Black sand vs White sand

[Moontanman]

I like black sand in my aquariums, it is not only expensive but it has about 1/2 the volume by weight than white sand. the black sand cannot contain metals because it's safe to use with marine invertebrates which are very sensitive to metal poisoning. White sand is primarily SiO2, what is black sand made of?

 

I may have found the answer to this....

 

Slica sand "SiO2" specific gravity 1.6

 

Ilmenite Sand "TiO2, Fe2O3, Cr2O3, MnO" specific gravity 4.6

 

Works for me....

[Ophiolite]

Silica has a specific gravity of 2.65. I don't know where you got the lower figure from.

 

Black sands that I am familiar with are generally formed from ferro-magnesian minerals: olivines, pyroxenes, or amphiboles. I don't know if they are the typical source for black acquarium sand.

[Moontanman]

You are thinking of quartz which has a specific gravity of 2.65, dry sand has a sp of 1.6, wet sand is 1.9

 

http://wiki.answers.com/Q/What_is_the_specific_gravity_of_sand

 

What is the specific gravity of the materials you suggested if they are in sugar sized sand particles? The black aquarium sand is at least twice as dense as silica sand by my estimate.

 

The black aquarium sand is sold as Tahitian Moon Sand, i my doubts about it coming from Tahiti or the Moon, volcanic sand can be black but it should have a similar specific gravity to silica sand...

Edited August 22, 2010 by Moontanman

[Ophiolite]

You are thinking of quartz which has a specific gravity of 2.65, dry sand has a sp of 1.6, wet sand is 1.9

Quartz is silica. You cannot really talk about the specific gravity of sand, but only of the sand grains. We have grain density (or specific gravity) and bulk density. The bulk density depends on the graind density and the porosity of the sand. The latter depends upon the shape, angularity, size and sorting of the grains. For an uncemented sand this could vary from below 20% up to about 40% (for cubic packing of equisized spheres).

 

What is the specific gravity of the materials you suggested if they are in sugar sized sand particles?

All these figures are off the top of my head. Olivine is about 3.4. Pyroxene and amphiboles can have more complex compositions and hence a wider range of densities. I think these can range from 3.0 up to 3.9, but the more typical values will be in the 3.2 to 3.5 range of the olivine. Again, please note, these are the grain densities, since you cannot apply specific gravity in a meaningful way to a loose sand.

 

The black aquarium sand is at least twice as dense as silica sand by my estimate.

As you can see the grain density is about 30% greater. However these ferromagnesian minerals are generally platy, so that they will pack more efficiently, yielding a lower porosity. Your estimate of double the bulk density would be entirely consistent with that.

 

The only black sand I can specifically recall is an olivine (?) sand on a Hawaiian beach. A little checking reveals that there are black sand beaches on Tahiti, at the site where both Captain Cook and Captain Bliigh alighted. So you may have a little piece of South Sea history.

[Moontanman]

I sent a message to the company who makes Tahitian Moon Sand and got this back....

 

 

TM is basically man made obsidian. This means it is very high in iron and other minerals. These give it its color. TM is does not contribute these minerals very readily as they are fixed at high heat and therefore not soluble.

 

So now I'm back to why is it so much heavier per volume than regular silica sand.... since obsidian is silica or volcanic glass....

 

The density of sand is figured by a unit volume not by individual grains, probably so the weight of truck loads can be figured

Edited August 23, 2010 by Moontanman

[Ophiolite]

The density of sand is figured by a unit volume not by individual grains, probably so the weight of truck loads can be figured

Understood, but the practice of calling this a specific gravity rather than a bulk density is simply wrong.

 

If the obsidian is from a basic igneous rock then the absence of free quartz, the preponderance of ferromagnesian minerals and the absence of any significant porosity in any of the individual grains would give you a fairly high density. I believe 3.0 gm/cc is typical.