AQUAMARINE, BERYL AND POSSIBILITY OF FINDING EMERALD AND ALEXANDRITE IN VIỆT NAM

NGUYÔN KINH QUèC, NGUYÔN QUúNH ANH

Việt Nam Association of Gemstone

Abstract: In Việt Nam, some aquamarine, beryl deposits and occurrences are distributed in the Xuân Lệ, Thạch Khoán areas and others. In general, beryl of Việt Nam belongs to the rare mineral pegmatite-genetic type, in which aquamarine is in greenish micaceous zone of the marginal part of pegmatite with surrounding rock-metapelite and/or felsic volcanics (greisenization type).

Currently, the occurrences and mineralization points of emerald and chrysoberyl have not been seen in the preliminary study on gemstones of Việt Nam. However, the geological premises and indirect indicators of emerald (aureoles) display plentifully. Definitely, the finding of emerald should be paid attention to the beryl-bearing pegmatite fields close related to the alteration zones yielding green mica, plagioclasite of ultramafic-mafic magmatic rocks, amphibolite as well as chrysoberyl occurrences in Mg skarn of metacarbonate rock largely distributed in Việt Nam, particularly, the Nậm Xe - Tam Đường (Phong Thổ, Lai Châu), Pia Oắc, Chợ Đồn and Sông Lô River (nearby Sông Chảy massif, Hà Giang province) areas.

This study should be done in a detailed research project in the next future.

INTRODUCTION

Precious and semi-precious stones of beryl group are related to the development of beryl- and tourmaline-bearing granitic pegmatite and pneumato-hydrothermal formation (greisenization) in mica-bearing ultramafic-mafic intrusion and carbon-bearing carbonate shale.

Beryl [Be₃Al₂(Si₆O₁₈)] is an alumina silicate mineral having 12 isomorphic minerals, in which emerald and alexandrite have the best quality (first class) for jewel.

The colour of beryl mineral group ranges by isomorphic mixture and some minor/rare elements. For example, emerald containing Cr₂O₃ (0.15-0.6 %) is green or light green, when yielding Fe³⁺ and/or V³⁺- sky-blue in colour with yellow lustre. Sky-blue and navy-blue aquamarines yield Fe²⁺ and violetish-red beryl yields Mn, Co in the octahedral coordination.

Chrysoberyl (BeAl₂O₄) is usually yellow, pale green or brown and contains a small amount of iron. Principal varieties are cat’s eye stone and transparent alexandrite with grass-green or emerald-green colour.

In the 1/1,000,000 scale gemstone map of Việt Nam [3], there are one deposit and 7 occurrences of aquamarine and beryl registered. They are mainly distributed in the Xuân Lệ (Thanh Hoá Prov.), Thạch Khoán (Phú Thọ Prov.) and rarely in Quỳ Châu (Nghệ An Prov.), Lục Yên (Yên Bái Prov.), Thiện Kế (Tuyên Quang Prov.), Pia Oắc (Cao Bằng Prov.), south Sông Chảy Massif and An Điềm (Quảng Nam Prov.) areas, except for many beryl and beryllium. aureoles and small beryl mineralization points.

Figure 1. Sketch map showing the distribution of regional metamorphic complexes metasomatic formation and pegmatite fields yielding corundum, aquamarine…

I. AQUAMARINE AND BERYL

Aquamarine gem of Việt Nam was first discovered in Xuân Lệ deposit (Nguyễn Văn Hương et al, 1985). Later, the KT.01.09 national gemstone project has discovered some aquamarine occurrences with prism crystals of 10-30 mm in size in Dị Nâu, Đồi Đao, Mỏ Ngọt areas (Thạch Khoán, Phú Thọ Prov.) [3] In the An Điềm area and Ca pass (Đồng Hới) there occur uncompleted grains of hexagonal prism type of aquamarine (2-4 mm). There are coarse grains of beryl in tourmaline pegmatite of Paleogene alkali-feldspar granite Complex of Quỳ Châu gem mine and long prism of aquamarine in ruby placer of Đồi Tỷ mine. Genesis of aquamarine in the above mentioned mine and occurrences are closely related to Late Cretaceous - Paleogene granitic pegmatite formation. Besides, beryl and aquamarine are of small size (2-4 mm) in greisens of Paleogene Sn, W-bearing granite located with Middle Triassic felsic volcanites in Thiện Kế area.

In the Xuân Lệ deposit, many pegmatite bodies yielding beryl, topaz, tourmaline and fluorite are 0.4-5.0 m thick and some tens metres long of NW-SE or sub-latitudinal direction. They are related to granosyenite, biotite granite of the Paleogene Sông Chu Intrusive Complex. This intrusion penetrated into Jurassic terrigenous sediments intercalated with felsic volcanites of the Mường Hinh Formation (J mh).

The average mineral composition (% vol.) of pegmatite consists of quartz (38-48), kali feldspar (35-36), plagioclase (18-24), muscovite (2.3-2.5), biotite (little-2) and accessory minerals with beryl, topaz, tourmaline, fluorite, zircon. Particularly, pegmatite yielding beryl in abundance usually contains a low content of small sized mica (0.5-1 cm²). Generally, beryl forms large druses and/or complete prism in pegmatite with the size of 0.5-1 x 1-2 cm². In deluvial-eluvial placers the local people have found big aquamarine prisms with the size of 1.5-2.5 x 6-10 cm², in navy-blue colour, relative transparent, non-fractured corresponding to high grade economic aquamarine. In alluvial-proluvial placers, aquamarine lies in cobble, pebble and sand layers of intermediate position under friable sediments (higher than the bottom layer) with the size of 1-6 x 5-20 cm². They could be exploited together with topaz. Although in many occurrences aquamarine has large-grained size and relative transparence but it has also horizontal noticeable joint, so its gem value is limited (Table 1).

After Nguyễn Văn Hương et al, (1985), the symmetrical zoning pegmatite bodies in Xuân Lệ area are poor in aquamarine, but rich in beryl and accompanied by many morions and sky-vein quartz druses.

Aquamarine prisms of the Thạch Khoán area are beautiful and of large crystals accompanied by beryl of big size. Average content of aquamarine and beryl in a quarry with the volume of 35 m³ cutting through the mica pegmatite body is of 0.8 g/m³ and 50 g/m³, respectively. This pegmatite body is unclearly zonated with the thickness of 5-10 m, composed of large grains (5-10 cm²) of bluish-white mica, black tourmaline, feldspar, quartz, aquamarine and beryl (1%). This vein penetrated into crystalline schist composed of quartz, feldspar, biotite, disthene, staurolite having NW-SE schistose direction. It is difficult to determine clearly the distribution of aquamarine and beryl in the pegmatite body, because a part of the surface layer of the quarry was turned upside-down by local people. However, generally in the deeper layer, aquamarine and beryl are distributed mainly in the upper part of pegmatite body and not far from the quartz-rich and small-grained mica zones. They are similar to the marginal greisen zone belonging to pneumato-hydrothermal alteration. This occurrence is different from very coarse beryl druses in the central zone usually associated with quartz druses. The beryl size is usually 2-4 cm long, sometimes to 10-20 cm. Beryl of the Thạch Khoán area was formed in equilibrium of temperature of 320-580⁰C and pressure of 2.5-2.6 bar.

Table 1. Characters of aquamarine, beryl in bedrocks of different occurences in Việt Nam

Area	Dị Nậu (Thạch Khoán)	Xuân Lệ	Đồi Tỷ (Quỳ Châu)	Thạch Khoán
Sample	D180/1		D1-A9	D3073
Form of crystal	Hexagonal prism	Hexagonal prism	Hexagonal prism (1:6)	Hexagonal prism
Dimension	11 x 31 mm (5 grains-48 cts)	6 x 20; 15 x 35 mm	5 x 33 mm (4.8 cts)	10-40 mm Esp. 15 x 45 cm
Clarity	Transparent, semi-transparent	Transparent, semi-transparent	Transparent (90%)	Semi-transparent
Colour	Light sky blue	Light-dark navy blue	light sky-blue	Sky-blue, dark sky-blue
Refractive index (Ng; Np; D)	1.602; 1.594 D=0.008	1.577; 1.570 D = 0.008	1.602; 1.594 D=0.008	1.590; 1.575 D=0.015
Hardness	7.5-8	8	8	6.8-7.5
Density	2.76-2.89		270	2.80
Fracture	Shell type, non-smooth	Irregular horizontal fracture	Shell type, non smooth	Non-smooth
Clustre	Vitreous	Vitreous	Vitreous	Vitreous
Cleavage	Non	Non	Non	Non
Fluores.spect. of ultraviolet: LW SW	Navy-blue Non coloured		Sky-blue Very weak	Weak sky-blue Weak sky-blue
Inclusions (mm)	Gas, cloud, quartz, feldspar		Gas, cloud	Black ore
Absorption spectrum (nm)	690, 650
Chemical com- position (% wt): SiO2 Al2 O3 FeO Na2O BeO Fe2 O3 Rare alkaline Rb2O Cs2O Li2O	Aquamarine 66.69 19.03 0.16 - ? - 0.12-0.15	Aquam. Beryl 64.16-65.71 66.00 17.76-19.58 18.89 1.10-3.81 0.73 Little-0.48 ? 13.70 0.49 -	Aquamarine 65.87 19.98 0.28 ?	Beryl 65.66-66.27 18.08-18.82 0.25-2.50 0.09-0.23 13.65-15.84 - 0.16-0.19 0.02-0.04 Mn-0.07 0.06-0.12 Ti-0.10 0.01-0,04 Cr-0.02

Table 2. Characteristics of lateral zoning structure of some pegmatite bodies
in the Thạch Khoán area

Number of zones	5 (margin)		4 Ü	3 Ü	2 Ü		1 Ü	0 (centre)
Main mineral composition of zone	Quartz, feldspar, disthene, staurolite, garnet		Plagioclas, quartz, small muscovite	Plagioclase, quartz, muscovite, little kali-feldspar	Microcline, quartz, muscovite		Microcline, quartz, muscovite of big size	Quartz druses. little feldspar and muscovite
Thickness (m)	0.2-1.5		0.5-1	.5-1	0.5-1		1-2	1-2
Size of crystals	Irregular		Small	Medium, irregular (1:6)	Intergrowth		Coarse-grained quartz, feldspar mass	Quartz core, druses
Concentration of minerals
-Plagioclase		1%	20-30%	25-30%		20%	4.6%	-
- Kali feldspar		Little	3-5%	2-4%		41%	50-58%	-
-Generations of muscovite			I-muscovite	II-muscovite		II-muscovite III-muscovite IV-muscovite (light sky blue)	IV-muscovite (later) V-muscovite	VI-muscovite
-Beryl and/or aquamarine			Aquamarine	Small crystal (aquamarine)		Big crystal and scarce (aquamarine)	Very coarse crystal and concentration
- Quartz			Small grain	Medium grain			Coarse grain	Block, mass
-Other minerals		Disthene, staurolite, garnet, sillimanite	Many garnet, disthene	Garnet (little), dark-brown tourmaline			Dark-black tourmaline	Brown tourmaline

Source: Lý Bá Tiến et al, 1988; Nguyễn Kinh Quốc et al, 1995.

The Table 2 shows the lateral zoning structure of a pegmatite body in the Thạch Khoán area. From central to marginal zone of pegmatite (0 Þ 5), the main mineral concentration is as follows: quartz druses, very coarse beryl crystal, coarse beryl crystal and/or aquamarine sparsely distributed, small beryl crystal, garnet-disthene and garnet-disthene-staurolite-sillimanite in the marginal zone.

When studying the vertical structure of the weathering crust of pegmatite body it suggests that beryl and/or aquamarine occurs in the very weakly weathered zone and weakly fractured structure zone of bedrocks; garnet and tourmaline - in the semi-weathered zone. Particularly, disthene and staurolite are in eluvial, deluvial and alluvial placers.

In the Đồi Tỷ Mine (Quỳ Châu area) the big grain of beryl (1-2 cm) has been discovered in body of pegmatite yielding dark tourmaline and corundum. Some long aquamarine prisms (3 x 22 mm) are in ruby placer. Mineral and gem characteristics of the Đồi Tỷ aquamarine and beryl are listed in Table 1, column 4, Photo 1.

II. POSSIBILITY OF FINDING EMERALD AND CHRYSOBERYL/ALEXANDRITE

In the World map of gem deposits (E.J. Gubelin, 1992), in general, and in the South-Southeast Asia gemstone metallogenic province, in particular, there are 6 regions containing beryl and chrysoberyl, such as Jakdalek (Afghanistan), Mysore (S. India), Elehara (Sri Lanka), Yunnan (SW China), Phrae and Bo Phloi (N. and Central Thailand), except for Việt Nam, Laos and Cambodia, where no occurrence of these gems has been found. In fact, there are some emerald occurrences in black schist in N. China, in halleflinta-quartz vein (quartzofeldspathic metavolcanic type rock) of chromium mica-biotite-diopside granulite of Dayakou zone in Malipo (SE Yunnan, China), an aquamarine mine namely Na Don in Sam Neua province of NE. Laos [7] and some above said aquamarine mine/occurrences in Việt Nam. So, up to now, there is not any occurrence of emerald in the S-SE Asia gemstone metallogenic province.

Photo 1. Crystal and cut aquamarine from the Đồi Tỷ Mine, Quỳ Châu area (above)
and the Dị Nậu Mine, Thạch Khoán area (under)

Chrysoberyl of industrial genesis is distributed in beryl bearing-pegmatite (Brazil, Madagascar, Sri Lanka) and concentrated mainly in phlogopite-biotite-emerald bearing metasomatic zone of ultramafite (Ural of Russia, S. Africa and India).

Currently, direct indicators of emerald and chrysoberyl occurrences in Việt Nam have not been seen in the preliminary study on gemstones in the past time. However, the geological premises and indirect indicators of emerald (beryllium element and beryl mineral aureoles) are manifested plentifully as follows:

- The Tạ Khoa dome-type structure including shale/biotite gneiss, green-schist and metadiabase, ultramafite; the Bản Xang ultramafic intrusion (Tạ Khoa, Sơn La Pov.) penetrated by many garnet-tourmaline bearing pegmatite bodies of Middle Triassic age. They formed some great metasomatic zones bearing mica (vermiculite, green mica...), pleonaste and very coarse-grained plagioclasite bodies yielding small grains of red rutile. After preliminary study on the materials from the Bản Xang intrusive bodies [3], beryl and/or emerald have not been seen by naked eyes seen in the field trips.

- In the Pia Ma - Chợ Đồn areas (Cao Bằng Prov.) there are some nepheline syenite, gabbroid and granitoid intrusive complexes and their pegmatite swarms with alkali metasomatic zones in marble, coaly schist and manganese siliceous shale of Pia Phương Formation of Early Devonian age.

- In the Pia Oắc area (Cao Bằng Prov.) area there is the Late Cretaceous tin-bearing granitoid complex and its pegmatite swarm yielding uranium, fluorite with the beryllium element aureole in the surrounding rocks of terrigenous-carbonate sediments and Cao Bằng dunite-gabbrodiabase-granite-granophyre complex.

- The existence of the Paleozoic dunite, gabbro and diabase, plagiogranite series of longitudinal direction with simultaneously tourmaline pegmatite activities in the volcanic sedimentary deposits of the Hà Giang and Pia Phương Formations along the Sông Lô area (E-SE Sông Chảy granitoid massif).

- Dark-green micaceous zone related to the metasomatic activities of pegmatite granite and amphibolite. Lenses, small ultramafic intrusions as well as metacarbonate rocks (calciphyre marble in the Núi Sõng (Hùng Temple, Phú Thọ Prov.), Bảo Yên - Bảo Hà (Lào Cai Prov.) areas etc. along the Paleoproterozoi Sông Hồng metamorphic belt.

- The Phan Si Pan Paleogene alkali intrusive series with their rare metal pegmatite dike swarms and metasomatites yielding REE-U ores and fluorite in the ultramafic-mafic and metacarbonate surrounding rock fields, widespread in the Nậm Xe - Tam Đường areas (Lai Châu Prov.).

- Many occurrences of talc schist, micaceous rock and tremolitite, diopside jade, nephrite type rock, metamafic-ultramafic intrusions, plagiogranite, garnet-tourmaline pegmatite in the Cò Phương and Nậm Giôm areas (Sơn La Prov.) of Early Paleozoic ophiolite association located along the Sông Mã suture zone in metamorphic grade of amphibolite facies with relative high pressure.

- In Đồng Hới dome-typed structure where occur corundum, andalusite-bearing banded metamarl, amphibole-diopside-olivine-calcite rock, magnetite-calcite-diopside skarn, Trường Sơn diorite-granite-granosyenite Complex and tourmaline-beryl bearing pegmatite (Ca Pass).

- Some occurrences of micaceous rock, tremolite-actinolite in alteration zones of the Chư To Dron, Làng Hồi, Dak Long ultramafic intrusions related to the adjective pegmatite and granite along the Hiệp Đức submeridional and Sa Thày - Plei Ut longitudinal suture zones.

- Many occurrences of micaceous zone in metaamphibolite. Lenses/thin layers of metacarbonate beds interbeded in crystalline schist of (high) amphibolite and granulite facies of the Kon Tum Archean-Paleoproterozoic uplifted geoblock with the completed activities of many pegmatite phases and granite complexes of various ages.

Differing from ruby, sapphire and spinel, which are rather stable and usually simultaneously formed in alluvial placer deposits, minerals of beryl group are changeable in the humid tropical weathering condition. So, gem beryl (aquamarine etc) especially derived from gem-bearing primary sources related to accumulation of relative light materials and minerals in the upper part of weakly weathered pegmatite and alteration zones and in deluvial-eluvial deposits.

The above mentioned areas should be paid attention to check by field trips in prior order when meeting beryllium element and/or overlapped beryl aureoles.

It is necessary to point out that emerald could be found in green micaceous metasomatite of ultramafite-mafite and Late Cretaceous - Paleogene pegmatite, granitoid, as well as chrysoberyl in their skarn and metasomatic zones of Việt Nam, particularly, in the Nậm Xe - Tam Đường (Phong Thổ, Lai Châu Prov.), Pia Oắc, Chợ Đồn and Sông Lô (near Sông Chảy Massif, Hà Giang Prov.) areas.

CONCLUSIONS

Among beryl gem the first to be paid attention are emerald and chrysoberyl (first class), and then, aquamarine belonging to semi-precious stone (second class). In Việt Nam, some aquamarine deposits and occurrences are distributed in the Xuân Lệ, Thạch Khoán areas and others. However, emerald and chrysoberyl occurrences are not met yet.

On the other hand, the geological premises and indirect indicators on the existence of emerald and alexandrite display clearly. It is necessary to point out that emerald could be found in green micaceous metasomatite of ultramafite and beryl-bearing pegmatite as well as chrysoberyl in their skarn and metasomatic zones largely distributing in Việt Nam, particularly, in the Nậm Xe - Tam Đường (Phong Thổ, Lai Châu Prov.), Pia Oắc, Chợ Đồn and Sông Lô (near Sông Chảy Massif, Hà Giang Prov.) areas.

This study should be continued in a detailed research project in the next future.

REFERENCES

1. Chijik O.E., Lejkukh Z.V., 1980. On genesis of emerald of micaceous type. In: Precious and coloured stones. Nauka, Moskva (in Russian).

2. Kievlenko E. I. , 1982. Geology of gemstone deposits. Nedra, Moskva (in Russian).

3. Nguyễn Kinh Quốc (Chủ biên), 1995. Báo cáo Nguồn gốc, qui luật phân bố và đánh giá tiềm năng đá quý, đá kỹ thuật Việt Nam. Lưu trữ Địa chất, Hà Nội.

4. Nguyễn Kinh Quốc, 2000. Gemstone metallogeny of Indochina and adjacent regions. Abstr. 31st Intern. Geol. Congr. Rio de Janeiro, Brazil.

5. Nguyễn Kinh Quốc, 2005. Discussion on aquamarine, beryl and possibly finding out emerald, alexandrite in Việt Nam. Intern. Workshop on Gem-Mat-Man, Hà Nội (Unpublished).

6. Nguyễn Văn Bình, Phan Tiến Dũng, Trần Văn Soát, Nguyễn Thị Hồng, 2005. The mineral beryl from Sông Chảy Massif and possibility to find there aquamarine and emerald. Proc. of Gem-Mat-Man Intern. Symposium, p. 146-158. Hà Nội.

7. Rook P.J., B. Vongdara, 1991. Geological and mineral occurrences of Lao PDR. 2^nd Conf. Geol. of Indochina. Hà Nội.

8. T.T.M. Nguyệt, B.T. Hiệp, T. Chot, 2001. The characterization of Vietnamese aquamarine by UV-VIS absorption spectroscopy. Workshop on Material characterization by solid state spectroscopy: gems and minerals of Việt Nam. Hà Nội.