『Abstract
Emerald, the green gem variety of beryl, is the third most valuable
gemstone (after diamond and ruby). Although it is difficult to
obtain accurate statistics, Colombia supplies most (an estimated
60%, worth more than $500,000,000 per year) of the world's emeralds.
However there is speculation that the emerald mines in Colombia
are becoming depleted. Brazil currently accounts for approximately
10% of world emerald production. Emeralds have also been mined
in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar,
Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the
United States, and Zimbabwe.
Because it is difficult to obtain accurate analyses of beryllium,
most published analyses of beryl are renormallized on the basis
of 18 oxygen and 3 Be atoms per formula unit. The color of emeralds
is due to trace amounts of chromium and/or vanadium replacing
aluminum at the Y site; in most cases the Cr content is much greater
than that of V. To achieve charge balance, the substitution of
divalent cations at the Y site is coupled with the substitution
of a monovalent cation for a vacancy at a channel site.
Beryl is relatively rare because there is very little Be in the
upper continental crust. Unusual geologic and geochemical conditions
are required for Be and Cr and/or V to meet. In the classic model,
Be-bearing pegmatites interact with Cr-bearing ultramafic or mafic
rocks. However in the Colombian deposits there is no evidence
of magmatic activity and it has been demonstrated that circulation
processes within the host black shales were sufficient to form
emerald. In addition, researchers are recognizing that regional
metamorphism and tectonometamorphic processes such as shear zone
formation may play a significant role in certain emerald deposits.
A number of genetic classification schemes have been proposed
for emerald deposits. Most are ambiguous when it comes to understanding
the mechanisms and conditions that lead to the formation of an
emerald deposit. Studies of individual emerald deposits show that
in most cases a combination of mechanisms (magmatic, hydrothermal,
and metamorphic) were needed to bring Be into contact with the
chromophores. his suggests the need for a more flexible classification
scheme based on mode of formation. Stable isotopes can be used
to estimate the contribution of each mechanism in the formation
of a particular deposit. Such estimates could perhaps be more
precisely defined using trace element data, which should reflect
the mode of formation.
Emerald may be identified in the field by color, hardness, and
form. It will tend to show up in stream sediment samples but because
its specific gravity is relatively low, it will not concentrate
in the heavy mineral fraction. In Colombia, structural geology,
the sodium content of stream sediment samples, and the lithium,
sodium, and lead contents of soil samples have all been used to
find emerald occurrences. Exploration for gem beryl could result
in the discovery of new occurrences of non-gem beryl or other
Be minerals that could become new sources of Be and Be oxide.
Future efforts should go towards creating a comprehensive data
base of emerald compositions (including trace elements), determination
of the role of metamorphism in the formation of some emerald deposits,
improved classification schemes, and more effective exploration
guidelines.
Keywords: Emerald deposits; Emerald occurrences; Crystal chemistry;
Geochemistry; Production; Stable isotopes; Classification; Exploration』
1. Introduction
2. The crystal chemistry of beryl
3. The geochemistry of Be, Cr, and V
4. Production
5. Emerald deposits and occurrences
5.1. Afghanistan
5.2. Australia
5.3. Austria
5.4. Brazil
5.5. Bulgaria
5.6. Canada
5.7. China
5.8. Colombia
5.9. Egypt
5.10. India
5.11. Kazakhstan
5.12. Madagascar
5.13. Mozambique
5.14. Nigeria
5.15. Norway
5.16. Pakistan
5.17. Russia
5.18. South Africa
5.19. Spain
5.20. Tanzania
5.21. USA
5.22. Zambia
5.23. Zimbabwe
6. Genetic classification
7. Stable isotopes
8. Exploration
9. Future research
Acknowledgements
References