New Mexico Mineral Symposium — Abstracts

Turquoise is one of those semi-precious gemstones that has never really been explored for, but instead almost always found as a secondary mineral of copper deposits. However, in some cases it occurs by itself for unexplained reasons in a non-copper deposit environment, such as at Cerrillos and Orogrande, New Mexico, northern Nevada, and Cripple Creek, Colorado.

When you look at the mineralogy and chemistry, turquoise becomes a gem stone possessing many properties based mostly on its chemistry, which in turn is responsible for all the varying hues of color that commonly occur in turquoise. The question becomes whether a true turquoise actually exists having a composition of: CuAl₆(PO₄)₄(OH)₈.4H₂O.

Turquoise forms a chemical series with the mutual substitution of Fe³⁺ for Al³⁺ forming chalcosiderite at the far end of the series with a composition of CuFe₆(PO₄)₄(OH)₈.4H₂O. There are four minerals in the middle of this group that are all hydrated phosphates and vary only by the amounts of Cu, Fe²⁺, Zn, and Ca, which can all substitute for Cu and still produce colors associated with "turquoise."

Most turquoise is deficient in Cu, and only in rare instances do materials called or labeled "turquoise" have Cu as the principal constituent in the divalent state. The x-ray diffraction data for all of the minerals in the turquoise group have almost identical d spacing, which makes identification difficult without corresponding chemical analysis. Because most samples of turquoise are cation deficient, the probability of a true turquoise mineral occurring in the western U.S. is doubtful.

With the popularity of turquoise as a jewelry stone and the lack of any real, untreated natural turquoise, many associated minerals and facsimile materials have entered the market place. Because of this, turquoise has had its mineralogy and popularity undermined by the introduction of treatments, imitations, and synthetics. Some of these facsimiles are difficult to detect even by the experts. Other minerals, which are sometimes called "turquoise," such as chrysocolla, variscite, and gaspeite, are attractive semiprecious stones in their own right. Gaspeite in particular is often more costly than turquoise. Over 60% of all turquoise currently being mined in the U.S. is stabilized to some degree with pricing increasing significantly for untreated natural turquoise.