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> From: Stephen.Pushak at saudan_HAC.COM
> Subject: Laterite & Aluminum Silicate
> To: Aquatic-Plants at actwin_com
> Date: Tue, 11 Mar 1997 18:29:03 PST
> Cc: psears at nrn1_NRCan.gc.ca, jpp at inforamp_net
> First let me get my question out of the way; this is to the chemists:
> Aluminum silicates (of various forms) are one of the most common
> structures found in natural soils. There seems to be some uncertainty
> over whether tropical, volcanic laterite (of the type mined by Dupla)
> actually contains aluminum or more specifically aluminum silicates.
> (I suppose it's only uncertain to me, not to geologists ;-)
I'll respond as a geologist rather than a chemist.
> 1) How probable is it that aluminum silicates could be leached from
> soils under tropical conditions (i.e. mildly acidic water in large
It's quite likely. Lateritic soils are fairly common in tropical climates
and by definition much of the silica originally present in the soil has
leached. While the aluminum silicates are largely gone, the aluminum is
not. Aluminum and iron oxides, hydroxides and oxyhydroxides are the
dominant minerals in lateritic soils. Kaolinite and halloysite are the
only aluminum silicates (clays, in this case) normally found in laterites,
and they are relatively minor constituents. At their ultimate
development, there are no silicates (no clays) in laterite.
Clays are extremely common in most soils. Their near absence in laterites
is a distinguishing characteristic.
A laterite with no aluminum might exist somewhere on earth, but it
wouldn't develop from a volcanic soil. Volcanic material pretty much
always contains substantial amounts of aluminum.
> 2) How reactive are aluminum silicates in clay form in the substrate
> under low redox conditions and with possibly low pH (~6 pH) ?
Clays are relatively stable minerals under most near-surface conditions.
Not all clays are stable under all conditions, but I doubt that you would
experience many reactions. Low redox conditions are irrelevant, because
none of the structural components of clays are reduced by normal soil
processes. Stability at pH near 6 depends on other factors such as the
amount of silica, sodium and potassium in the water and the exact nature
of the clay in question. If a reaction does occur it would happen rather
slowly - probably on the scale of years to decades under aquarium
conditions. Laterites develop over periods of thousands of years.
> We can assume that regular clays and probably lateritic clays
> contain large proportions of aluminum silicates. Some material I've
> been given states that aluminum only becomes toxic (or soluble)
> below pH of 5.5 and that in the presence of organic matter, the free
> aluminum is rapidly complexed and de-toxified. This suggests that the
> whole issue of the presence of aluminum silicates in clays is
All clays are aluminum silicates with varying amounts of other components.
Laterites should not contain much clay. The aluminum will be present
largely as aluminum hydroxide (gibbsite and its relatives), which is one
of less soluble materials around. Dissolved aluminum in equilibrium
with gibbsite shouldn't exceed 100 parts per trillion at normal pH
levels. Reactions involving gibbsite probably would be rather slow,
so if you do get higher concentrations of aluminum in your water (from a
drop of condensate falling off the aluminum reflector in your light hood,
for instance) then it may remain elevated afterwords.
I'm a little unclear on the toxicity problem, myself. The US drinking
water standard (normally lower than toxicity-based environmental
standards) was 0.05 to 0.2 mg/l, effective 7/30/92 - orders of magnitude
higher than the levels of dissolved aluminum you are likely to get from
> Some other points:
> Sand, quartz, gravel are all primarily crystalline forms of aluminum
Quartz contains no aluminum. Usually sand is mostly quartz. But aside
from quartz, the vast majority of rock-forming minerals are aluminum
silicates, so most all gravels will contain some aluminum silicates.
> Glass is another form of aluminum silicate and glass vessels can be
> used to store some of the most concentrated and powerful acids without
Roger S. Miller