10th Asian Regional Conference of IAEG (2015)

Study on the chemical weathering of black shale in northern Guangxi, China
Xin LIAO (1), Jinhang ZHANG (2), Jian LI (3), Masahiro CHIGIRA(4), Xiyong WU (5)(1)Department of Geological Engineering, Southwest Jiaotong University, China
E-mail:xinliao@swjtu.edu.cn(2)Department of Geological Engineering, Southwest Jiaotong University, China(3)Yibin city Education and Sports Bureau, Sichuan, China(4) Disaster Prevention Research Institute, Kyoto University, Japan(5)Department of Geological Engineering, MOE Key Laboratory of High-speed Railway Engineering,Southwest Jiaotong University, China
Abstract
The black shale rocks are widely distributed in northern Guangxi, China. Some engineering problems
are caused by the black shale chemical weathering, in which pyrite oxidation plays important role in theprocess. The identification of minerals and chemical constituents as well as the observation ofsubmicroscopic morphology was conducted to provide insight into the nature of the black shale rock.
Comparing to the materials produced by the black shale weathering in the field and laboratory, it isevidenced that the acid solution and Fe-sulfate minerals as the same weathering products have significant
effects on the rock’s properties and ecological environment.
Keywords: Black shale, Chemical weathering, oxidation

1.

Introduction

formation black shale are studied to reveal its chemicalweathering features.

Black shale weathering is typical geologicalprocesses dominant by chemical weathering. The

2.

sulfide minerals contained in the black shale can beoxidized to produce corrosive acidic water and some

Geological setting
The study area is located in the Sanjiang county,

minerals, such as gypsum and jarosite with poor

Guangxi province (Fig. 1). It is the hilly area withengineering property in the oxidation environment.

some mountains and less flat ground, where the terrain

Therefore, the black shale is one of the typical strata

tilts from north to south. The Qingxi formation strata

which can cause engineering problems (Chigira andare mainly carbonaceous shale, black shale andOyama, 1999; Zheng et al, 2007). The Cambrian black

carbonaceous siliceous shale with gray-black laminate

shale is widely distributed in Guizhou, Hunan andor lenticel limestone interlayer in the Rongshui andGuangxi regions, China. The Qingxi formation black

Quanzhou counties; In the Rong’an area, gray andshale as one type of Cambrian black shale is turbidity

black carbonaceous shale and sandy shale are in thedeposition mainly distributed in the northern Guangxi

lower part, gray-black fine-grained quartz sandstone,region. Some previous studies have been conducted toblack shale and sandy shale with a small amount ofunderstand the causes of ore-forming, petrological andlenticel limestone are in the middle and upper parts,

geochemical characteristics of the Cambrian black

while the black carbonaceous shale is in the top part.

shale in northern Guizhou and western Hunan regions

On the whole, the Qingxi formation strata show

(Wei et al, 2006; Liao et al, 2013). In this paper, thedecrease in sandy and siliceous rocks and increase inmineralogy, chemical composition, sub-microstructure

shale and carbonaceous shale from southeast toand water-rock interaction characteristics of the Qingxi

110th Asian Regional Conference of IAEG (2015)

northwest on the cross section (Xiang, 1999).

Fig. 1 Distribution map of cambrian Qingxi Formation in Guangxi
3.

Sampling and methods
The Qingxi formation black shale samples were

X-ray diffraction (XRD; Rigaku Geigerflex RAD-IIB)

collected at the outcrop near a power station (Fig. 2).

using Cu Ka radiation at 20 kV and 50 mA from 3° toSome fine-grained quartz crystal particles are visible

40°. The scanning speed is 2°/ min and the stepper

from the dark black and hard rocks. The pyrite particles

angle is 0.1°. Chemical analyses were conducted with

were found rich along the bedding plane oran X-ray fluorescence spectrometer (ARL Advant’ XP).

accumulated to pyrite nodule locally. The sedimentary

Seven rock samples including slightly weathered (SW)structure

showingand relative fresh (RF) shale were chosen for analysis.

laminated structure. Seven rock samples were collected

The rock powder samples after grinding were pressedfrom slightly weathering to fresh. Two precipitates

into piece. Polished thin sections of black shale wereoccurred on the rock surface and two fissure water

analyzed by scanning electron microscopy (SEM;

sample were collected as well.

HITACHI S3500). The hydrochemical analyses wereisparallel

bedding

Four samples including two black shale rocks andconducted with the inductively coupled plasma mass

two weathering precipitates were selected to grind less

spectrometry (ICP-MS) for the water samples under thethan 200 mesh. Mineral analyses were conducted withconditions of 23℃ and humidity 53%.

Fig 2. Outcrop in the study area
(Note: 1#、2#、3#—rock sampling location, 4#—white precipitate, 5#—yellow precipitate, A、B—water samples)
The mineral analysis results determined by XRD

4 Results

were shown in Table 1. Black shale samples were

4.1 Mineralogy

mainly consist of illite, muscovite, quartz and small2

10th Asian Regional Conference of IAEG (2015)

amount of pyrite. The white weathering product wascrystal is tabular, flaky or granular, which cleavage

mainly rozenite (FeSO4·4H2O) and small amounts ofsurface can be clearly visible with the magnification ofillite, quartz, pyrite as well as trace amounts of jarosite

2000. The diameter and thickness of the flaky crystal isand gypsum. The yellow weathering product was

3-5μm and 0.3μm, respectively. Generally, the crystal

2+3+mainly copiapite (Fe Fe 4(SO4)6(OH)2·20H2O) andis quite small and can aggregate into sulfur-yellow tosmall amount of quartz, ferricopiapite, jarosite andlemon-yellow powdered or hemispheric particle (Figmagnetite.

3.a). The rozenite is monoclinic and its crystal structure
is tabular with the thickness of 2-3μm. The crystal canaggregate into white sheet and granular powders with

4.2 Chemistry

quantities of interior pores (Fig 3.b). The white
The results of the chemical analyses are reported inaggregates have glassy luster and white Stripes byTable 2. It is shown that the highest major element is

naked eyes.

SiO2 ranging from 67.0 to 74.5%. The following is
Al2O3 and K2O ranging from 12.0 to 14.0% and 4.2 ~4.4 Hydrochemistry

5.1%, respectively. The content of Fe2O3 and MgO areclose, ranging from 0.5 to 2.6% and 0.5 to 2.6%,

The hydrochemical analyses results are shown inrespectively. The contents of other chemical

Table 3. The pH value of the water samples were about

components are relative lower.

2.5 which indicated the solution was strongly acidic.
The total salinity was between 3 ~ 10 g/ L on behalf ofmoderate mineralized water. Thus, the type of the water

4.3 Sub-microscopic observations

samples were SO42- —Fe2+/3+, Ca2 + and Mg2+. Besides,
The sampled two weathering products werethe content of the sulfate and total iron ions were very

observed using the SEM. The copiapite is triclinic andhigh, which was inferred to be the result of the pyrite

parallel double-side crystal. The structure of the single

oxidation.

Table 1 Mineral composition of the samples
SampleIlliteMuscovite

PyriteCopiapite

Rozenite

Jarosite

GypsumMagnetite

RF++

+-(*)

(*)

(*)

(*)

(*)

SW+++-(*)

(*)

(*)

(*)

(*)

WP

-(*)

-(*)

++

----

(*)

YP

(*)

(*)

(*)

++

(*)

-(*)

-

SW: slightly weathered. RF: relative fresh. WP: white precipitate. YP: yellow precipitate++: abundant, +: medium, -: small amount. --: trace amount. (*): none. Quartz is contained in all samples.
Table 2 The major element contents of the Qingxi Formation black shale
Sample

SiO2

Fe2O3

Al2O3

CaO

MgO

K2O

Na2O

P2O5

TiO2

MnO

SO3+S

RF1

74.53

0.57

12.11

0.30

1.31

4.26

0.05

0.00

0.70

0.10

0.09

RF268.56

0.61

12.03

2.88

1.28

4.36

0.05

0.00

0.79

0.25

0.09

SW166.98

1.80

13.97

0.07

1.33

5.04

0.05

0.06

0.77

0.10

0.19

SW267.29

2.00

13.52

0.78

1.38

5.07

0.07

0.04

0.71

0.16

0.61

SW3

67.45

2.29

13.50

0.09

1.34

4.97

0.04

0.09

0.69

0.14

0.17

SW467.02

2.56

13.64

0.10

1.32

4.99

0.05

0.10

0.76

0.09

0.19

SW567.35

2.60

13.26

0.24

1.33

4.78

0.09

0.05

0.68

2.19

0.49

SW: slightly weathered. RF: relative fresh.

3

10th Asian Regional Conference of IAEG (2015)

Fig 3. Representative SEM micrographs of weathering products. a) copiapite, b) rozenite.
Table 3. Chemical compositions of fracture waterSample

CO32-ClSO42-

SiO2

A

0.00

<25977B

0.00<22234-

2-

HPO42+

Ca

Mg

99.5

0.40

114

32.4

0.61

51.6

2+

unit: (mg/L)
+Na+

Al3+

TFe

pH

265

1.11

0.61

377

1367

2.46

33.4

1.15

0.39

69.3

678

2.43

K

5. Half-immersion test in laboratory

produced in strong acidic environment. The

5.1 weathering behaviors under laboratory

yellowish-brown precipitate was inferred to be ironobservationoxides and hydroxide, which were produced by ferric
ion hydrolysis and attached on the rock surface. The

The indoor half-immersion experiment lasted aboutsignificant differences between the upper and lower

six months. Because the experimental time wasrock surfaces were that there were many visible whitebetween autumn and winter, the temperature andquartz minerals but no copiapite or rozenite on thehumidity ranged about 8 ~ 28 ℃ and 60 ~ 70% in thelower rock surface. Besides, it is also worth noting thatlaboratory. The samples were placed in a transparent

the immersion solution gradually changed from nearplastic vessel and immersed in deionized water. The

neutral to strongly acidic during the wholeimmersion height was about one-third to one-half ofhalf-immersion period.

the height of the samples. After the water lever was
5.2 Electrical conductivity and pH

below the tick line about 1/3 by evaporation, deionizedwater was supplied to return to the initial water lever.

The pH and electrical conductivity (EC) of theFrom the morphological observations on the side
and cross profiles after the experiment (as shown in Fig.

immersion solution were measured at the interval of4), it is shown that yellow and white precipitates

one day after 7 days. The results showed the immersion

occurred in patches on the upper surface exposed to air

solution was acidic and the pH ranged from 2.4 to 4.0.

and yellowish-brown precipitate formed on the lower

The EC values ranged from 0.09 to 1.63 mS/cm. Basedsurface immersed in water under the influence ofon the correlation analysis of these two parameters, itwater-rock interactions. Furthermore, the yellow and

was indicated that they showed exponential relation inwhite precipitates were examined to be copiapite andthe half-immersion process (Fig. 5).

rozenite by XRD. Both of them were soluble and4

10th Asian Regional Conference of IAEG (2015)

Fig 4. Weathering phenomenon after half-immersion. a) weathering products on the rock surface exposed in the air;b) the profile represented for the rock surface above and under the water lever (the dashed line).
2.0sample
ExpDecEC( ms/cm)

1.6

1.2(-x/0.50)

y = 173.70e
2
R = 0.9786

0.8-0.0058

0.4

0.0
2.0

2.4

2.8

3.2

3.6

4.0

4.4

pHFig 5. The pH - EC relations of the immersion solution

6 Discussion and conclusions

sulfate minerals. Generally, they are prone to

6.1 Black shale weathering process and products

precipitate on the rock surface at outcrop, where therocks are rich in the pyrite or marcasite particles(Jambor et al, 2000). It is speculated that they may

Based on the mineralogical and chemical analysis,

originate directly from pyrite in study area. The overall

it indicates that the sulfide mineral is mainly pyrite.

reaction can be represented as equation (1) and (2).

The iron and sulfur element content increases with theIn addition, with the development of the external

development of weathering. It is inferred that the ironcondition and reaction process, the sulfate ionsand sulfate ions produced by the water-rock interaction

produced by pyrite oxidation could react with the otherof the fresh shale migrate upwards under the capillaryelement in rocks or other ions contained in theeffects of the pore water. Therefore, the two elements

underground water to form different kinds of sulfate

content in the weathered rock samples were relativeminerals, such as the jarosite and gypsum.

higher than that in the fresh rock samples. From thechemical formula of the yellow copiapite and whiterozenite, it shows the two precipitates are both ferrous
2FeS2+7O2+10H2O→ 2FeSO4·4H2O +2H2SO42+(1)

3+5 FeS2+ 37/2 O2 + 25H2O → Fe Fe 4 (SO4)6(OH) 2·20H2O + 4H2SO45(2)10th Asian Regional Conference of IAEG (2015)

6.2 The acidic environment and its engineering

remarkable swell-shrink characteristics as theimpact caused by the black shale weathering

crystal water gained and lost in the wet-dry cycling

Sulfide minerals as one of the importantenvironment. These minerals generally occur on thecomponents of the black shale, could produce strong

fissure surface, which can cause constant expansion of

corrosive acid water through hydrolysis, oxidation andthe micro-cracks. Second, the acidic water can react

free organic acids during the oxygen-rich surface waterwith the feldspar minerals to produce clay mineral soleaching process. This acidification could bringthat it could reduce the mechanical strength of theargillization and failure to the rock masses. Based onrocks.

the hydrochemical analysis of the water samples

Therefore, the study of the chemical weathering ofcollected in the field, the pH - EC relationship of thethe black shale makes an important theoretical andimmersion solution and the reaction formulas (1) andpractical significance to the reaction mechanism itself(2), the chemical weathering of black shale is an acidand the evaluation of influence on the engineering

producing process which has been proved by previousconstructions.

studies (Lowson, 1982; Nesbitt et al, 1998; Yue et al,

Acknowledgements

2004; Liao et al, 2014). It is noteworthy that theelectrical conductivity and pH values have no obvious

This research was supported by the National Natural

mathematical relation. Because the EC value is

Science Foundation of China (Project No. 41172261

relevant to the property of the ions and pH value only

and Project No. 41472256), the Key Technology

reflects the concentration of the hydrogen ion.

Research and Development Program of Sichuan

However, the pH and EC values have exponential

Province (Project No. 2012SZ0051), the Fundamental

relationship for our immersion solution. It is inferred

Research Funds for the Central Universities (Projecton one hand, the evaporation caused the hydrogen ion

No. 2682014BR013), the People’s Republic of China.

concentration increasing and pH value decreasing sothat the EC value increases correspondingly. On theReferences

other hand, the hydrogen ion produced by pyrite

Chigira, M., Oyama, T. (1999): Mechanism and effectoxidation reacts with the clay minerals to release moreof chemical weathering of sedimentary rocks,

soluble ions, which makes EC value increase. Besides,Engineering Geology, Vol. 55, pp. 3-14.

the exponential relationship between the pH and EC

Jambor, J.L., Nordstrom, D.K., Alpers, C.N. (2000):

value might be associated with theSulfate minerals-crystallography, geochemistry,ionization equilibrium constant of the reaction products

and environmental significance. Reviews in

as well (Shi et al, 2013). The detailed mechanism

mineralogy and geochemistry, Vol. 40, pp.

should be further studied.

303-350.

On account of the fast neutralization andLiao, X., Wu, X., Zhu, B. (2013): Chemical weatheringdecomposition of the acidic water, the quick changes incharacteristics of Cambrian black shale inrock’s composition and structure cause by acidic

northern Guangxi, China, Journal of Central South

corrosion not only occur in the rock itself, but also

University, Vol. 44, No. 12, pp. 4980-4987. (inprompt the corrosions to the adjacent rocks andChinese)

relevant construction material. These effects could

Liao, X., Chigira, M., Matsushi. Y., et al. (2014):

bring a series of engineering problems (Pye and Miller,

Investigation of water-rock interactions in1990). What’s more, the various products formed byCambrian black shale via a flow-throughblack shale weathering have adverse effect on the rockexperiment, Applied Geochemistry, Vol. 51, pp.

mass integrity and engineering mechanical property as65-78.

well. First, the sulfate minerals such as jarosite,Lowson, R.T. (1982): Aqueous oxidation of pyrite bycopiapite and gypsum produced by the chemicalmolecular oxygen, Chemical Reviews, Vol. 82, pp.

weathering of the black shale have610th Asian Regional Conference of IAEG (2015)461-497.

Guizhou province, China, Journal of Guizhou

Nesbitt, H.W., Bancroft, G.M., Pratt, A.R., Scaini, M.J.

University, Vol. 23, No. 4, pp. 356-360.

(1998): Sulfur and iron surface states on fractured

Xiang, L. (1999): Stratigraphic lexicon of

pyrite surfaces, American Mineralogist, Vol. 83,

China-cambrian, Beijing: Geological Publishing

pp.1067-1076.
Pye, K., Miller, J.A. (1990): Chemical and biochemical
weathering of pyritic mudrocks in a shaleembankment, Quarterly Journal of Engineering
Geology, Vol. 23, pp. 365-381.

House, pp. 51-52.
Yue, M., Zhao,F., Ren, D., et al. (2004): The
environment geochemistry information of the coalmine acid mining drainage, Coal Geology &Exploration, Vol. 32, No. 3, pp. 46-49. (inShi, C., Deng X., Wu B., et al. (2013): Characteristics

Chinese)Zheng, G., Lang, Y., Miyahara, M., et al. (2007): Ironoxide precipitate in seepage of groundwater froma landslide slip zone, Environmental Geology, Vol.
51, No. 8, pp. 1455-1464.

of Precipitation pH and Conductivity at Mt.
Huang, Environmental Science, Vol. 34, No. 5, pp.
1964-1972. (in Chinese)Wei, H., Yang, R. Bao M., et al. (2006): Geochemical
characteristics of Early Cambrian black shale in7