(1) Chemical composition and trace elements of emerald
Emerald is a cyclic beryllium aluminum silicate. Its crystal structure is that the silicon-oxygen units are ring-shaped along the c-axis. Distributed in a tubular shape. The empty tubes in these structures play a very important role in connecting ions that cannot participate in the beryl lattice. Some foreign ions, such as sodium and cesium, cannot participate in the crystal lattice due to their size, but can exist in the empty tubes of the structure. At the same time, the structural empty tubes also play an important role in the molecules inside, such as water molecules or carbon dioxide molecules.
Beryl varieties are classified according to color. Beryl with pure chemical composition is colorless. Colors are produced because of foreign elements. Iron makes beryl appear blue, green and yellow, manganese makes beryl pink and red, and chromium and vanadium make beryl green. The main color-causing element of emerald is chromium, and the different shades of the same color are produced by vanadium. Containing iron or iron salts can give emeralds an undesirable blue tint.
Other elements such as magnesium and sodium can be present in emerald, but will not affect its color. The content of these elements within the emerald varies greatly, sometimes reaching several weight percent. High magnesium emeralds come from metamorphosed schist. Sodium is an important element accompanying magnesium, and they both replace aluminum. When the positive divalent magnesium ions in the octahedron replace the positive trivalent aluminum ions, a cation hole appears in the mineral structure, which requires a positive monovalent atom, such as sodium, lithium or cesium, to fill the hole and maintain charge balance. It is usually distributed along one or two water molecules within the pipe.
The chemical composition of emerald reflects the geological environment during the mineralization, including the composition of the fluid, the composition of the host rock, and the temperature and pressure conditions during the mineralization. For example, emeralds from the Swat Valley in Pakistan and the Santa Terezinha mining area in Brazil both come from iron- and magnesium-rich talc-carbonate schists, and they have the highest content of foreign elements. In contrast, Colombian emeralds from black shale have lower levels of exotic elements.
The emeralds produced in Zambia, Zimbabwe, Madagascar, Egypt, South Africa, Russia and Brazil belong to schist type deposits, which shows that the emeralds from these origins are composed of biotite-phlogopite, actinolite-tremolite Stone-mafic amphibole, talc, chlorite, carbonate related. Based on this formation environment, the w (MgO) content of emeralds from these origins is between 1% and 3%. There is a negative linear relationship between w(Al2O3) and w(MgO). This is because magnesium enters the crystal structure of beryl by replacing aluminum, and magnesium ions or iron ions replace aluminum ions. In order to maintain charge balance, sodium ions need to be introduced. See the following chemical equation:
Research on the world's major colored gemstone origins
In the octahedral position, Al3+ is replaced by trivalent transition metal ions or divalent transition metal ions such as Mg2+. Divalent transition metal ions must enter the empty tube together with alkali metal ions to maintain charge balance.
Research on the world's major colored gemstone origins
In the tetrahedral position, Be2+ is replaced by divalent transition metal ions or Li+. Li+ needs to enter the empty tube together with alkali metal ions to maintain charge balance.
Research on the origins of major colored gemstones in the world
However, emerald does not completely comply with the principle of homogeneous and homogeneous substitution. If according to the rules of crystal structure O=18, some natural beryl exhibits silicon. Defect (Si6). The content of alkali metals is not always related to the defects of beryllium and aluminum, that is, a certain amount of Be2+ is replaced by Li1+ or a certain amount of Al3+ is replaced by Mg2+ and other divalent transition metal ions. Therefore, between beryllium, aluminum and silicon The isomorphous substitution that occurs between the two may follow the following substitution principles:
Research on the world's major colored gemstone origins
(2) Chemical composition and trace element analysis of emeralds from major origins< /p>
1. South America
1) Colombia and Brazil
The contents of exotic elements chromium, vanadium, iron, sodium, magnesium, gallium and cesium can be used as chemical Compositional fingerprinting identifies emeralds from different origins and the geological environment in which they were formed. After measurement, the color of emeralds from Brazil and Colombia is related to the content of chromium and vanadium. In light-colored low-chromium and dark-colored high-chromium emeralds, the chromium content is between (100~7000)×10-6gμ/g between.
The only exception is the emerald from Salininha, Bahia, Brazil, which has a chromium content of only (5~20)×10-6. The vanadium content of emeralds varies greatly due to different production environments. The vanadium/chromium ratio provides a favorable indication of the origin of the emerald. When the vanadium/chromium ratio is high (170:600, high vanadium content and low chromium content), it indicates that the emerald comes from Brazil. Salininha, Oia. The vanadium/chromium ratio of emeralds from other sources in Brazil is lower (0.01:0.7, with higher chromium content and lower vanadium content), which also shows that vanadium plays a minor role in the color of Brazilian emeralds.
In Colombian emeralds, the vanadium/chromium ratio is medium (0.2~10), which shows that the two elements play an equal role in the cause of color.
Compared with emeralds from other origins, emeralds from Colombia are relatively pure, with few foreign elements, accounting for only 2%. The contents of the color-causing elements chromium and vanadium are approximately equal, with the chromium content being (100~5000)×10-6 and the vanadium content being (400~6000)×10-6. Most Colombian emeralds contain low levels of magnesium, sodium and cesium, and the iron content is (200~1000)×10-6 (this number is considered relatively low).
Alkali metal elements mainly refer to sodium and magnesium, which are the most abundant foreign elements found in emeralds, usually in a few weight percent. Brazilian emeralds can be divided into two categories according to their alkali metal content: one is from Itabea (Goias State), Santa Terezinha (Goias State) and Salininha (Bay). Emeralds from Asia) have a high alkali metal content, with an average of (25000~40000)×10-6±2500×10-6; the other type is from Itabira-New Ella (Minas Emeralds from Gilas State) and Kanaiba-Sokoto (Bahia State) have low alkali metal content, with an average of (15000~25000)×10-6±7000×10-6. Colombian emeralds have the lowest alkali metal content, with an average of 8400×10-6±3500×10-6.
The gallium content in emeralds from Itabira-Nueira (Minas Gerais) and Kanaiba-Sokoto (Bahia) ranges from low to Medium (5-25) ×10-6. The gallium content of Colombian emeralds varies greatly, ranging from (5~50)×10-6.
The iron content of most Brazilian emeralds is medium to high, (4500~15000)×10-6. The emerald from Monte Santo (Tocantins) has a very high iron content, which is (12000~26000)×10-6. The iron content in Colombian emeralds is low, (150~2200)×10--6.
According to the cesium content, Brazilian and Colombian emeralds can be divided into four categories: those with high cesium content are emeralds from the Kanaiba-Sokoto mining area, with a content of (500~2300)×10 -6; those with medium to high cesium content are emeralds from Santa Teresinia (Goias), with a content of (50~800)×10-6; those with medium to low cesium content are emeralds from other areas of Brazil Emerald, the content is (20~160)×10-6; the emerald with low cesium content is from Colombia, the content is (5~30)×10-6.
Because the iron content is very low, Colombian emeralds and Brazilian emeralds from Canaiba-Sokoto (Bahia) only overlap in a very small area (Figure 3-114) . Accordingly, Colombian emeralds can be clearly separated from other Brazilian emeralds. When comparing emeralds from different regions of Brazil, the role of iron is limited. There is a wide range of overlap in the gallium content of Brazilian and Colombian emeralds.
Figure 3-114 Contents of gallium and iron in Brazilian and Colombian emeralds
Cesium plays a very important role in identifying emeralds from different mining areas in Brazil: On the one hand, cesium The content of elements is significantly different from that of Colombian emeralds; on the other hand, the entire area of ??elemental content overlaps only slightly.
Generally speaking, Colombian emeralds have low cesium content, while Brazilian emeralds have different cesium content. Emeralds with high cesium content come from the Kanaiba-Sokoto mining area (Bahia state); medium to high cesium content The emeralds with high cesium content come from Santa Teresinia (Goiás state), with a content of (250~800)×10-6; the emeralds with medium to low cesium content come from Itabira-Nuevo Ella (Mina Skiras State) (Figure 3-115).
Figure 3-115 Contents of cesium and scandium in Brazilian and Colombian emeralds
2. Asia
1) Afghanistan
Emeralds from the Panjshir Valley in Afghanistan have a medium content of foreign elements (about 3%), the content of the color-causing element chromium is (1000~7000)×10-6, and the content of vanadium is (200~6000)×10-6 , the contents of magnesium, sodium and cesium are low to moderate, and the content of iron ranges from (900~20000)×10-6.
2) Pakistan
Pakistan’s Swat Valley emeralds have a high content of foreign elements, about 3.5%, and the content of the color-causing element chromium is (800~25000)×10- 6. The content of vanadium is (300~1000)×10-6, the content of magnesium, sodium and cesium is low to medium, and the content of iron ranges from (2500~10000)×10-6 (rarely can reach 20000×10- 6).
3) China
The content of color-causing elements in Chinese emeralds varies greatly. The chromium content in emeralds from Malipo, Yunnan is (30~100)×10-6 The content of vanadium is very high, (4000~8000)×10-6; the content of iron is (5000~7000)×10-6; the content of cesium is mostly 2000×10-6. The chromium content of emeralds from Xinjiang is high, (1000~3000)×10-6, and some even reach (8000~9000)×10-6; the vanadium content varies greatly, (4000~11000)×10-6; The content of cesium is low, (30~50)×10-6.
4) Russian Ural Mountains
The lithium content in Russian emeralds from the Ural region is medium to high, (320~1000)×10-6; the sodium content is medium to high High, (6100~15000)×10-6; magnesium content is medium to high, (4800~16000)×10-6; potassium content is medium, (90~1000)×10-6. The content of calcium varies greatly, ranging from low to medium to high, (60~660)×10-6; the content of scandium is low to medium, ranging from (1~140)×10-6; the content of titanium is medium to High, (50~150)×10-6; vanadium content is low to medium, (80~430)×10-6; chromium content is medium to high, (580~6600)×10-6; manganese The content of iron is medium, (10~50)×10-6; the content of iron is low, (1900~4700)×10-6; the content of cobalt is medium, (1~3)×10-6; nickel and zinc The content of nickel is (5~50)×10-6, zinc is (5~25)×10-6; the content of gallium is (5~20)×10-6; the content of rubidium is low, with (10~60)×10-6; the cesium content is medium to high, (330~1500)×10-6.
Figure 3-116 Charts of gallium and iron content in emeralds from Afghanistan, Pakistan, Russia and China
Through Figure 3-117 it can be clearly distinguished from the Swat Valley (Pakistan) ), Panjshir (Afghanistan) emeralds, the curves of emeralds from Xinjiang and Panjshir basically overlap completely.
Figure 3-117 Chart of cesium and scandium content in Asian emeralds
The chemical fingerprint identification of emeralds reflects the chemical composition and characteristics of their host rocks. For example, emeralds from the Swat Valley in Pakistan are produced in iron-rich and magnesium-rich talc-carbonate schist, so the emeralds contain higher iron, magnesium, and sodium contents. For another example, emeralds from the Panjshir of Afghanistan and the Cordillera Mountains of Colombia come from black shale, so they have low iron, magnesium, and sodium content.
3. Africa
The following is the chemical composition analysis of African emeralds. The origins include Madagascar, Mozambique (Ligonia), Nigeria (Prato State), Tanzania (Manchester) Yala), Zambia (Ndola and Solwezi) and Zimbabwe (Sandawana-Machinwe).
1) Zimbabwe
The lithium content of Zimbabwean emeralds is medium to high, (110~660)×10-6; the sodium content is high, (8800~18000)×10 -6; the magnesium content is medium to high, (7200~17000)×10-6; the potassium content is low to medium, (80~370)×10-6; the calcium content is medium, (70~230)×10- 6; The scandium content is low to medium, (5~150)×10-6; the titanium content of African emerald is mostly medium, (10~100)×10-6; the chromium content is medium to high, ( 940~7200)×10-6; The content of vanadium in African emeralds is slightly lower than that of chromium. The vanadium content of most African emeralds is low to medium, <100×10-6 to 1000×10-6; The manganese content varies greatly, (5~110)×10-6; the iron content is low to medium, (3800~6300)×10-6; the cobalt content is (1~3)×10-6; the African emerald’s The nickel content is low to medium, (2.5~20)×10-6; the zinc content is low to medium, (5~80)×10-6; the gallium content of African emeralds is less than 50×10-6. The gallium content of Zimbabwean emeralds varies greatly, ranging from (5~30)×10-6; the rubidium content varies greatly, from (10~320)×10-6; the cesium content is medium, ranging from (230~970)×10- 6.
2) Zambia
The lithium content of Zambian emeralds is low, (70~110)×10-6; the sodium content is medium to high, (8000~19000)×10 -6; the magnesium content is medium to high, (4800~17000)×10-6; the potassium content is medium to high, (130~840)×10-6; the calcium content is medium, (75~440)×10- 6; The scandium content of Ndola emerald is low to medium, (5~280)×10-6, and the scandium content of Solwezi emerald varies greatly, (50~720)×10-6; African emerald The content of green titanium is mostly medium, (10~100)×10-6; the chromium content is medium to high, (800~9400)×10-6; the content of vanadium in African emeralds is slightly higher than the chromium content. Low, the vanadium content of most African emeralds is low to medium, ranging from <100×10-6 to 1000×10-6. The vanadium content of Solwezi emerald is (820~4000)×10-6; The manganese content varies greatly, (5~70)×10-6; among African emeralds, the lowest iron content is the emerald from Solwezi, which is (1200~2500)×10-6, and the Ndola emerald has the lowest iron content. The content of green iron is medium to high, (5100~13000)×10-6; the cobalt content of Solwezi is very low, (0.05~0.3)×10-6, and the cobalt content of Ndola emerald is ( 1~5)×10-6; the nickel content of African emeralds is low to medium, that of Ndola is (6~30)×10-6, and that of Solwezi emeralds is (1.5~30)×10-6 6; Solwezi’s emerald has the lowest zinc content, which is (0.5~1.5)×10-6, while Ndola’s emerald’s zinc content varies greatly, from 10×10-6 to 160×10-6 (this means that the zinc content is from low to high); the gallium content of African emeralds is less than 50×10-6, and the Ndola and Solwezi emeralds are (15~35)×10-6. Solwezi emerald has the lowest rubidium content, (1.5~3.5)×10-6, Ndola emerald has a medium rubidium content, (10~130)×10-6; African emeralds have the lowest cesium content The emerald from Solwezi is (5~15)×10-6; the emerald from Ndola has a medium to high cesium content, which is (350~1900)×10-6.
3) Mozambique
The lithium content of Mozambique emerald is low, (70~100)×10-6; the sodium content is medium to high, (9100~11000)×10- 6; High magnesium content, (11000~12000)×10-6; Medium potassium content, (360~860)×10-6; Medium calcium content, (250~350)×10-6; African emerald The content of titanium is mostly medium, (10~100)×10-6; the chromium content is low to medium, (550~1700)×10-6; the content of vanadium in African emeralds is slightly lower than that of chromium, absolutely The vanadium content of most African emeralds is low to medium, ranging from <100×10-6 to 1000×10-6; the manganese content is relatively high, ranging from (45~75)×10-6.
The iron content is medium to high, (12000~15000)×10-6. Generally speaking, the cobalt content of African emerald does not exceed 5×10-6, which is (2~2.5)× 10-6; the nickel content is low to medium, (4~6)×10-6; the zinc content is low to medium, (20~25)×10-6; the gallium content is less than 50×10-6, (5~30)×10-6; the rubidium content is medium, (45~75)×10-6; the emeralds in Ligonia, Mozambique, have the highest cesium content, (1500~3000)×10-6.
4) Nigeria
Nigerian emeralds have low lithium content, (40~120)×10-6; low sodium content, (640~1500)×10-6; The magnesium content is the lowest, (230~740)×10-6; the potassium content is low to medium, (25~140)×10-6; the calcium content is low, (30~45)×10-6; the scandium content is low , is (10~80)×10-6; most African emeralds have a medium titanium content, which is (10~100)×10-6. Only some Nigerian emeralds or green beryls have high titanium content. Reaching 150×10-6; the chromium content is the lowest, (40~820)×10-6; the vanadium content in African emeralds is slightly lower than that of chromium, and the vanadium content of most African emeralds is low to medium. From <100×10-6 to 1000×10-6; the manganese content is low to medium, (3~15)×10-6; the iron content is low to medium, (2500~7500)×10-6; overall It is said that the cobalt content of African emerald does not exceed 5×10-6, the cobalt content of Nigerian emerald is very low, (0.2~0.75)×10-6; the nickel content of African emerald is low to medium, and the nickel content of Nigerian emerald is low to medium. The nickel content is (1~4)×10-6; the zinc content is low to medium, (20~80)×10-6; the gallium content varies greatly, (15~40)×10-6; Nigeria’s general The rubidium content of emeralds from Lato State is low, (5~30)×10-6; the cesium content is low to medium, (40~490)×10-6.
Nigerian emeralds or beryls are extremely low in magnesium and sodium compared to emeralds from schist deposits. This shows that most of them come from younger granites among alkaline granites. Most Nigerian emeralds or beryls contain fluorite and iron-rich mica, consistent with the low magnesium content of younger granites. The chemical signature of this rare emerald or green beryl may be related to the chemical signature of their host rock, which is also low in magnesium.
The ferromagnetic minerals of Nigeria's alkaline granites contain iron or sodium, so these elements must change during the albiteization stage. However, despite the abundance of sodium in the environment, both Nigerian emeralds and beryls have very low sodium content. This phenomenon is the same as for emeralds in the eastern Cordillera of Colombia. The sodium in Colombian emeralds exists in the empty tubes of the emerald structure, but in order to seek charge balance, a divalent ion needs to be introduced at the position of Al3+. If Al3+ is not replaced by metal ions, then the sodium content in the emerald will still be there. will be very low, even if the ore-forming solution is rich in sodium.
Alkaline granite in Nigeria contains beryllium and fluorine. Compared with the content of beryllium and fluorine in global granite, Nigerian emeralds are higher. The color-causing elements of emerald, chromium and vanadium, are rare in Nigerian granite and are (0~10)×10-6.
However, most beryl mineralization occurs at the top of granite, where granite fluids can react with surrounding rocks. Therefore, the chromium and vanadium originate from schist basement or newer volcanic rocks, both of which contain high chromium and vanadium content. in granite. Emeralds from the eastern Cordillera Mountains of Colombia have low content of color-causing elements in host rocks, but can form green beryl and emerald. This shows that the formation of Nigerian emeralds does not require a large amount of color-causing elements.
The iron content of Nigerian emerald or beryl varies greatly, and w (FeO) can reach 1.2%. According to the absorption spectrum of Nigerian emerald, the presence of iron replaces Al3+ in the form of Fe3+, so emerald does not need sodium to maintain charge balance. Analysis of the color of some Colombian emeralds or beryls revealed that areas where color-causing elements are concentrated, especially iron, have enhanced blue-green tones. The color change is independent of magnesium and sodium.
5) Madagascar
The lithium content of Madagascar emeralds is low to medium, (55~160)×10-6; the sodium content is medium to high, (10000~17000) ×10-6; the magnesium content is medium to high, (10000~19000)×10-6; the potassium content is medium to high, (270~2200)×10-6; the calcium content is medium to high, (130~560 )×10-6; the scandium content is low, (5~130)×10-6; the titanium content of African emeralds is mostly medium, (10~100)×10-6; the chromium content is low to medium, (370~3200)×10-6; The content of vanadium in African emeralds is slightly lower than that of chromium. The vanadium content of most African emeralds is low to medium, ranging from <100×10-6 to 1000×10- 6; The manganese content varies greatly, ranging from (10~75)×10-6; the iron content is medium to high, ranging from (6000~12000)×10-6; generally speaking, the cobalt content of African emerald does not exceed 5 ×10-6, which is (2~6)×10-6; the nickel content of African emerald is low to medium, which is (10~60)×10-6; the zinc content is low to medium, which is (15~70)× 10-6; the gallium content of African emeralds is less than 50×10-6, (5~25)×10-6; the rubidium content is medium to high, (35~230)×10-6; cesium content Medium, is (100~700)×10-6.
6) Tanzania
The lithium content of Tanzania emeralds is low to medium, (80~200)×10-6; the sodium content is medium to high, (7700~16000) ×10-6; the magnesium content of African emerald varies greatly, (8900~16000)×10-6; the potassium content is medium to high, (240~1200)×10-6; the calcium content is medium, (170~ 410)×10-6; the scandium content is low, (5~30)×10-6; the titanium content of African emeralds is mostly medium, (10~100)×10-6; the chromium content is low to medium, being (210~2700)×10-6; The content of vanadium in African emeralds is slightly lower than that of chromium. The vanadium content of most African emeralds is low to medium, ranging from <100×10-6 to 1000×10- 6; The manganese content is low to medium, (5~25)×10-6; the iron content is low to medium, (3700~7000)×10-6; overall, the cobalt content of African emerald does not exceed 5× 10-6, which is (2~4)×10-6; the nickel content of African emerald is low to medium, which is (10~25)×10-6; the zinc content is low to medium, which is (15~30)×10 -6; the gallium content of African emeralds is less than 50×10-6, (5~15)×10-6; the rubidium content is medium to high, (65~270)×10-6; the cesium content is medium When it is high, it is (500~1500)×10-6.
Figure 3-118 The content of gallium and iron in African emeralds
Figure 3-119 The content of cesium and scandium in African emeralds