[Aztlan] Mayapan clay

[Bruce Rogers]

Listeros,

In reply to Brad Russell's recent questions about 
offerings of clay samples found in his Mayapan 
work, the following is a quick and dirty (pun 
intended) pocket guide to clay.


Clay - a few comments about a sticky subject

Clay forms the finest of sediments. To paraphrase 
the Glossary of Geology, clay is a natural 
material with a very fine texture, usually 
plastic when damp to wet, and hard and  compact 
when dry.  Clay's "stickiness" comes as a result 
of both its large surface area that allows very 
weak chemical forces to act over a large surface 
and also as a result of  (to be tactful) 
bacterial action by-products.  Its particles are 
less than 1/256 millimeter in diameter (that's 
about 0.ooo16 inches in diameter or 1/5 of the 
thickness of a sheet of paper).  Clay is the 
result of weathering of larger particles of 
minerals and/or rocks, be they other clays, silt, 
sand, boulders, or mountains. Most clays are made 
up of parallel sheets of silica minerals 
("phyllosilicates"), except the 
Palygorskite-Sepiolite group, which has a 
structure of alternating tooth-shaped and sheeted 
structures ("chain silicates").

There are seven large groups of clay minerals. 
These seven groups are: kaolinite-serpentine, 
often the result of feldspar weathering, 
pyrophyllite-talc, made of metamorphic minerals, 
monmorillonite-smectite-saponite, the 
"shrink-and-swell" clays; vermiculite, made of 
altered mica minerals; mica, as very fine 
particles; brittle mica, made of a few, more 
brittle minerals closely related to the true 
micas; and chlorite, made of several 
greenish-colored minerals.

The limestone making up the Yukatán Peninsula and 
Campeche Bank (the northern and eastern, larger, 
submerged part of the structure) is fairly pure 
with only a little original sandy or clayey 
sediment.  Over the last hundred million years or 
so, vast banks of limestone were deposited in 
shallow seas.  Occasionally throughout this time, 
the banks were elevated-or sea level dropped-and 
semi-arid conditions allowed saline deposits of 
salt and anhydrite to form.  As frosting on the 
cake, over the last 24 million of years or so, 
between 8,000 and 10,000 m (25,000-35,000 ft.) of 
limy sediment capped the platform and adjacent 
ocean slopes and basins.  To the west, toward 
Tabasco State, deep basins filled with shale and 
sandstone, and the occasional volcanic ash.  Dead 
things sank to the bottom, decayed, were buried, 
and cooked, leading to large oil and gas reserves 
in the Bay of Campeche and, indeed, bordering the 
entire Gulf of Mexico.

Above the older Cretaceous Period (144 million to 
65 million years old) carbonate rocks is the 
Chichen Itza Formation (Eocene age; 45 to 36 
million years old) and above it, the Carillo 
Puerto Formation (11 to 5.5 million years old). 
Above these rocks are unnamed limestone units of 
"Carillo Puerto-type" that apparently are between 
5 million and 220,000 years old.  Along the coast 
is a belt of narrow ridges made up of calcareous 
"sand" dunes dating to the last high sea level 
stand between 125,000 and 120,000 years ago; the 
base of these rocks lie on a pre-middle 
Pleistocene (ca. 85,000 years old) land surface 
marking the last low sea stand.  Topping this 
pile are even younger "beach ridges" of 
calcareous sand dating to only 3,000 yeas ago.

When these several formations weather, most of 
the limestone dissolves (= hard water) and is 
eventually either transported into the sea in 
solution or sometimes deposited as speleothems in 
caves (stalactites, stalagmites, etc.).  This 
solution leaves behind only a small amount of 
residue.  This is mostly clay, very fine sand, 
and a few other minerals such as gypsum 
(essentially anhydrite with incorporated water 
molecules; you are most familiar with this as 
plaster of Paris or the wallboard in your home or 
office).

The limestone at Mayapan (actually at Xocchel 
["Rainbow Shark"], some 40 km north), has been 
described as light-colored, microcrystalline to 
coarse-grained matrix with many mollusk and 
foraminifer fossils and some phosphate mineral 
pellets.  Limestone weathers to a white-colored, 
powdery, tufa-like texture...in other words, it 
dissolves easily; leaving little residue.

The clay-rich soils one usually finds in 
limestone terrains are varied depending on 
whether or not they are simply "leftovers" from 
solution of the limestone or have some added 
material brought in from streams or the ocean. 
Most clays identified from karst regions 
(limestone terrains shot through with cenotes, 
caves, and such) are mostly made of  kaolinite or 
montmorillonite/smectite, plus a few percent of 
hydrated iron oxide minerals (also called 
"rust").  Locally, where the ground water has a 
high acidity (low pH), the clay of choice is 
attapulgite/palygorskite.  This is the clay 
famous for its use in creating Maya Blue paint. 
Most of the limestone residual clays are white, 
cream, or slightly yellow in color.  Those 
ocherous red are stained with a small amount of 
goethite or other similar minerals.  One does not 
need much iron oxide to color something.  A 
friend told me of a new employee in a glass 
factory he worked in who threw a full shovel-full 
of iron oxide into 5 tons of glass, hoping to 
make a lovely light brown hue.  The amount he 
should have used was about a pint at most; the 
glass came out nearly black.

Since most of the Yukatán has only been drained 
of seawater over the last 10,000 to 7,000 years 
since seal level rose and stabilization, the 
soils, and thus clays, are pretty recent in age. 
Mary Hopkins' off the cuff comment about clay 
stability is well taken.  As far as stability is 
concerned, clays are fairly stable over geologic 
time.  Hot water solutions, pressure and 
temperature-driven metamorphism, and a few other 
oddball environmental conditions can alter clays, 
but these conditions are largely absent in the 
Yukatán.  Hence they will be stable over a 
human-oriented time span.

The following is a nice little website explaining a bit about weathering:

<http://geology.csupomona.edu/drjessey/class/Gsc101/Weathering.html>


Cheers,
  Bruce Rogers, earth scientist on a good day