Ametrine

Ametrine, also known as trystine, golden amethyst, or by the trade name bolivianite, is a variety of quartz with alternating zones of purple and yellow-orange coloration. Its name is a portmanteau of amethyst and citrine. While ametrine is commonly referred to as a combination of these two quartz varieties, some sources claim this is not accurate. Almost all commercially available ametrine is mined in Bolivia.

Ametrine

General

Category Tectosilicates, quartz variety

Formula Silica (silicon dioxide, SiO2)

Crystal system Trigonal

Identification

Formula mass 60.08 g/mol

Color Purple, Yellow to Orange

Crystal habit 6-sided prism ending in 6-sided pyramid (typical)

Twinning Dauphine law and Brazil law

Cleavage none

Fracture Conchoidal

Mohs scale hardness 7

Lustre Vitreous

Streak white

Diaphaneity Transparent to translucent

Specific gravity 2.65

Optical properties Unixal (+)

Refractive index nω = 1.543–1.553

nε = 1.552–1.554

Birefringence +0.009

Pleochroism Amethyst section shows weak to moderate purple/reddish purple

"Citrine" section shows very weak yellow/orange 

Melting point 1650±75 °C

Diagnostic features Distinct segments that are purple and yellow

Solubility Insoluble in common solvents

Common impurities Iron

Etymology

The word ametrine comes from the contraction of the words amethyst and citrine, from which we took the first two and last two syllables respectively.

Mineralogy

The mixture of the two stones that make up ametrine, amethyst and citrine, occurs naturally through hydrothermal processes, where quartz is mixed with various impurities of iron ions.

In particular, the purple or violet color of the stone is generated from iron ions of formula Fe 3+ oxidized and converted to Fe 4+, while the golden hues come from stable iron ions Fe 3+. 

Structure

Ametrine is composed of silicon dioxide (SiO2) and it is a tectosilicate, which means it has a silicate framework linked together through shared oxygen atoms.

Color

Ametrine, as its name suggests, is commonly believed to be a combination of citrine and amethyst in the same crystal. However, sources do not agree that the yellow-orange quartz component of ametrine may properly be called citrine. Like the purple color of amethyst, the yellow color in ametrine originates from trace amounts of iron in the crystal structure. The difference in coloration is due to the iron existing in different oxidation states. The yellow segments have been oxidized by heat, while the amethyst segments remain unoxidized. Some sources define citrine solely as quartz with its color originating from aluminum-based color centers, similar to those of smoky quartz. This definition excludes any amethyst that has been oxidized by heat. Accordingly, the yellow quartz in ametrine would not be considered true citrine.

History

Legend has it that ametrine was first introduced to Europe by a conquistador's gifts to the Spanish Queen in the 1600s, after he received a mine in Bolivia as a dowry when he married a princess from the native Ayoreos tribe.

As a form of commercial promotion, the names bolivianita, ayoreita, and anahita have been used, and a legend has emerged that ametrine was formed from the tears of Princess Anahí of the Ayorea tribe, who fell in love with a handsome stranger with a shining chest from distant lands (an armored Spanish conquistador). The happiness that love brought them was marred one day when the Spaniard got lost in the depths of the Amazon rainforest. She wept inconsolably for the return of her beloved. Perched on the highest point of the hill, her tears fell and glittered on the two-colored crystals, resulting in tears of unparalleled beauty that guided the lost lover to his beloved. Thus was born the legend of Princess Ayorea and the tears of honey and violet.

Color-zoned quartz distributed in a sectoral manner has been known since the 19th century, although the specimens were not of sufficient quality for use as gems. In the late 1970s, very good quality, relatively large quartz gems with zones of two well-differentiated colors, amethyst and citrine, began to appear on the world market. These gems were attributed to an undetermined locality in the mining area of Rio Grande do Sul, although it was soon determined that the locality of origin was located in Bolivia. 

Initially, the concealment and the discrepancy between the localities led to suspicion that the specimens that had appeared on the market could be the product of artificial coloring, since materials with similar characteristics could be obtained by combining heating and irradiation processes of some types of amethyst. Finally, it was determined that the specimens were authentic, analyzing in detail their mineralogical characteristics and the geology of the deposit. 

Until 1989, Bolivian law prohibited mining in the area, which was considered a fiscal reserve, so all ametrine extraction was carried out illegally, clandestinely transporting the mineral to Brazil, since its import was also prohibited by that country, and registering it in Corumbá as Brazilian production. 

In 1989, Bolivian law changed, authorizing mining exploitation, and Brazilian law authorized imports, so that the company Minerales y Metales del Oriente SRL was able to register the concession, giving the mine the name Anahí. The mineral was exported raw to Brazil, to be worked in Belo Horizonte, Governador Valadares, Teófilo Otoni, or re-exported raw to the Far East. 

On January 12, 2009, President Evo Morales Ayma enacted Law No. 3998 which declared bolivianite as an emblematic gem of Bolivian identity in the world. In addition, he declared the industrialization, processing and carving of bolivianite to be of national interest, whose existence as a natural resource in Bolivia is unique in the world, and prohibiting the export of the same in its raw form for ten years, so that the added value of the transformation of the product remains in Bolivia.

Occurrence

Naturally-occurring ametrine is sourced almost exclusively from a single mine in Bolivia. Deposits also exist in Brazil and India. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation. However, most ametrine on the market is created by partially heating or irradiating amethyst.

Ametrine in the low price segment may stem from synthetic material. Green-yellow or golden-blue ametrine does not exist naturally. Synthetic ametrine is grown with the hydrothermal method using solutions doped with specific elements, followed by irradiation of the created crystals.

The Anahí mine, Bolivia

The Anahí mine is located in the Germán Busch province, near the Mandioré and La Gaiba lagoons in the Puerto Quijarro municipality, Santa Cruz department, Bolivia. This mine, and other similar deposits in the area, are located on a regional fault system that extends for about 70 km. The mineralization is located in the northern part of a hill formed by limestones and dolomites, and has been generated by silicification of the limestones belonging to the Murciélago Group, deposited in the Precambrian, between 500 and 900 million years ago. Ametrine crystals are found in hydrothermal breccias. 

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