SERICITE MINERALIZATION IN VIÖT NAM AND ITS ECONOMIC SIGNIFICANCE

TRÇN TRäNG HUÖ, KIÒU QUý NAM

Institute of Geology, NCNST, Hoµng Quèc ViÖt, CÇu GiÊy, Hµ Néi

Abstract: Sericite has been studied and largely used in chemo-cosmetic industry in many countries, such as South Korea, Japan, France, etc.. However, at present sericite has not been studied and evaluated yet in our country as an useful mineral.

Basing on collected materials, for the first time the authors present some research results on sericite as a new source of mineral raw material of great economic significance and potentiality in ViÖt Nam, at the same time suggest some works needing to be invested for studying in the time to come.

I. GENERAL OBSERVATION

Long ago, minerals of the mica group have been known as a technical raw material having special qualities, such as heat insulate, sound insulate and electricity insulate, therefore, they are largely used in the electric, electronic, construction, etc. domains. And when one speaks about mica as an ordinary mineral, biotite and muscovite are mainly touched upon [1, 2, 4]. In our country mica has been found, evaluated and exploited for use, although with a limited level.

The technic and economic values of mica increase in accordance to the dimensions of its crystals in the plate form. Therefore, the prospecting and finding of mica sources of great value are concentrated mainly in the objects having favourable premises for the formation of large mica crystals, such as granite pegmatite bodies located in Proterozoic rocks distributed along the Red River metamorphic zone in Lµo Cai, Yªn B¸i, Th¹ch Kho¸n in the North, and Qu¶ng Nam, Qu¶ng Ng·i, Kon Tum in the South. So, the premises of favourable formation of other microcrystalline and cryptocrystalline forms of mica (sericite) are usually thrown away, although they have not less important economic significance.

Sericite is the cryptocrystalline form of muscovite, having the same chemical formula KAl2(OH)2(AlSi3O10) and the colour changing in accordance with mixtures. In its appearance an assemblage of monomineral sericite is difficult to be distinguished with kaolin, therefore, in the field it is usually wrongly determined as kaolin or white clay. The most salient differentiation between sericite and kaolin is its fineness and fat feeling similar to talc that kaolin has not.

In our country sericite is usually spoken about in geological documents as other rock making minerals in metamorphic formation, such as clay-sericite schist, quartz-sericite schist, etc., but completely not studied on the mineral resource side.

In practice, "monomineral" sericite assemblages are largely used long ago in the chemo-cosmetic industry, and the economic value of sericite is higher when its granulity is finer, and especially its whiteness is higher. In the world market the price of sericite varies in accordance with its quality, and can reach 40,000 USD/t for prossessed products. In Asia, Japan and South Korea are the first countries in exploitation, process and use of sericite.

 

III. SERICITE MINERALIZATION IN VIÖT NAM

Aiming to contribute new informations to the study on non-metal mineral resources for the reasonable use of resources and raising the economic efficiency of minerals, enlarging the list of useful minerals the Institute of Geology, NCNST for the first time carried out the investigation of some areas of the country having favourable premises in the sericite mineralization of commercial value in ViÖt Nam, and has been obtaining first satisfactory results.

1. Sericite mineralization in the La Vang area, Qu¶ng TrÞ Province

The La Vang sericite mineralization area lies on the territory of the H¶i Phó Commune, H¶i L¨ng District, Qu¶ng TrÞ Province, in 2 km southwest of the Qu¶ng TrÞ Town.

In this locality sericite was found in investigation well (Fig.1) in various depths from 4 to 10 m in the form of mineral bodies of various sizes, and of vein form crosscutting surrounding thick-bedded sedimentary members of the Middle Long §¹i Subformation (O-S1 ld2) with the dip of about 50-70o. These members consist of claystone, silty claystone with kaolinite as the main mineral.

The mineral bodies consist of white, loose sericite assemblage mixed with much quartz grit of 0.5 to 1 cm in size. Through the X-ray diffractometric analysis realized on equipment SIEMENS D5000 at 30 kV, 30 mA in the lab temperature we can see clearly the change of mineral composition in descending order of this well. In surrounding rocks, beside the main minerals as kaolinite and sericite which have insignificant content, but tending to increase in the vein side, there still are microcline and kalifeldspath. Combining with existed geological materials of the studied area, we can preliminarily suppose on the forming mechanics of sericite which is related to the alteration of feldspar-rich geological bodies; they are granite (diorite) pegmatite dykes and veins very widespread in the Long §¹i Formation which occurs largely in the area. They are of different time phases with the impact of hydrothermal flows of the later phase, causing the change of existed formations by the following reaction equation:

3K(AlSi3O8) + H2O = KAl2(OH)2(AlSi3O10) + 6SiO2 + K2O

(kalifeldspath) (sericite)

 

 

Photo 1a. Japanese commercial sericite sample Magnification x 3,500

 

Photo 1b. South Korean crude sericite sample Magnification x 3,500

 

 

 Photo 1c. La Vang sericite sample Magnification x 3,500

2. Sericite mineralization in the B×nh Liªu area

In the B×nh Liªu area, sericite mineralization occurs mainly in 2 km east of the B×nh Liªu Townlet [5].

There, sericite is closely connected with sediments of the Middle Hµ Cèi Subformation (J1-2 hc2) which is composed of white-grey, white-greenish claystone, fine silty claystone. The results of Rx analysis show that the mineral composition consists of, beside sericite, kaolinite and quartz, however, differing from Qu¶ng TrÞ samples, this formation still remains its typical sedimentary bedding.

In our opinions, the strong hydrothermal activities in the distribution area of the TÊn Mµi Formation that have been touched upon in former works [5] are the main factor causing the contact metasomatic process of hydrothermal type for kaolinite-rich claystone formation which were transformed into bedded sericite by the supplementation of K2O from the hydrothermal flows according to the following reaction equation:

3Al4(OH)8(Si4O10) + K2O = 4KAl2(OH)2(AlSi3O10) + 8H2O

(kaolinite) (sericite)

The result of composition analysis of sericite sample from the B×nh Liªu area are presented in the Fig.2 and Table 3.

Table 3. Results of granulity analysis of sericite sample from B×nh Liªu

Grain size (mm)

0.4

0.315

0.25

0.2

0.16

0.125

0.1

0.08

0.063

<0.063

Content (%)

1.32

7.6

5.4

5.0

3.6

20.2

7.4

13.4

16.4

17.0

Table 4. Results of chemical composition analysis of sericite sample from B×nh Liªu

No

Composition

Content (%)

Sample 1

Sample 2

1

SiO2

65.32

64.32

2

Al2O3

19.44

17.51

3

MgO

1.36

2.01

4

Fe2O3

2.70

2.08

5

FeO

0.54

0.62

6

Pb

2.03 ppm

2.07 ppm

7

Hg

1.93 ppm

2.00 ppm

8

As

<1 ppm

<1 ppm

9

K2O

1.52

1.49

10

Humidity

0.56

0.75

11

LOI

6.15

6.66

III. USING POSSIBILITY OF VIETNAMESE SERICITE

Comparing the quality of Japanese processed commercial sericite with Vietnamese sericite one can easily find that the Vietnamese sericite, basically, can answer the requirement on some important indices. Especially, the La Vang sericite has very fine granulity, usually reaching about 1-2 m m, while the commercial sample requires only the size of 5 m m (Fig. 3, Photo 1a, 1b, 1c); with the restricted composition and limited content of harmful elements, such as Pb (in average 0.922 ppm), Hg (in average 0.99 ppm), As (< 1 ppm). For the B×nh Liªu sericite the content of harmful elements is also smaller or equivalent to the minimum permitted content of commercial sericite (Pb: 5 ppm; Hg: 1 ppm; and As: 3 ppm). Besides, the not processed yet La Vang sericite has high whiteness (equivalent to the Japanese commercial sericite), therefore not requiring complicated technical processing. The above estimations have been checked by the South Korean SHINEKO Co. which has been proposing further studies. However, the sericite content of the collected samples is usually low (occupying about 50% of the clay component), that cannot compare with the quality of the commercial sericite. For reaching more satisfactory results, the beneficiation technology needs to be paid more attention in the time to come parallelly with the basic investigation.

CONCLUSION

With above preliminar informations the authors hope that in the near future sericite will be paid more attention in its study for having an all-round evaluation on its potentiality and quality, as well as in the beneficiation technology for this new, but full of economic potentiality, mineral type.

During the process of investigation and evaluation of sericite as a new mineral raw material which still is not yet cared to study the authors would like to express their thanks to Prof.Dr Ph¹m V¨n An of the University of Mining and Geology, Assoc.Prof. Dr NguyÔn §Þch Dü, Dr §ç Tù, Dr NguyÔn Xu©n H·n, Eng. NguyÔn ThÞ S¸u, Eng. NguyÔn ¸nh D­¬ng of the Institute of Geology for their precious recommendations and help in the realization of this paper.

REFERENCES

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2. Industrial Grade. CAS.12001/26/2. Mica. 2000.

3. NguyÔn Xu©n D­¬ng (Chñ biªn), 1996. §Þa chÊt vµ kho¸ng s¶n tê LÖ Thñy - Qu¶ng TrÞ (Geology and mineral resources of the LÖ Thñy - Qu¶ng TrÞ Map sheet). ThuyÕt minh tê B¶n ®å ®Þa chÊt 1:200.000 LÖ Thñy - Qu¶ng TrÞ. Côc §Þa chÊt vµ Kho¸ng s¶n ViÖt Nam, Hµ Néi.

4. Pough Frederick H., 1996. Rocks and minerals. New York, Boston.

5. TrÇn Xu©n To¶n, 1983. Quartzit thø sinh vïng TÊn Mµi vµ mét sè kho¸ng s¶n liªn quan ®Õn chóng (Secondary quartzite in the TÊn Mµi area and some related minerals). LuËn ¸n TiÕn sÜ, Th­ viÖn Quèc gia. Hµ Néi.