily reactive with oxygen.
•     Even though magnetite is itself an iron oxide, it’s already in an oxidized state and doesn’t undergo further oxidation under normal surface conditions.

[Norvic]

There ya go now you have the answer from MSs AI. Aye the tin and gold alluvial miners lose a lot of value as the buying foundry/mint $ penalises them if the magnetite is prevalent in their concentrate, costs $s to remove. A wee like aluminium oxide that forms a thin tough outer layer that stops further oxidization thus our unpainted fishing tinnies can tackle sea water even. Now I`m diverging from the thread just a wee bit.

[mn90403]

We all have to 'deal' with it and it is somewhat unimportant until it is.  The Manticore, Algo and 7000 aren't too affected by it where I go but I wonder what it might be masking.  Detector companies have dealt with it rather than succumb to its properties.

I'll have to do more research to find out why it granulizes.  

[Erik Oostra]

6 hours ago, Norvic said:

Sea beach "black sand" I do not know if it`s the same stuff as I`m a "20 minute" beach detectorist.

The video is on Magnetic Island's pristine 'white' beach at Alma Bay.. Although it is white because of the coral sand it still has a lot of black sand in it.. This has been washed down from the hills where the eight old gold digging are.. The island was formed from molten granite that pushed through the Earth's crust about 250 million years ago, followed by more recent volcanic eruptions (200 million years ago) that formed gold-bearing quartz reefs/veins.. As you said the slopes down from them are hotter ground than the creeks they flow into.. The dry gullies up there also have a lot of black sand.. 

 

[geof_junk]

5 hours ago, phrunt said:

Our black sands are mostly magnetite, and near impossible to detect.  Some detectors do better than others. 

Simon in those condition I found no trouble when using the Old Garretts Deepseeker vintage 1979. It had a true TR ability to balance out the Magnetite when discriminating by ground balancing in discrimination mode with the discrimination knob. I did well in areas that had others completely stumped. 

[King-Of-Bling]

FWIW on So.Cal beaches and hunting them all my life :  most all our beaches have some form of blacksand or another. Up high and dry there can be little or no blacksand as indicated by my attached magnet in my scoop and it is specifically beach dependant. As I swing closer to the water where there is "medium" blacksand and dry , I'm less successful at finding gold jewelry. Yet my PI works smoother with more depth. When on a slope the blacksand up high can be literally black but also separated or layered in "bands" as I dig deeper. These bands can very greatly. They seem to be "supercharged" when saturated with saltwater , rendering some detectors useless. Using a PI is very difficult and requires patience and some adjustments and "tricks" I've learned. But these areas I've also been more successful finding Gold compared to less heavier blacksand. And the depth is less. It is weird but also the best way to describe it. The noise and falsing can be unbearable but doable with a PI compared to say a Manticore where the frustration level is off the charts and you got to quit. Waiting until these areas breakdown from surf activity are oftentimes  productive and easier to deal with when searching for Gold.