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Re: Laterite: where what and why (long post)

Hello fishfolks,

Time to add my $.02 to the laterite discussion (groan).  First a
disclaimer:  I have
not yet used "laterite" in my tanks and so have little experience on the
nuances of
their use in aquaria.  But, rocks and dirt are my business so here goes. 

One of the main reasons that the occurrence of laterite is so hard to
pin down is 
that it is one of those wastebasket terms that geoscientists have grown
to
HATE.  I quote:

	"The use of the term laterite and its derivative latosol bedevilled
soil 
nomenclature elsewhere in the world for many years and this word is now 
restricted in its use to features which result from iron and aluminum 
segregation."   (E.M. Bridges, "World Soils").

	To confound matters further, there are multiple soil classification
schemes 
used in different parts of the world.  The two most common now are the
U.S. Soil Taxonomy scheme and the FAO/UNESCO World Soil Legend.  For
good reasons 
related to details of soil structure and evolution, there is not a
one-to-one 
correspondence between the terms, but in general most soils that might
have 
been called laterite at one time are now in the Oxisol or Ultisol groups
(US 
Taxonomy) or Ferralsol/Nitosol groups (FAO/UNESCO).  

	Under the technical definition, only a soil material which has an
accumulation 
of sesquioxidic material (usually Fe and Al).  As others have posted,
laterite is often
part of a relict soil (formed during a past episode of soil genesis and
not in 
equilibrium with current conditions).  Laterite horizons form below the
ground 
surface, where groundwater movement concentrates Fe and Al oxides into a 
restricted layer.  Often, erosion strips off the original surface,
leaving the laterite 
exposed, where it dries and hardens irreversibly.  Thus the famous
Western 
Australia laterites are remnants of soil that developed in the Miocene
and Pliocene 
(23.7 to 1.6 million years before present)!

	So these soils are composed primarily of kaolinite and Fe/Al
sesquioxide clays,
and sometimes small amounts of quartz present as sand; the Ultisols also
typically
contain aluminum chlorite.  In terms of fertility they are low in
nutrients (they've been 
leached out).  They have a low cation-exchange capacity and a low
exchangeable 
base content.  Ergo they are weakly buffered, and only a little bit
acidic.  These are
the kinds of soils that, for agriculture, require liming to increase
CEC; adding
lime (commonly powdered dolomite) will also raise pH of the soil,
reducing the
mobility of both Al and Mn  (But Al toxicity is only really an issue at
pH less than 6.0).

	A look at the world soils map shows large extents of Oxisols in the
tropics,
as you'd expect  (Amazonia, central and southern Africa) and Ultisols in
the subtropics
(SE USA including part of Texas), Chile, Uruguay, Uganda and
Zaire,India, SE Asia
and Oceania.  I have samples of relict laterite from southern Oklahoma,
West Africa,
and the east African highlands.  There are known relict Oxisols in
Australia,  Hawaii 
and Puerto Rico and presumably elsewhere.  Some 	of the most Fe-rich are
formed 
from basic igneous rocks (eg basalt) under tropical conditions.   But
not all of these 
are necessarily suitable for aquaria.

	There are other Fe-rich soils that are not technically lateritic, for
example the
"B" horizon of Spodosols (soils which typically form from glacial
sediments
and are found in association with boreal forests).  These soils often
have the same
sesquioxide nodules as laterites, but the Fe is confined to the B
horizon and sometimes
found in combination with high levels of humus.  Spodosols are very
acidic (as low
as 3.5!!!), and grow much better timber and blueberries than they do
major food crops.  Spodosols are common from the Great Lakes to the
maritime regions of North 
America, in Scandinavia and across northern Asia.  

	OK, switching from hard data to opinion here:

	I would not hesitate to collect "wild" laterite (and since I live where
there is
some, I think I will do).  I would make sure that a) it is a legal
collection, ie not a
national park or such; and b) that I knew what the land use of the
collection area
had been.  Some of these clays do readily adsorb pesticides, herbicides
and 
organic compounds including hydrocarbons, and they can release these 
compounds under certain conditions.

	So I would not collect in a recent agricultural area or from a yard
where 
turfgrass is grown in the typical (chemically-intensive) manner. 
Finally, t'were it
me, if the stuff passed the above tests I would still vet a sample in an
isolation 
tank with a non-precious plant and a hardy fish or two before risking my
main
tanks. YMMV.

	Oh, one last thing that I cannot let pass:  

	Glass does not usually contain aluminum.  It is made of amorphous SiO2, 
silicon dioxide that's been melted and cooled quickly so crystals don't
form.  
Source material for the SiO2 is often a very pure, white sandstone. 
IOW, sand. 
Some types of lab glass contain boron, and crystal and art glass often
contains traces 
of metal ions (e.g. gold,lead), but I know of no glass that contains
even trace aluminum.


Pax vobiscum,
Pat (jellison at ceps_nasm.edu)
(Who will go back to her "real" soil work now   ;-}   )

References:	

  Bridges, E.M., 1978 "World Soils", 2nd ed., Cambridge Univ. Press,
Cambridge, 128 pp.
  Fairbridge, R.W. and Finkl, C.W. Jr., 1979 "The encyclopedia of soil
science, part 1",
	Dowden, Hutchinson and Ross Inc, Stroudsburg, 646 pp.
  Hausenbuiller, R.L ., 1985 "Soil Science: Principles and Practice",
Wm.C.Brown
	Publishers, Dubuque, 610 pp.
  Lamey, C.A., 1966, "Metallic and Industrial Mineral Deposits",
McGraw-Hill, New
	York, 567 pp.


Aquatic-Plants-Owner at ActWin_com wrote:
> 
> 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 ;-)
> 
> 1) How probable is it that aluminum silicates could be leached from
> soils under tropical conditions (i.e. mildly acidic water in large
> quantities)?
> 
> 2) How reactive are aluminum silicates in clay form in the substrate
> under low redox conditions and with possibly low pH (~6 pH) ?
> 
> > From: "James Purchase" <jpp at inforamp_net>
> 
> > Iron and aluminium are two metals which seem to remain in clays which have
> > been subjected to extended weathering over long periods of time. That is
> > why a lot of laterite has high concentrations of both.
> 
> 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
> unimportant.
> 
> Some other points:
> 
> Sand, quartz, gravel are all primarily crystalline forms of 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
> contamination.
> 
> Steve
>