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Feldspar -> clay (kaolin, bentonite, etc.) -> laterite

The geological processes that create laterite may follow several
different paths. IDK.

One I'm a bit familiar with is the sequences partially on the way to
laterite. Feldspar, making up more than 50% of the earth's crust is an
alumino silicate also containing calcium, sodium or potassium.
Geothermal action can remove the other materials to leave an aluminum
silicate, which develops clay-like properties depending on the water of
hydration in the final crystalline (or platelet) forms. White kaolin
china clay is a particularly pure form. [Bentonite usually goes via
volcanic action and the conversion of glassy lavas.] 

Nature, being messy, often leaves other "stuff" behind, too, like iron
oxides or sulfides, etc. Many red clays used for pottery and adobe fit
that area. they vary in their "fluxing" ability (how high firing is
required to fuse the particles together) and the final color of the
fired ceramic. The fluxes are often lower-melting point compounds that
can get the aluminum silicate partially into solution at high
temperatures, without allowing complete meltdown. The cooled product
then is nearly totally insoluble and withstands years of use in the
kitchen. "High Fire" ceramics also render lead oxide, etc. insoluble,
too, so they are safer for eating utensils.

Clay particles are easily washed out of the hills where they were formed
around geothermal springs. They become (contaminated) sediments in the
bottom of canyons and river valleys. Rain then can begin the long
ultra-slow leeching process, whereby anything soluble is carried out to
sea. In rain-forest (tropical rainy) regions, the soil quickly becomes
infertile and nearly inert as all the solubles are grabbed by flowing
rain water. Slowly, the silicates can even be reduce to oxides as SiO2
is gradually dissolved out of them. That's the process that possibly
takes thousands of years. Silicon dioxide (pure amorphous quartz glass)
isn't very soluble, even in highly corrosive pure distilled water.

[This last paragraph is mostly conjecture on my part, while the earlier
ones actually have some basis in my experience.]

Firing any ordinary clay to form a true ceramic (even if crushed later)
probably ruins it for what we want. The porosity probably goes down like
mad. [Partial firing, at lower safe levels, is what I suspect is done
with "Profile" and similar products.]

I have had excellent results using red potter's clay, baked at a way low
temp. compared to normal firing.

By rolling long cylinders and cutting them into 1/2" long x 1/2"
diameter plugs, I could bake them to a semi-insoluble state in the oven
at about 350F. I imagined a 2" square grid on the floor of a 55G tank.
Placing one plug at each intersection, I covered the bottom. Gravel to
just cover, was added, then a thin layer of peat. A final thick gravel
layer built the substrate up to about 3". I sloped it a little from
front to back, so the front was about 2" deep.

That tank has been moved twice, and the plants still go crazy in it
after 3 or 4 years. When I root through the substrate, the plugs are a
ball of slimey clay, but uprooting plants, etc., has never given me a
cloudy tank. I added too much peat, so I got some noxious gas burps
during the first year or so. [*Never* add more than 5% of your substrate
as organics. :-)]

I use less light (80W) than many of you, and have slowed plant growth by
dropping CO2 injection during the past couple of years. Nevertheless, it
is a way to go that's pretty easy and gave me good results.


Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot com

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