A Study on Beryl in the Cuonadong Be-W-Sn Polymetallic Deposit, Longzi County, Tibet, China A study on Beryl in the Cuonadong Be-W-Sn Polymetallic Deposit, Longzi County, Tibet, China

: Recently, aquamarine was discovered in the Cuonadong Be-W-Sn Polymetallic Deposit, Longzi County, Tibet. Longzi aquamarine is being extracted and is expected to be available over the next decade. This study provides a full set of data through standard gemmological properties, including scenes, color characteristics and advanced spectroscopic and chemical analyses, including micro ultraviolet–visible–near-infrared (UV–Vis–NIR), Raman and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The main inclusions in Longzi aquamarine are gas–liquid inclusions and a great number of quartz inclusions. The content of type I H 2 O is greater than that of type II H 2 O because of the low-alkali metal content, and “tetrahedral” substitutions and “octahedral” substitutions exist at the same time. Abstract: Recently, aquamarine was discovered in the Cuonadong Be-W-Sn Polymetallic Deposit, Longzi County, Tibet. Longzi aquamarine is being extracted and is expected to be available over the next decade. This study provides a full set of data through standard gemmological properties, including scenes, color characteristics and advanced spectroscopic and chemical analyses, including micro ultraviolet–visible–near-infrared (UV–Vis–NIR), Raman and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The main inclusions in Longzi aquamarine are gas– liquid inclusions and a great number of quartz inclusions. The content of type I H 2 O is greater than that of type II H 2 O because of the low-alkali metal content, and “tetrahedral” substitutions and “octahedral” substitutions exist at the same time.


Introduction
Beryl is a common beryllium-bearing cyclosilicate mineral with high transparency, similar to many gem-quality minerals. The iron-bearing green-blue, blue-green or light blue-blue beryl is known as aquamarine. Most aquamarine is generated in highly evolved leucogranites, such as in the Aracuai orogen in Brazil [1].

Introduction
Beryl is a common beryllium-bearing cyclosilicate mineral with high transparency, similar to many gem-quality minerals. The iron-bearing green-blue, blue-green or light blue-blue beryl is known as aquamarine. Most aquamarine is generated in highly evolved leucogranites, such as in the Aracuai orogen in Brazil [1].

Figure 1.
Aquamarine-producing areas (grey parts). The red dot locates Longzi County. Other sources are . The Cuonadong dome is near the South Tibet Detachment System. The dome con of three parts: core, mantle and edge (a ring-banded structure). The dome core is for by granite gneiss and leucogranite and is interspersed by a large number of pegm dikes, which are usually blocky texture cut by granite or veins that cut through the g [32] (Figure 2b). Pegmatite is composed of K-feldspar, plagioclase, quartz, muscovit otite, and apatite. The widespread apatite consistent with most porphyries in Tibet [33 has relatively high Be and Li contents [35] and has beryl formed in it ( Figure 3). The Cuonadong dome is near the South Tibet Detachment System. The dome consists of three parts: core, mantle and edge (a ring-banded structure). The dome core is formed by granite gneiss and leucogranite and is interspersed by a large number of pegmatite dikes, which are usually blocky texture cut by granite or veins that cut through the gneiss [32] ( Figure 2b). Pegmatite is composed of K-feldspar, plagioclase, quartz, muscovite, biotite, and apatite. The widespread apatite consistent with most porphyries in Tibet [33,34], has relatively high Be and Li contents [35] and has beryl formed in it ( Figure 3). We can see feldspar, mica, tourmaline and other minerals with coarse particles. The beryl crystals are typically hexagonal, ranging in length from 1 to 10 cm, they are pale blue, translucent to microtransparent, and acicular and columnar tourmaline can be seen in the surrounding adjacent rock.

Materials and Methods
Three beryl crystals (L1 to L3) and two pegmatite slices (LZ-1 and LZ-2) of beryl in Longzi County were examined using standard gemmological techniques.
The gemological tests for the samples were conducted at the Gemological Research Laboratory of China University of Geosciences (Beijing) to determine their optical properties, hydrostatic specific gravity, UV fluorescence and microscopic and macroscopic features.
The refractive index (RI) and birefringence (DR) of the crystal polishing surface were measured with a refractometer (Xueyuan Jewelry Technologies, Wuhan, China) using diiodimethane as refractive oil. The specific gravity (SG) of the sample was obtained by hydrostatic weighing method. The samples was observed by Chelsea Color Filter (CCF) (Baoguang Technologies, Nanjing, China) and it's fluorescence was observed by the ultraviolet light with main wavelengths of 365 nm and 254 nm.
Crystals 2021, 11, 777 5 of 23 In this experiment, a laser sheet of LZ-2 was selected as a sample. Uniform parts with no inclusions in beryl were selected for the test to obtain more representative element contents.
Major-element compositions of minerals were measured using a four-spectrometer Jeol JXA 8100 electron probe microanalyzer (JEOL Ltd., Akishima, Japan) in the Key Laboratory of Submarine Geosciences, State Oceanic Administration, Second Institute of Oceanography, Ministry of Natural Administration, with an accelerating potential of 15 kV, a beam current of 20 nA, a counting time of 20 s and a spot size of 10 µm. Element peaks and background were measured with counting times of 10 s and 5 s. NaAlSi 2 O 3 (Na), KAlSi 3 O 8 (k), Cr 2 O 3 (Cr), Diopside (Si, Mg, Ca), Fe 2 O 3 (Fe), Pyrope (Al), Mn 2 O 3 (Mn), TiO 2 (Ti) and Ca 5 P 3 F (P) have been used as standards. ZAF correction schemes were used for calibration.
Trace-element compositions of minerals were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) (Agilent Technologies, Santa Clara, CA, America) in the Ore Deposit and Exploration Centre, School of Resources and Environmental Engineering, Hefei University of Technology. The laser ablation system was CetacAnalyte HE, and the ICP-MS model was Agilent 7900. The ablation was carried out in He gas, which was then mixed with Ar gas to introduce the samples into the ICPMS.
In determining the mineral trace element content, we used multiple reference standards such as BCR, SRM610 and SRM612, and multiple external standards without internal standards for quantitative calculation. The results were processed and exported using ICPMSD 12.0 [36].

Visual Appearance and Gemmological Properties
The beryl samples are light blue to green-light blue and transparent to translucent with incomplete cleavage and a glassy luster. The fracture has a conchoidal fracture or uneven shape and has a glassy to resin luster. The crystal is hexagonal columnar and hexagonal thick plate-like. The thick parallel plate shows completely parallel doubly terminated (0001) faces. Aquamarine sizes range from 5 mm to 3 cm (Figure 4a-d).
In this experiment, a laser sheet of LZ-2 was selected as a sample. Uniform parts with no inclusions in beryl were selected for the test to obtain more representative element contents.
Major-element compositions of minerals were measured using a four-spectrometer Jeol JXA 8100 electron probe microanalyzer (JEOL Ltd., Akishima, Japan) in the Key Laboratory of Submarine Geosciences, State Oceanic Administration, Second Institute of Oceanography, Ministry of Natural Administration, with an accelerating potential of 15 kV, a beam current of 20 nA, a counting time of 20 s and a spot size of 10 μm. Element peaks and background were measured with counting times of 10 s and 5 s. NaAlSi2O3 (Na), KAlSi3O8 (k), Cr2O3 (Cr), Diopside (Si, Mg, Ca), Fe2O3 (Fe), Pyrope (Al), Mn2O3 (Mn), TiO2 (Ti) and Ca5P3F (P) have been used as standards. ZAF correction schemes were used for calibration.
Trace-element compositions of minerals were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) (Agilent Technologies, Santa Clara, CA, America) in the Ore Deposit and Exploration Centre, School of Resources and Environmental Engineering, Hefei University of Technology. The laser ablation system was CetacAnalyte HE, and the ICP-MS model was Agilent 7900. The ablation was carried out in He gas, which was then mixed with Ar gas to introduce the samples into the ICPMS.
In determining the mineral trace element content, we used multiple reference standards such as BCR, SRM610 and SRM612, and multiple external standards without internal standards for quantitative calculation. The results were processed and exported using ICPMSD 12.0 [36].

Visual Appearance and Gemmological Properties
The beryl samples are light blue to green-light blue and transparent to translucent with incomplete cleavage and a glassy luster. The fracture has a conchoidal fracture or uneven shape and has a glassy to resin luster. The crystal is hexagonal columnar and hexagonal thick plate-like. The thick parallel plate shows completely parallel doubly terminated (0001) faces. Aquamarine sizes range from 5 mm to 3 cm (Figure 4a   The transparency of the sample is poor under 40× magnification and dark-field illumination ( Figure 4b). A large number of cracks are observed, and there are dark inclusions inside and pits on the surface. The longitudinal striation are visible on the crystal face parallel to the c-axis.
The refractive index of the polished A-1 specimen is measured using a gem refractometer. The sample refractive index is 1.578 to 1.583. All samples are inert to longwave The absorption spectrum of aquamarine is studied using a UV-Vis spectrophotometer. The spectrum of the slices perpendicular and parallel to the c-axis of the sample crystal A-1 is shown in Figure 5.
The transparency of the sample is poor under 40× magnification and dark-field illumination (Figure 4b). A large number of cracks are observed, and there are dark inclusions inside and pits on the surface. The longitudinal striation are visible on the crystal face parallel to the c-axis.
The refractive index of the polished A-1 specimen is measured using a gem refractometer. The sample refractive index is 1.578 to 1.583. All samples are inert to longwave and shortwave UV radiation. The specimens are all blue-green under the Chelsea color filter (CCF). The specific gravity (SG) values varied between 2.59 and 2.65.

UV-Vis Spectrum
The absorption spectrum of aquamarine is studied using a UV-Vis spectrophotometer. The spectrum of the slices perpendicular and parallel to the c-axis of the sample crystal A-1 is shown in Figure 5. One wide peak is observed between 700 and 900 nm in both directions. However, the absorption peaks of the slices parallel to the c-axis are wider than those of the slices perpendicular to the c-axis. According to Wood and Nassau [37] and Goldman [38], the absorption peaks in the No (the index of refraction of the ordinary-ray) direction are related to the six coordinated Fe 2+ ions that replace the Al 3+ ions in the octahedron. The absorption peaks in the Ne (the index of refraction of the especial-ray) direction are caused by the Fe 2+ ions in the tunnel structure.

Raman Spectrum
By comparing the Raman spectrum of the beryl on the slice with that in the literature [8,39], we found the following characteristic peaks in the range of 200-1500 cm −1 : a strong absorption peak at 685 cm −1 caused by the Si-O-Si internal vibration of deformation, an absorption peak at 1068 cm −1 caused by the Si-O internal vibration of non-bridging oxygen expansion, an absorption peak at 1011 cm −1 caused by the Be-O outer vibration of nonbridging oxygen expansion, an absorption peak at 398 cm −1 caused by the Al-O outer vibration of deformation and an absorption peak at 323 cm −1 caused by the Al-O outer vibration of bending. There are some impure peaks around the range of 400-700 cm −1 (Figure 6a). One wide peak is observed between 700 and 900 nm in both directions. However, the absorption peaks of the slices parallel to the c-axis are wider than those of the slices perpendicular to the c-axis. According to Wood and Nassau [37] and Goldman [38], the absorption peaks in the N o (the index of refraction of the ordinary-ray) direction are related to the six coordinated Fe 2+ ions that replace the Al 3+ ions in the octahedron. The absorption peaks in the N e (the index of refraction of the especial-ray) direction are caused by the Fe 2+ ions in the tunnel structure.

Raman Spectrum
By comparing the Raman spectrum of the beryl on the slice with that in the literature [8,39], we found the following characteristic peaks in the range of 200-1500 cm −1 : a strong absorption peak at 685 cm −1 caused by the Si-O-Si internal vibration of deformation, an absorption peak at 1068 cm −1 caused by the Si-O internal vibration of non-bridging oxygen expansion, an absorption peak at 1011 cm −1 caused by the Be-O outer vibration of non-bridging oxygen expansion, an absorption peak at 398 cm −1 caused by the Al-O outer vibration of deformation and an absorption peak at 323 cm −1 caused by the Al-O outer vibration of bending. There are some impure peaks around the range of 400-700 cm −1 (Figure 6a). The Raman spectrum of the aquamarine channel water is in the 3590-3620 cm −1 range ( Figure 6b). A band at 3610 cm −1 (related to type I H2O) is narrow and strong. However, a band at 3600 cm −1 (related to type II H2O) is weak and wide. According to the area ratio, the proportion of type I H2O and type II H2O contents can be calculated: the type I H2O content of Longzi aquamarine is approximately 70-80%.

Inclusions
The fluid inclusions of beryl in the polished chips are observed under a 50× objective single polarizing microscope (Figure 7c). There are abundant of gas-liquid two-phase needle-like fluid inclusions parallel to the c-axis in the Longzi beryl. They are approximately 10-40 μm long and densely arranged.
There are also a large number of colorless and transparent solid inclusions, whose relief is the same (but slightly lower) as that of the main crystal of beryl ( Figure 7c). Because of the thickness limitation of the sheets, the solid inclusions are mostly sheet-like. However, the crystal edges of the solid inclusions are partly visible. Therefore, they may be crystal inclusions. The sizes of such inclusions are approximately 20-80 μm, appearing individually or in groups.

Inclusions
The fluid inclusions of beryl in the polished chips are observed under a 50× objective single polarizing microscope (Figure 7c). There are abundant of gas-liquid two-phase needle-like fluid inclusions parallel to the c-axis in the Longzi beryl. They are approximately 10-40 µm long and densely arranged.
There are also a large number of colorless and transparent solid inclusions, whose relief is the same (but slightly lower) as that of the main crystal of beryl ( Figure 7c). Because of the thickness limitation of the sheets, the solid inclusions are mostly sheet-like. However, the crystal edges of the solid inclusions are partly visible. Therefore, they may be crystal inclusions. The sizes of such inclusions are approximately 20-80 µm, appearing individually or in groups. The Raman spectrum of the aquamarine channel water is in the 3590-3620 cm −1 range ( Figure 6b). A band at 3610 cm −1 (related to type I H2O) is narrow and strong. However, a band at 3600 cm −1 (related to type II H2O) is weak and wide. According to the area ratio, the proportion of type I H2O and type II H2O contents can be calculated: the type I H2O content of Longzi aquamarine is approximately 70-80%.

Inclusions
The fluid inclusions of beryl in the polished chips are observed under a 50× objective single polarizing microscope ( Figure 7c). There are abundant of gas-liquid two-phase needle-like fluid inclusions parallel to the c-axis in the Longzi beryl. They are approximately 10-40 μm long and densely arranged.
There are also a large number of colorless and transparent solid inclusions, whose relief is the same (but slightly lower) as that of the main crystal of beryl ( Figure 7c). Because of the thickness limitation of the sheets, the solid inclusions are mostly sheet-like. However, the crystal edges of the solid inclusions are partly visible. Therefore, they may be crystal inclusions. The sizes of such inclusions are approximately 20-80 μm, appearing individually or in groups.  By comparing the Raman spectrum with the standard spectrum, we determined that the solid inclusions are quartz. The characteristic peaks and corresponding vibrations of the quartz inclusions are listed in Figure 7b. The peak at 1161 cm −1 is considered the characteristic peaks of Si-O asymmetric stretching vibration. Two peaks at 808 and 697 cm −1 are caused by Si-O-Si symmetric stretching vibration. The peaks at 466, 396, 353 cm −1 are considered the characteristic peaks of Si-O bending vibration, and the peak at 265 cm −1 is related to the vibration of SiO 4 [40]. In addition to the needle-like fluid inclusions parallel to the c-axis, some separate large fluid inclusions can be found in Longzi aquamarine, varying in shapes: rectangular ( Figure  By comparing the Raman spectrum with the standard spectrum, we determined that the solid inclusions are quartz. The characteristic peaks and corresponding vibrations of the quartz inclusions are listed in Figure 7b. The peak at 1161 cm −1 is considered the characteristic peaks of Si-O asymmetric stretching vibration. Two peaks at 808 and 697 cm −1 are caused by Si-O-Si symmetric stretching vibration. The peaks at 466, 396, 353 cm −1 are considered the characteristic peaks of Si-O bending vibration, and the peak at 265 cm −1 is related to the vibration of SiO4 [40]. In addition to the needle-like fluid inclusions parallel to the c-axis, some separate large fluid inclusions can be found in Longzi aquamarine, varying in shapes: rectangular (Figure 8a  These non-c-axis parallel inclusions isolated and randomly distributed, they exhibit primary textures. The length of these fluid inclusions is approximately 40-50 μm, and the width is approximately 20 μm ( Figure 8).
Four fluid inclusions with different shapes and sizes were selected for Raman spectroscopy. These fluid inclusions are two-phase inclusions in which the gas volume is less than that of the liquid one. The gas and liquid compositions are subjected to Raman spectroscopy. The results are as follows.
These non-c-axis parallel inclusions isolated and randomly distributed, they exhibit primary textures. Under the microscope, some inclusions in the beryl are selected for the Raman spectrum test. The results are shown in Figure 8. A band at 3406 cm −1 is considered the characteristic peak of H2O in the inclusions. The peaks at 3600 and 3609 cm −1 are con- These non-c-axis parallel inclusions isolated and randomly distributed, they exhibit primary textures. The length of these fluid inclusions is approximately 40-50 µm, and the width is approximately 20 µm (Figure 8).
Four fluid inclusions with different shapes and sizes were selected for Raman spectroscopy. These fluid inclusions are two-phase inclusions in which the gas volume is less than that of the liquid one. The gas and liquid compositions are subjected to Raman spectroscopy. The results are as follows.
These non-c-axis parallel inclusions isolated and randomly distributed, they exhibit primary textures. Under the microscope, some inclusions in the beryl are selected for the Raman spectrum test. The results are shown in Figure 8. A band at 3406 cm −1 is considered the characteristic peak of H 2 O in the inclusions. The peaks at 3600 and 3609 cm −1 are considered the characteristic peaks of type I and type II H 2 O in the beryl. The N-N bonding Raman spectrum peak at 2331 cm −1 shows that the inclusions contain N 2 . Two peaks at 1238 and 1385 cm −1 are caused by the Fermi coupling resonance in the CO 2 molecule, and the small peaks on both sides are the hot bands of CO 2 . The peak at 686cm −1 is caused by Si-O-Si bending vibration. The peaks at 1011 and 1069 cm −1 indicate the existence of HCO 3 − and CO 3 2− , respectively [41]. By analyzing the gas and liquid parts of the same inclusion, it can be seen that the gas part has strong CO 2 and N 2 peaks, whereas the liquid part has strong H 2 O peaks.

Major and Trace Elements
The major and trace elements of LZ-2 are listed in Tables A1 and A2a,b in Appendix A. According to the test results and calculation of the major and trace elements, the major element contents constituting the beryl chemical formula are SiO 2 = 65.045 wt%-65.025 wt%, Al 2 O 3 = 17.180 wt%-18.106 wt%. Be is a major element in beryl, so the results measured by LA-ICP-MS are inaccurate. We used the data measured by EPMA and calculated the ratio of ideal chemical formula to obtain BeO = 12.928 wt%-13.555 wt%. Through calculation, it was found that the chemical formation of LZ-2 is: ch (Na 0.079 ,K 0.002 ) T2 [13]. A thorough analysis is presented in Section 5.4.

Gemmological Characteristics
The test results, compared with existing aquamarine data from other places (Table A3), show that the optical and physical properties of almost all aquamarine are approximately the same. The color of Longzi aquamarine varies from light green-blue to blue and is lighter than that of other aquamarines. A better color may be obtained through heat treatment and so forth. Most crystals have hexagonal columns and thick plates. The particle size varies from 5 mm to 3 cm, smaller than the size of aquamarine in other places, but larger ones may be found in future development. Most of the crystals have low transparency and many cracks. Moreover, the values of N o and N e of Longzi aquamarine are the same as those of other producing areas.

Water Molecules in the Channels
Beryl has an open honeycomb hexagonal ring structure [42]. This hexagonal ring structure is cumulatively stacked into open cavities along the c-axis [43] (Figure 9). In beryl, H 2 O molecules are ordered and oriented in crystal cavities [44].

Figure 9.
Stacked-ring structure of beryl [45]. According to the experimental results of Wood and Nassau [37], there are two types of H2O molecule occupancy in the open cavities of a beryl. When H2O is not paired with an alkali metal ion, the H-H vector is parallel to the c-axis (type I water). When an extra-framework cation is present near H2O molecules, the H2O molecules above and below the cation orient with their H-H vectors vertical to the c-axis because of the electrical attraction between the oxygen of the water molecules and the cation (type II water) [45].
With the increased alkali content in beryl, the proportion of type II H2O and intensity of its spectrum rose [37]. Zou's [46] research on beryl with different colors in different producing areas shows that the components of the standard blue, blue-green, yellowgreen and green beryl are the same, and the only difference is between the proportion of type I H2O and type II H2O. Green, yellow-green and blue-green beryl will become pure blue aquamarine when the ratio of type I and type II H2O becomes 1:1 [46]. The type I H2O content of Longzi aquamarine is approximately 70-80%. By controlling the temperature and pressure to transfer some type I H2O molecules to the position of type II H2O molecules to increase the proportion of type II H2O, the color of Longzi aquamarine will be more vivid, which can increase the commercial value of aquamarine significantly [46].

Inclusions
Generally, aquamarine is rich in fluid inclusions. Multiphase inclusions of different shapes are frequently seen in aquamarine from various origins. As distinguishing features of aquamarine, short needle-like two-phase inclusions are also important in Longzi aquamarine. There are a large number of solid inclusions in Longzi aquamarine, and quartz is one of such characteristic inclusions.
Longzi aquamarine abounds with needle-like fluid inclusions approximately 10-40 μm long. Moreover, there exist some fluid inclusions approximately 40-50 μm long and 20 μm wide. The fluid composition is H2O, CO2 and their solution. An N2 characteristic peak is found in the gas portion, indicating that it may contain N2, H2O and CO2 as volatile components, which can be indicators of fugacity when the host crystals formed [45]. This indicates that Longzi aquamarine is formed in a fluid-rich environment. CO2 is the highest valence state of C, and CO2 and H2O in the inclusions are the final products of oxidation. The presence of strong reducing components such as CH4 is not detected in beryl, so the CO2 and H2O in beryl should be generated by the oxidation of CH4 [47], and the magmatic melt may have evolved from a reducing environment to an oxidizing environment. . Stacked-ring structure of beryl [45]. According to the experimental results of Wood and Nassau [37], there are two types of H 2 O molecule occupancy in the open cavities of a beryl. When H 2 O is not paired with an alkali metal ion, the H-H vector is parallel to the c-axis (type I water). When an extra-framework cation is present near H 2 O molecules, the H 2 O molecules above and below the cation orient with their H-H vectors vertical to the c-axis because of the electrical attraction between the oxygen of the water molecules and the cation (type II water) [45].

Major and Trace Elements
With the increased alkali content in beryl, the proportion of type II H 2 O and intensity of its spectrum rose [37]. Zou's [46] research on beryl with different colors in different producing areas shows that the components of the standard blue, blue-green, yellow-green and green beryl are the same, and the only difference is between the proportion of type I H 2 O and type II H 2 O. Green, yellow-green and blue-green beryl will become pure blue aquamarine when the ratio of type I and type II H 2 O becomes 1:1 [46]. The type I H 2 O content of Longzi aquamarine is approximately 70-80%. By controlling the temperature and pressure to transfer some type I H 2 O molecules to the position of type II H 2 O molecules to increase the proportion of type II H 2 O, the color of Longzi aquamarine will be more vivid, which can increase the commercial value of aquamarine significantly [46].

Inclusions
Generally, aquamarine is rich in fluid inclusions. Multiphase inclusions of different shapes are frequently seen in aquamarine from various origins. As distinguishing features of aquamarine, short needle-like two-phase inclusions are also important in Longzi aquamarine. There are a large number of solid inclusions in Longzi aquamarine, and quartz is one of such characteristic inclusions.
Longzi aquamarine abounds with needle-like fluid inclusions approximately 10-40 µm long. Moreover, there exist some fluid inclusions approximately 40-50 µm long and 20 µm wide. The fluid composition is H 2 O, CO 2 and their solution. An N 2 characteristic peak is found in the gas portion, indicating that it may contain N 2 , H 2 O and CO 2 as volatile components, which can be indicators of fugacity when the host crystals formed [45]. This indicates that Longzi aquamarine is formed in a fluid-rich environment. CO 2 is the highest valence state of C, and CO 2 and H 2 O in the inclusions are the final products of oxidation. The presence of strong reducing components such as CH 4 is not detected in beryl, so the CO 2 and H 2 O in beryl should be generated by the oxidation of CH 4 [47], and the magmatic melt may have evolved from a reducing environment to an oxidizing environment.

Major and Trace Elements
In the beryl crystal structure, there are "octahedral" cation substitutions: Al 3+ can be replaced by divalent cations such as Mg 2+ , Fe 2+ and Mn 2+ or trivalent cations such as Fe 3+ , Cr 3+ , V 3+ and Sc 3+ [19]. Fe is the most important substitute for Al in Longzi aquamarine. By comparing the Fe-Al substitution to other areas, it can be seen from Figure 10a that there is a significant negative correlation between Al and Fe. Figure 10b shows Al versus the sum of its substituents at the Y site of aquamarine from Longzi and other areas, The data points are evenly distributed near the straight line with Al = 2 apfu and ∑ substitutions in Y site = 1 apfu [13] as the endpoints. It can be seen that there are "octahedral" substitutions in Longzi aquamarine. The substitutional trend in Longzi aquamarine is consistent with most producing area which has relatively high content of Al. In Canada and Vietnam, the Y site values are very high and significantly different from aquamarine from other producing areas. In the beryl crystal structure, there are "octahedral" cation substitutions：Al 3+ can be replaced by divalent cations such as Mg 2+ , Fe 2+ and Mn 2+ or trivalent cations such as Fe 3+ , Cr 3+ , V 3+ and Sc 3+ [19]. Fe is the most important substitute for Al in Longzi aquamarine. By comparing the Fe-Al substitution to other areas, it can be seen from Figure 10a that there is a significant negative correlation between Al and Fe. Figure 10b shows Al versus the sum of its substituents at the Y site of aquamarine from Longzi and other areas，The data points are evenly distributed near the straight line with Al = 2 apfu and ∑ substitutions in Y site = 1 apfu [13] as the endpoints. It can be seen that there are "octahedral" substitutions in Longzi aquamarine. The substitutional trend in Longzi aquamarine is consistent with most producing area which has relatively high content of Al. In Canada and Vietnam, the Y site values are very high and significantly different from aquamarine from other producing areas. Figure 10. (a) Relationship between the major octahedral site ion (Al), and the principal substituent at the octahedral site (Fe), consistent with major Fe-Al substitution：There is a negative correlation between them, in atoms per formula unit (apfu). (b) Al versus the sum of its substituents in Y site, in atoms per formula unit (apfu). Other sources are [3,18,19,21,22,48,49].
Li + mainly replaces Be 2+ in [BeO4] tetrahedron, whereas Na + , CS + and other ions with large radii are distributed in the channel of the beryl structure [50] to compensate for the charge imbalance caused by Li + replacing Be 2+ . Li + mainly replaces Be 2+ in [BeO 4 ] tetrahedron, whereas Na + , CS + and other ions with large radii are distributed in the channel of the beryl structure [50] to compensate for the charge imbalance caused by Li + replacing Be 2+ .
Since divalent cations substitute trivalent Al 3+ in octahedron, the monovalent cations in the channel need to balance the electricity price. Therefore, the sum of divalent cations in octahedral site (R 2+ ) and monovalent cations in channel (A + ) should be proportional to the ideal state, and the slope is 1 [13,19]. It can be seen from Figure 11 that the sum of R 2+ and A + of Longzi aquamarine are close to 1:1, but some points are above the 1:1 line, which indicates that there is a certain amount of Li substituting Be in the tetrahedral T1 position, and there may be "tetrahedral" substitutions in Longzi aquamarine. Since divalent cations substitute trivalent Al 3+ in octahedron, the monovalent cations in the channel need to balance the electricity price. Therefore, the sum of divalent cations in octahedral site (R 2+ ) and monovalent cations in channel (A + ) should be proportional to the ideal state, and the slope is 1 [13,19]. It can be seen from Figure 11 that the sum of R 2+ and A + of Longzi aquamarine are close to 1:1, but some points are above the 1:1 line, which indicates that there is a certain amount of Li substituting Be in the tetrahedral T1 position, and there may be "tetrahedral" substitutions in Longzi aquamarine. After Li + replaces Be 2+ , Na + enters the channels of the structure to maintain charge balance, but Cs + becomes the main charge balance cation in the pegmatite samples with a higher evolution degree. Generally, when the Na/Li value is 2 to 4, the charge balance changes from Na-dominant to Cs-dominant compensation [51,52]. Using the Na/Li versus Cs diagram for late primary beryl from granitic pegmatites by [53] as the basic model and taking Cs (wt%) content as the abscissa and Na/Li value as the ordinate based on the analysis of the Na, Li and Cs contents of Longzi aquamarine, we conclude that Longzi aquamarine is located in area A in Figure 12, indicating that the pegmatite producing Longzi aquamarine is a barren and geochemically primitive beryl-type pegmatite (including most pegmatites of the rare-earth type).
In Longzi aquamarine, the experimental results also show that the NaO content is between 0.41 and 0.70 wt%. The K2O content is between 0.01 and 0.45 wt%. The Cs2O content is between 0.161 and 0.247 wt%, and the ratio of Na2O/K2O to Na2O/Cs2O is greater than 1. According to the ratio relationship of Na2O/K2O to Na2O/Cs2O by ČErný P et al. [52], because of the ratio value greater than 1, Longzi aquamarine classified as "sodic lithian beryls". After Li + replaces Be 2+ , Na + enters the channels of the structure to maintain charge balance, but Cs + becomes the main charge balance cation in the pegmatite samples with a higher evolution degree. Generally, when the Na/Li value is 2 to 4, the charge balance changes from Na-dominant to Cs-dominant compensation [51,52]. Using the Na/Li versus Cs diagram for late primary beryl from granitic pegmatites by [53] as the basic model and taking Cs (wt%) content as the abscissa and Na/Li value as the ordinate based on the analysis of the Na, Li and Cs contents of Longzi aquamarine, we conclude that Longzi aquamarine is located in area A in Figure 12, indicating that the pegmatite producing Longzi aquamarine is a barren and geochemically primitive beryl-type pegmatite (including most pegmatites of the rare-earth type).
In Longzi aquamarine, the experimental results also show that the NaO content is between 0.41 and 0.70 wt%. The K 2 O content is between 0.01 and 0.45 wt%. The Cs 2 O content is between 0.161 and 0.247 wt%, and the ratio of Na 2 O/K 2 O to Na 2 O/Cs 2 O is greater than 1. According to the ratio relationship of Na 2 O/K 2 O to Na 2 O/Cs 2 O byČErný P et al. [52], because of the ratio value greater than 1, Longzi aquamarine classified as "sodic lithian beryls".
Li and Cs contents are positively related. The Li versus Cs content shows two concentrated areas: the "low-alkali beryl" and "high-alkali beryl" areas. As observed in Figure 13, aquamarine in most areas belongs to the "low-alkali beryl" area, including Longzi aquamarine. Figure 13. Chemical fingerprint diagram of the log-log plot of Li versus Cs. On the basis of the Li and Cs contents, beryls from different areas are classified as "high-alkali beryl" and "low-alkali beryl". Other sources are [18,[20][21][22]55].
For aquamarine, Fe is the chromogenic element. By analyzing the Cr-Fe-V content of aquamarine ( Figure 14) based on the Cr-Fe-V ternary diagram of the emerald chromogenic element by [56], we conclude that the Cr and V abundance of aquamarine is very low, indicating that as little as 0.04-wt% Cr2O3 is locally sufficient to generate an emerald green color [54]. By partially enlarging the circled area in Figure 14a, the Figure 14b was acquired. It can be seen from Figure 14b that the aquamarine barely contains Cr or V. Compared to other aquamarine, the Longzi aquamarine hardly contains Cr, but it contains a little V which differentiates it from aquamarine in Namibia, Italy and Argentina. Li and Cs contents are positively related. The Li versus Cs content shows two concentrated areas: the "low-alkali beryl" and "high-alkali beryl" areas. As observed in Figure 13, aquamarine in most areas belongs to the "low-alkali beryl" area, including Longzi aquamarine. Li and Cs contents are positively related. The Li versus Cs content shows two concentrated areas: the "low-alkali beryl" and "high-alkali beryl" areas. As observed in Figure 13, aquamarine in most areas belongs to the "low-alkali beryl" area, including Longzi aquamarine. Figure 13. Chemical fingerprint diagram of the log-log plot of Li versus Cs. On the basis of the Li and Cs contents, beryls from different areas are classified as "high-alkali beryl" and "low-alkali beryl". Other sources are [18,[20][21][22]55].
For aquamarine, Fe is the chromogenic element. By analyzing the Cr-Fe-V content of aquamarine ( Figure 14) based on the Cr-Fe-V ternary diagram of the emerald chromogenic element by [56], we conclude that the Cr and V abundance of aquamarine is very low, indicating that as little as 0.04-wt% Cr2O3 is locally sufficient to generate an emerald green color [54]. By partially enlarging the circled area in Figure 14a, the Figure 14b was acquired. It can be seen from Figure 14b that the aquamarine barely contains Cr or V. Compared to other aquamarine, the Longzi aquamarine hardly contains Cr, but it contains a little V which differentiates it from aquamarine in Namibia, Italy and Argentina. Figure 13. Chemical fingerprint diagram of the log-log plot of Li versus Cs. On the basis of the Li and Cs contents, beryls from different areas are classified as "high-alkali beryl" and "low-alkali beryl". Other sources are [18,[20][21][22]55].
For aquamarine, Fe is the chromogenic element. By analyzing the Cr-Fe-V content of aquamarine ( Figure 14) based on the Cr-Fe-V ternary diagram of the emerald chromogenic element by [56], we conclude that the Cr and V abundance of aquamarine is very low, indicating that as little as 0.04-wt% Cr 2 O 3 is locally sufficient to generate an emerald green color [54]. By partially enlarging the circled area in Figure 14a, the Figure 14b was acquired. It can be seen from Figure 14b that the aquamarine barely contains Cr or V. Compared to other aquamarine, the Longzi aquamarine hardly contains Cr, but it contains a little V which differentiates it from aquamarine in Namibia, Italy and Argentina.

Conclusions
Southern Tibet, China, has always been a significant resource reserve base. The discovery of aquamarine in Longzi County in southern Tibet is not only of great research value but also of great commercial value. Generally speaking, the particle size of Longzi aquamarine is small, and the color ranges from light green-blue to light blue. The fluid inclusions in aquamarine indicate that it is formed in a fluid-rich environment. There is an obvious negative correlation between Fe and Al, and the existence of Li substitutions for Be, indicating that both tetrahedral substitutions and octahedral substitutions exist in Longzi aquamarine. The pegmatite producing Longzi aquamarine is a barren and geochemically primitive beryl-type pegmatite (including most pegmatites of the rare-earth type).
There exists more type I H2O than type II H2O, which is related to the low-alkali metal content. By changing the ratio of the two kinds of H2O, we can improve the color of Longzi aquamarine.

Conclusions
Southern Tibet, China, has always been a significant resource reserve base. The discovery of aquamarine in Longzi County in southern Tibet is not only of great research value but also of great commercial value. Generally speaking, the particle size of Longzi aquamarine is small, and the color ranges from light green-blue to light blue. The fluid inclusions in aquamarine indicate that it is formed in a fluid-rich environment. There is an obvious negative correlation between Fe and Al, and the existence of Li substitutions for Be, indicating that both tetrahedral substitutions and octahedral substitutions exist in Longzi aquamarine. The pegmatite producing Longzi aquamarine is a barren and geochemically primitive beryltype pegmatite (including most pegmatites of the rare-earth type).
There exists more type I H 2 O than type II H 2 O, which is related to the low-alkali metal content. By changing the ratio of the two kinds of H 2 O, we can improve the color of Longzi aquamarine.  Acknowledgments: This is the third contribution of BX for National Mineral Rock and Fossil Specimens Resource Center. Thanks to the two reviewers and the editors for their comprehensive and professional suggestions.

Conflicts of Interest:
The authors declare no conflict of interest. Table A1. Chemical compositions with the structural formulas of aquamarine and tourmalines from Longzi occurrence analyzed by EMPA (in wt.%).