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Re: Aquatic Plants Digest V2 #577

> From: spushak at CCGATE_HAC.COM
> Date: Thu, 13 Mar 97 18:40:30 PST8
> Subject: Terrastone pottery clay analysis
> More info on Terrastone. Apparently it's not as rich in
> iron as we thought. Only 4%! anybody who can get on their
> web page, please send us whatever you can find out.
> How come everything is an oxide? Why isn't it classified
> according to the types of aluminum silicates etc.? What is
> LOI?

The analysis lists everthing as an oxide because this is a standard means
of reporting analyses.  It doesn't imply that the chemicals are present as
oxides.  It is relatively difficult to determine what the actual mineral
content is.

> 20 Kg Terrastone iron rich pottery clay
> Cost:     ~$18 Cdn for 20 Kg
> Address:  Plainsman Clays Ltd       Phone: (403)527-8535
>           Box 126                    FAX: (403)527-75086
>           Medicine Hat, Alberta
>           T1A 7M9   CANADA
> WWW: http://www.memlane.com/business/imc
> Chemical Analysis:
>  BaO     0.4%
>  CaO     0.5
>  K2O     3.2
>  MgO     2.5
>  Na2O    0.7
>  TiO2    0.7
>  Al2O3  16.1
>  P2O5    0.3
>  SiO2   71.5
>  Fe2O3   4.0
>  LOI     6.5

The analysis above contains too much SiO2 to be a pure clay.  It must
contain some free SiO2.  The SiO2 and some of the other constituents are
important to the way the clay responds when its fired.  Aside from the
SiO2, the composition suggests an illite clay.  Illite is a very common
clay mineral.  Following is an analysis of illite from "An Introduction to
Rock Forming Minerals"  (Deer, Howie and Zussman, 9th edition, 1976).

SiO2	56.91%
TiO2	 0.81
Al2O3	18.50
Fe2O3	 4.99
FeO	 0.26
MgO	 2.07
CaO	 1.59
Na2O	 0.43
K2O	 5.10
H2O+	 5.98
H2O-	 2.86

I believe that LOI stands for "Loss On Ignition", and represents the
amount of free water and other volatile components in the mix.  In the
illite analysis, the difference in water before heating (H2O+) and after
heating (H2O-) is partly comparable to LOI.

> Maybe there's something important about pottery clays that they have to
> contain a lot of silica in order to bond when fired. Even though this is
> considered iron rich for pottery clay, by the standards for laterite, it
> must be very low in iron. Maybe the quest for a low cost alternative to
> laterite needs to continue or we need to find a way to enrich the iron
> content of this clay.
> Steve

Chlorite is a common clay mineral with substantial reduced iron content.
It is found in soils and in many rocks, particularly shales, where it is
often mixed with illite.  Deer, Howie and Zussman list compositions with
12% to 38% reduced iron (FeO) and 0% to almost 9% oxidized iron (Fe2O3).
Despite the reduced nature of the iron and its rather high concentration,
I doubt that it would be readily available to plants.  The iron is rather
tightly held in a stable crystalline structure.  Also, I don't think that
chlorite is a commonly used pottery clay, possibly because the iron
content is so high.

Vermiculite can contain substantial amounts (say around 10%) of oxidized
(Fe2O3) iron, but little or no reduced iron and always less iron than
aluminum.  Again, this iron would be part of a stable crystalline
structure and may not be readily available to plants.

You know, the iron in laterite isn't chemically much different from

Roger Miller