Ametrine

Ametrine is a variety of quartz that contains both amethyst and citrine sectors in the same crystal. Both amethyst and citrine are colored by small amounts of iron (approx. 40 parts per million). Amethyst color develops when iron-containing quartz is exposed to ionizing radiation. In nature, gamma rays from the decay of potassium-40 are the most likely source of ionizing radiation. The model currently accepted is that radiation oxidizes Fe3+ to Fe4+. There is still uncertainty about the site of the iron. Both interstitial sites in the c-axis channels, and the silicon tetrahedral sites have been proposed as the site of the amethyst center. Citrine color is from Fe3+. The properties of the Fe3+ spectra suggest that the Fe3+ ions are aggregated and hydrated in clusters of unknown size.


Natural Occurrences

Bolivia

The only significant source of natural ametrine is the AnahÝ mine, in eastern Bolivia. The mine is operated by Minerales y Metales del Oriente, S.R.L., Santa Cruz, Bolivia, and employs about 70 workers at the mine site. It is the source of all the natural ametrine currently on the world's market. In the early days of production, there was much mis-information about the locality with Brazil and Uruguay frequently being mentioned as the source of ametrine.

Here is a map showing the approximate location of the ametrine mine.

The ametrine occurs in veins in a dolomitic limestone. Crystals of Bolivian ametrine range from 10 cm to 30 cm in length with diameters ranging from 4 to 12 cm. The interior of the crystals as seen in a slice of Bolivian ametrine show the typical sector zoning. Another slice of Bolivian ametrine shows the same sector zoning.

Inclusions of the fluid from which the crystals grew can be found in most of the crystals. A small gas bubble can be seen in the center of the largest (0.4 mm wide) fluid inclusion.

In a new area of the mine recently opened (1999) called Pozo Rico, a 4 x 3 x 1 meter cavern of crystals was discovered.  Here are two pictures of the large pocket provided by the company. Note the miner for scale:  picture 1,    picture 2.

Here are other images concerning the mining of ametrine in Bolivia.

The best quality production from the mine is used for faceted ametrine, amethyst, and citrine.  Some specimen material and ametrine slabs are also produced.  Mine run material is used for colorful carvings (49 K).
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India

Minor amounts of citrine occur in some amethyst from Hyderabad, India. The 5 cm wide crystal contains only a minor amount of citrine color. Citrine color is found in few specimens. In the magnified image the view across is about 2.5 cm.

Brazil

An occasional specimen of Brazilian bicolored quartz has been found which contains both amethyst and citrine zones. The magnified view shows that it does not approach the quality of the Bolivian material. The view is about 2.5 cm across.


Synthetic Ametrine

Synthetic ametrine is now produced in limited quantities in Russia. The original details of the development of bicolored quartz were reported by Balistky and Balistkya (1986). Dr. Balistky developed the process for producing synthetic ametrine at the Institute for Experimental Mineralogy in the Chernogolovka Science Center, Russia, northeast of Moscow.

Commercial production takes place in a factory at Alexandrov, Russia, in large hydrothermal vessels. The crystals are grown on racks of rectangular seed crystals. As initially grown, the crystals crystals do not have the amethyst color. It will be developed in a later step. The shape of the seed results in a final crystal which has a different morphology from the natural material. In synthetic ametrine, the colorless seed crystal can be seen in the center of the crystal running its length from left to right. The citrine is in the interior and amethyst is at the rim of the synthetic ametrine crystal. Here are slabs from a crystal which show the crystal as grown and after the outer zones are converted to amethyst with ionizing radiation from cobalt-60.  The interior of these crystals, seen in a slice of synthetic ametrine, show the sharp demarcation between colored zones.

Crystals, grown experimentally on a seed of a different orientation may have different arrangements of the colored sectors as seen in this slice looking down the c-axis.  This compares to the arrangement of colors seen in a slice looking perpendicular to the c-axis.

The crystals of synthetic ametrine are now available on the international market and are fabricated into carvings and faceted gemstones and jewelry items.


The color of ametrine

Citrine is a term applied to a variety of different quartzes that range from yellowish green to orange brown. These colors have a number of different origins. Some are due to changes brought about by exposure to naturally occuring gamma rays and some are due to minor differences in the chemical composition. The citrine color in Bolivian ametrine appears to come from the incorporation of very small aggregates of Fe3+, most likely in the form of a hydrous iron oxide. Microprobe analyses have shown that the orange-yellow citrine sectors have, on average, about 70 ppm of iron compared to the amethyst sectors that have iron concentrations that average from 20 to 40 ppm.

Amethyst color develops in a two step process. First, individual Fe3+ions replace Si4+ ions in the quartz structure. Quartz with only these ions is nearly colorless. To develop the amethyst color, the crystal must be exposed to ionizing radiation to oxidize the iron to the 4+ state. In nature, gamma rays from the naturally occurring isotope potassium 40 (40K) are probably the most important source of the radiation. Ametrine crystals found on the surface in sunlight have lost their color in the sectors which were originally amethyst. As is the case for all amethyst, the amethyst color center in ametrine is somewhat photosensitive and will be lost upon prolonged exposure to bright light.

The association of amethyst color to radiation is easily proven through the synthesis of amethyst in the laboratory. The development of amethyst color in synthetic ametrine after irradiation is a case in point.

The growth of ametrine in the laboratory demands that the oxidation state of iron be carefully controlled and also that the rate of growth falls within a critical set of conditions. Quartz can also be grown under different conditions which result in the incorporation of Fe2+.  In quartz, Fe2+ causes a green color.  It is even possible to prepare crystals with iron in all three oxidation states in a single crystal as this slice of a Russian synthetic crystal shows.


Literature References

The most complete reference to the Bolivian ametrine is:

Vasconcelos PM, Wenk HR and Rossman GR (1994) The AnahÝ Ametrine Mine, Bolivia. Gems & Gemology, 30, 4-23.

The bicolor in synthetic Russian ametrine is described in:

Balistky VS and Balitskaya OV (1986) The amethyst-citrine dichromatism in quartz and its origin. Physics and chemistry of Minerals 13, 415-421.

Commercial synthetic ametrine production is briefly described in:

Balitsky VS, Makhina IB, Mar'in AA, Dorogovin BA, Shigley JE, Rossman GR (1997) The first commercial synthetic ametrine from Russia ( Abstract ). 26th International Gemmological Conference, Idar-Oberstein, Germany, Sept.27-Oct.3, 1997.

A detailed description of the production and properties of synthetic ametrine is in:

Balitsky VS, Lu T, Rossman GR, Makhina IB, Mar'in AA, Shigley J, Elen S, Dorogovin GA (1999) Russian Synthetic Ametrine. Gems & Gemology 35, 122-134.

A more general technical discussion and review of color in quartz is found in:

Rossman GR (1994) Colored varieties of the silica minerals. Reviews in Mineralogy, 29, 433-468.


last revised: 27-Apr-2012