[Aztlan] Jiskairumoko, Lake Titicaca, Peru gold necklace

[Bruce Rogers]

Comment on the Jiskairumoko, Lake Titicaca, Peru gold necklace

Listeros,

First, geologists, mineralogists, petrologists, archaeologists, and 
gemologists may have different "field" definitions for some common 
terms.  One must consider this when giving a name to worked stone, 
precious and semi-precious stones, and gems found in an 
archaeological context. 

To many gemologists and archaeologists the term "turquoise" generally 
means a pale, robin's egg blue stone.  There is a suite of minerals 
and rocks that fit into this general term.

To mineralogists, and as defined in the Amer. Geol. Inst.'s Glossary 
of Geology, the term turquoise is specific to the mineral whose 
chemical make up is copper aluminum phosphate hydrate with 4 attached 
water molecules.  This mineral allows the substitution of iron for 
the aluminum, thus making a "solid solution" series of minerals 
ending in the total replacement of the aluminum with iron.  This 
mineral is the chalcosiderite mentioned by Lennert van Oorschot. 
(Trivia aside: "chaco" usually means copper while "sider" means iron 
in a mineral's chemical make up.)

The Jiskairumoko material is a pale greenish-blue to intense blue in 
color and is present as irregular veins and blebs in the pale 
pinkish-gray matrix.  This material fits the color and occurrence 
mode of turquoise. 

After looking closely at the several of the necklace photos on line, 
they all seem to have one important aspect in common.  The pale 
greenish-blue coloration is present as irregular veins and blebs 
(yes, "bleb" is a real live mineralogical term...) in a faintly 
pinkish gray matrix.  Irregular veins of turquoise and other copper 
minerals are often present in the high silica, grayish-colored rocks 
of the Andes - termed andesite by petrologists who study rocks (and, 
yes, it was named after the Andes). 

Turquoise is not a terribly common mineral; chalcosiderite is less 
common yet.  Another pale greenish-blue copper mineral, chrysocolla 
(hydrated acid copper silicate) is more common in the same 
geochemical environs as turquoise and chalcosiderite.  Chrysocolla is 
soft, brittle, and slowly dehydrates to a chalky, greenish-blue white 
color; not ideal for use as a gemstone.  In the huge copper deposits 
of the Andes chrysocolla is fairly widespread. 

The term lapis lazuli is correctly applied by petrologists (who study 
rocks) to a rock (not a mineral) that is composed mainly of the 
diagnostically deep "azure" blue mineral lazurite that has a complex 
chemical formula (sodium-calcium aliminum-silica oxygen-sulfur 
sulfate-chloride hydroxide).  This rock is composed of varying 
amounts of lazurite, calcite, and pyrite, sodalite feldspar, and 
other minor minerals.  The rock is very uncommon and is always a deep 
blue color (a diagnostic feature of this semi-precious stone).  (More 
trivia: minerals are inorganic, naturally occurring materials with a 
definite set of physical and chemical attributes; rocks are mixtures 
of minerals and have varying attributes.)

While my cursory observations on minimal resolution internet 
photographs don't preclude the necklace being composed of other 
minerals or rocks, it appears that the name turquoise -  or better 
yet turquoise-colored - material is a good description at present 
until more analytical work could be performed on the beads.  (Even 
more trivia: usually diagnostic identification of a mineral involves 
destruction of a tiny bit of the material; unfortunately the amount 
would be nearly a third of one of the beds . . . However, recent 
advances in Black Box technology allows for a non-destructive 
analysis . . . both requite more than a small bag of gold/silver coin 
to perform.)

As far as who brought the first turquoise to Mexico, be it from Peru 
or the American southwest, perhaps we shall never know. 

Cheers,
Bruce Rogers, earth scientist on a good, robin's-egg blue day