Page 264 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

stone: oriental and occidental turquoise. The names are associated with either the source of the
stone or its quality, according to Palache, Berman, and Frondel (1951).
The name turquoise is thought to be derived from the word Turkish, possibly because the
stone was originally imported into Europe from Persia through Turkey. In antiquity turquoise
of very fine quality was obtained from the southern slopes of the Ali-Mirsa-Kuh Mountains,
northwest of Maden in the Khorasan province of present-day Iran. Minerals that are thought
4
to refer to turquoise were called "chalchihuitl" by the Aztecs and "calláis" and "callaina" by
Pliny, who records the following observations on this semiprecious gem:

Subsequently the stone is shaped by the drill, being in other respects an easy stone to deal
with. The best stones have the colour of emerald (smaragdus), so that it is obvious, after all,
that their attractiveness is not their own. They are enhanced by being set in gold, and no
gem sets off gold so well. The finer specimens lose their colour f they are touched by oil,
i
unguents or even undiluted wine, whereas the less valuable ones preserve it more stead­
fastly. No gemstone is more easily counterfeited by means of imitations in glass. 5

The chemistry and The turquoise group of minerals is actually quite complex
mineralogy of turquoise and was recently reevaluated by Foord and Taggart (i998).
The general formula for these minerals can be expressed as
^0-i^6(PO 4 ) 4 _ JC (PO 3 OH) x (OH) 8 -4H 2 O, where A and Β are different cationic species and χ
ranges from 0 to 2. The group comprises six members: planerite, turquoise, faustite, ahey-
lite, chalcosiderite, and an unnamed ferrous-ferric analogue. Planerite has the formula
Z 1 Al 6 (P0 4 ) 2 (P0 3 OH) 2 (OH) 2 (OH) 8 -4H 2 0, where X may represent a variety of elements such
as copper or iron. It was first described in 1862 and has been reaffirmed as a valid mineral species
after more than a century of uncertainty. A complete solid solution is thought to exist between
turquoise and planerite. Foord and Taggart state that many occurrences of turquoise described
in earlier literature are, in fact, of planerite. There also appears to be a significant solid solution
between turquoise, CuAl 6 (P0 4 )(OH) 8 -4H 2 0, and chalcosiderite, CuFe 6 (P0 4 )(OH) 8 -4H 2 0,
and between turquoise and faustite, (Zn,Cu)Al 6 (P0 4 ) 4 (OH) 8 -4H 2 0. The name "planerite"
hardly has the romantic cachet of "turquoise," so redolent of a rare and precious material, and
the name is unlikely to be changed any time in the near future. Nonetheless, the revision of this
mineral group is potentially of considerable interest to future studies of ancient artifacts. Some
members of the group, such as aheylite, Fe Al (P0 4 ) 4 (OH) 8 -4H 2 0, do not contain any cop­
2 +
6
per at all in their pure or uncorrupted forms, although the analyses listed by Foord and Taggart
show that aheylite may contain some zinc. 6
Minerals of the turquoise group are found in the ferruginous gossan of ore deposits.
Turquoise usually has a massive habit, often with conchoidal fracture, and with a Mohs hard­
ness of 5- 6. The specific gravity is 2.6-2.8. The luster of turquoise may be vitreous or waxy; the

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