[Aztlan] Changes in surface appearance of stone

[Bruce Rogers]

Listeros,

A few thoughts about surface changes in stone from a geologists point of view.

I believe the word most applicable might be "alteration."  This is a 
general term only indicating something has happened the material in 
question.  There can be several "flavors" of surface or limited 
penetration alteration applicable to archaeological materials such as 
onyx and greenstone.  As Elaine Schele pointed out in her Oct. 16 
post, the terms hydration, hydrolysis, and oxidation are generally 
all processes of surface or shallowly penetrating alteration.

Oxidation usually takes place on/in metals as the free elements 
combine with oxygen to form more stable oxide materials.  Hydration 
usually occurs in silica materials in which water molecules are 
incorporate into the surface layers of the original material, but can 
also occur in metallic objects.  Hydrolysis usually occurs in silica 
materials wherein the chemical reaction produces silicic acid and the 
entire original material disintegrates into other materials or is 
dissolved away.

As to what happens with onyx, first one must define what kind of 
material one is calling "onyx."  Classically, onyx is a variety of 
silicon dioxide - the same material, chemically, as quartz.  However, 
gemologists and stone workers also call travertine - a variety of 
calcite/limestone/marble - "onyx."  Usually it is preceded by a 
geographical qualifier such as Mexican onyx, thus alerting one to the 
possibility that it may a material different than the original 
material the name was applied to.  Most semi-precious gems and stones 
with such a geographical modifier are, indeed, not what they purport 
to be, but a similar appearing material.  "Bohemian Ruby," for 
instance, is usually pyrope garnet, a semi-precious, transparent 
gemstone with a pleasingly deep red to purplish red color.

If the material one is calling onyx is silicon dioxide, then one must 
realize it could be one of several varieties of the chemical 
compound.  Silicon dioxide comes in several "flavors," each with a 
slightly different crystal structure.  The more tidy the structure 
the more "ordered" the variety of silicon dioxide.

Amorphous silica is the most disordered material (and is not really 
amorphous; it's just that the crystallites in that particular 
material are smaller than the length of an x-ray wave used in the 
X-ray diffraction process that most materials are identified by). 
This is also called natural glass or lechatelierite, and is a common 
component of obsidian, a natural silica glass.

The next most "ordered" compound is opal.  Opal comes in four 
flavors: opal, opal a, opal a', and opal C-T.  These names indicate 
the increasingly ordered structure of the material.  The structure of 
opal is a stack of tiny spheres that reflect light, thus giving them 
their characteristic opalescence.  Reflective clothing, reflective 
paint, and reflective highway signs all have a thin film of silicon 
dioxide spheres similar those of opals and thus reflect light.

Quartz is next on the scale of more ordered compounds.  It has a 
great number of varieties, mostly based on color, which are, in turn, 
based on included materials.  Stishovite is a special high-density 
variety formed by very extreme pressures at meteor impact sites.

Finally there are several very high-temperature varieties, coesite 
(stable at about 700 degrees C, but only formed under extreme 
pressure), tridymite (stable between 870 and 1470 degrees C), and 
cristobalite (stable above 1470 degrees C).

Obsidian or natural glass will, over time, absorb water and hydrate. 
One can measure the thickness of this hydration rind and assign an 
age to the piece under study.  The rate of hydration varies with 
climate, rain/snow fall and several other factors so is particular to 
a general area.  Hydration rates in the Pacific Northwest, for 
instance, are different than those in central Sonora State in Mexico.

Thus one can usefully use the term hydration for objects made of onyx 
if their surface sheen is altered from their original.  Changes in 
opalescence, dullness, color changes, or other changes can be covered 
under this process.

Greenstone is, in the archaeological world, a natural material 
composed of a collection of greenish materials that can be either 
minerals or rocks.  These can be composed of nearly anything from 
jade to chlorite mica schist, but in a geological sense these are 
rocks of greenish (usually) silica minerals.  These rocks are rather 
high-grade metamorphic rocks composed of greenish minerals such as 
chlorite mica, actinolite (one of several green, needle-like 
minerals), or pyroxene and/or epidote.  Since all of these minerals 
include iron, the alteration process changing the color or luster of 
these materials usually is oxidation of the iron in the original 
minerals to more stable oxide forms such as the mineral goethite 
(commonly called "rust"...).

Dave Hixson's comment of a material developing a patination is also a 
good, over-all term for such a change in archaeological materials, 
but does not indicate what sort of change has taken place.  The term 
corrosion suggested by Miguel Covarrubias is usually applied to 
metals or, perhaps to a lesser degree, building stone, but involves a 
significant amount of material removal, thus altering the color, 
luster, and actual mass of an object.

Cheers,
Bruce Rogers, earth scientist on a good day