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[wildeone]

continued

Just had to get that off my chest...
PS -- As for the health risk to humans, silicosis is a respiratory disease resulting from the inhalation of crystalline silica. It typically occurs after decades of exposure (but can occur quickly in cases of massive infusion of freshly crushed silica), during which crystalline silica is trapped in the lungs, because the crystalline particles repeatedly injure the lining of the lung wall, and we mount an immune response (technically a nodular fibrosing pneumoconiosis, if anyone cares ). There is more to the disease than simple cutting, however. When crystalline silica is deposited in the lung, SiOH groups on the surface of the particle eventually form chemical bonds to membrane proteins and phospholipids, and these chemical reactions with the lung lining lead to denaturation of the membrane proteins and phosopholipids. During crushing of crystalline silica, surface free radicals are formed (half life is about 30 hours), and this makes the effects *much* worse in freshly ground silica than aged silica. That is why people who do sandblasting, mining or other rock work with quartz are at risk. Our fish, cukes and the like are generally not crushing the sand grains, they simply pass them through the gills or gut, and the particles are certainly aged by the time that you could get them (unless you're crushing your own rock to make sand ). Furthermore the surface is completely coated by a bio-organic film (primarily bacteria and their by-products) within hours to days of being introduced to seawater, and the surface of the particles are unlikely to ever come into direct contact with the gills or gut of the sand fauna for long enough to start the chemical reactions that are damaging to the tissues.

Addendum 4 Aug 2000

I fired that message off one morning without a lot of thought and never really intended it to become the reference article it has ended up being. After some pretty extensive discussions of this subject with a number of people (I especially thank Randy Holmes-Farley and Craig Bingman for their input), I wanted to post a clarification of the original comments.
First of all, I was speaking specifically of quartz sand in that post, and I must emphasize that the comments in it apply only to > 99% quartz sand that does not contain a significant amount of feldspar (or any other particularly soluble silica compound) contamination. If you plan to include a quartz sandbed in your tank, and are worried about possible diatom growth, it is advisable to get a chemical analysis from the company (many list this on their web pages), or get a good silica test kit and soak the sand for a couple of weeks in a little DI/RO water before testing it to see if there is any measurable amount of silica liberated by the sand. Randy and I discussed this at length in the Reefs.Org archives if you're interested in the nitty-gritty details.
Second, Craig Bingman has corrected me on a couple of points that I made in my original post. It turns out that the folks in the glass shop gave me 2 bad pieces of information: 1) that silicone rubber contains a lot of aluminosilicate (which is among the most soluble forms of silica) impurities from the manufacture, and 2) flat glass is simply melted quartz sand. Both of these are incorrect. The first may be true of certain cheap silicone sealants, but is apparently not true of anything that would be considered for use in aquaria, so that comment ought to be ignored. The second point turns out also to be untrue, and Craig tells me that almost all glass (which is really a supercooled liquid where the viscosity has become high enough for them to be mistaken for a solid) have other things dissolved in them to alter its physical properties. For example, most plate glass that is used by humans is soda lime glass (roughly 2/3 - 3/4 SiO2), which has sodium oxide and calcium oxide dissolved in the melt. These compounds lower the melting temperature of the glass, thereby making it easier to work with, but also make the glass much more soluble in water. So even though diatoms can pry silicic acid or silicate out of glass aquarium walls fairly easily, the same is not true of quartz sand grains. Craig's comment was that it may be possible for diatoms to get their silica requirement from quartz, but "quartz is the ultimate hard case for silica-loving animals, and it turns out to be much easier for critters to pull silicates out of the walls of the aquarium than out of a quartz sand bed." In fact, depending on the source of the aragonite added to an aquarium, a carbonate bed with a reasonable inclusion of biogenic opal (in the form of ancient diatom frustules and sponge spicules, etc.) could well provide more available silicic acid to the system than would a really clean, crystalline quartz substrate...
Craig is working on a detailed article on the chemistry of silica in aquariums, and in the next issue of Aquarium Frontiers, he will argue that silica is not the bugbear that we have all come to worry about. I would strongly suggest people ought to read that article to find out a lot more detail.
Anyhow, my personal feeling here is that the bottom line is still the same, but some of the details are a little fuzzy there (sorry folks) and I wanted to correct them for the record.

Bottom line, with only 5% of the weight being the Calcium Formate, and the silica sand seeming to be a limited issue due to the fact that it is insoluble, and the portland cement being used elswhere, I am going to go forward and use it.
I will let you know if anything ever comes up from it.