Pyrite in Sliding Mud: A Potential Indicator of Landslide Development

Pyrite can be formed through either abiogenic (geothermal) or biogenic (low temperature) processes (Ohfuji and Akai, 2002; Chigira and Oyama, 2000; Donald and Southam, 1999; Cripps and Edwards, 1997; Wilkin and Barnes, 1997, 1996; Berner, 1984, 1970; Lin and Chen, 1983), and is found in many types of rocks, coal seams, and depositional environments. We have observed pyrite as newly formed crystals in sliding mud within a slip zone of Tertiary formation type of landslide (TFTL) in Himi City, Northwest Japan (Fig. 1). TFTL occupies 67% of all landslides throughout Japan and are normally very harmful because of their close locations to towns and cities, and also traditionally industrial districts and agricultural lands (JLS, 1996; Yang et al., 1995). The Tertiary sequences are dominated by tuff and metamorphic rocks in the landslide area studied and are mostly distributed along the front slope of high mountains and low hills in Toyama Prefecture, Japan. At present, human actions are becoming much more intensive in these areas and have aggravated the problems with time; examples include new construction works in the form of residential and commercial buildings, road networks, and many other public and private facilities. Because of the rapid and huge accumulation of residential properties in urban areas together with collective transportation lines or zones, modern human actions have, in turn, resulted in the loss of many more properties due to simulated faster progression and more activities of landslides (JLS, 1996).

Figure  1.  A map showing the distribution of Tertiary sequences and TFTLs in the Toyama Prefecture, Japan (after JLS, 1996). Almost all landslides (in circles) occurred within the distributing area of Tertiary sequences. Black circles indicate the landslides studied. Fm.. Formation.

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We previously studied redox conditions within slip zones in relation to landslide development based mainly on vertical variations of iron speciation and total Fe contents in landslide profiles. For example, the profile of iron species and total iron contents revealed variations in redox conditions on the Tsuchikura landslide (Fig. 2). A relatively stronger reducing condition was observed in the landslide slip zones, of which ferrous Fe dominated in the sliding mud, whereas ferric Fe species dominated in the corresponding debris rocks and bedrocks (Zheng et al., 2002a, b). Moreover, pyrite has also been identified in the Mössbauer spectra of most sliding mud samples such as the Kitayama landslide (Zheng et al., 2002a), although no corresponding peaks of pyrite appeared in the XRD spectra. Here the sliding mud indicated that newly-formed muddy materials were composed of clay minerals and fine amorphous and organic matter. Such sliding mud is mostly easily distinguished in a landslide profile in boring cores or excavation drainage well walls because of altered lithological characteristics such as color and structure. More recently, pyrite crystals barely visible to the naked eye (Fig. 3) were obtained from sliding mud from an excavation-drainage well in the Nakadaura landslide in Himi City (Zheng et al., 2006). The well was bored out through the lower central part of the debris body and into the bedrock. Field investigation and laboratory studies identified the pyrites as neogenetic minerals associated with sliding mud accumulation.

Figure  2.  Vertical profile of iron speciation and tot-Fe on a typical landslide of Tertiary formation type in Japan. There is a layer of sliding mud in deep gray and black of color, which is always matured with percolated groundwater in the landslide slip zones. These two typical matters are developed along the slip zone between the debris rock and bedrock. The slip zone is actually like a special aquatic system in which geochemical reactions take place along with landslide occurrence and development.

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Figure  3.  SEM of pyrite from the Nakataura landslide, Japan.

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The crystals were dominantly of small to micro size, less than 0.5 mm, and were mostly in a cubic shape and/or balls with multi-faces. They always occurred in a collective complex as a fine aggregate of various single crystals. As shown in Fig. 3, there are several small ball-shaped crystals distributed on the surface of a larger cubic-like particle, probably indicating various generations of crystals. One crystal of pyrite was interpreted as growing during the argillization in the slip zone. On the other hand, some pyrite crystals were formed after fracture occurrence and during the development of the slip zone. Some pyrites aggregated into large particles in collective mineral structures whereas some biomass (e.g., the remains of grass roots and other black carbonaceous breakers) remained within the fractures. There were large variations in crystal shape and particle size among the different parts of the sliding mud layer.

Various kinds of methods confirmed the pyrite characteristics. An automated powder diffractometer, using Cu Kα radiation, was used to characterize the bulk samples by means of X-ray diffraction. The sliding mud identified by XRD was mainly composed of clay minerals, quartz, and feldspar. Typical reflecting peaks of pyrite were obtained clearly in the XRD spectra, confirming the occurrence of crystal pyrite in the sliding mud (Fig. 4). Chlorite was the major component of clay minerals whereas smectite was much less abundant in the sliding mud of the Nakadaura landslide, which is similar to other landslide profiles studied previously (Zheng et al., 2002a, b). However, the relative contents of clay minerals vary among different landslides. Considering the estimated age of landslide formation (Xu et al., 2003), there is a positive correlation between landslide age and chlorite content in the sliding mud. This factor strongly suggests a mineralization of chlorite and/or conversion of smectite to chlorite under special reducing conditions in the sliding mud along with landslide development.

Figure  4.  XRD patterns for typical sliding mud samples from the Nakataura landslide, Japan. Sm. smectite; Ch. chlorite; Ka. kaolinite; Qz. quartz; Fe. feldspar; Py. pyrite.

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Mössbauer spectra for various kinds of iron compound were identified with an Austin Science S-600 Mössbauer spectrometer using a γ-ray source of 1.11 GBq ⁵⁷Co/Rh at 293 K. Half-width and peak intensity of each quadruple doublet was constrained to be equal. Isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil. As shown in Fig. 5, there are mainly two iron species described as paramagnetic ferrous iron (para-Fe²⁺) and iron in pyrite (pyr-Fe²⁺) in the sliding mud samples, whereas a paramagnetic ferric iron (para-Fe³⁺) was detected in the slope washed deposits as the debris of this landslide. Such a vertical variation pattern of iron species indicates that a relatively stronger reducing condition occurred within the landslide slip zone in contrast to oxidative conditions in the host rock, including the sliding debris and the bedrock. It is probably just the favorable environment for argillic alteration within the landslide slip zones in which percolated groundwater with organic matter might be the essential factor for stimulating such processes along with landslide development.

Figure  5.  Typical Mössbauer spectroscopy of sliding mud from the Nakataura landslide. Raw data. measured results; para-Fe²⁺. paramagnetic ferrous iron species; pyr-Fe²⁺. iron in pyrite; sum. the summary for all calculated species.

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Total sulfur content was measured by means of a wet chemical digestion followed by ion chromatography analysis. All sliding mud contains much higher sulfur than debris and bed rock, supporting the idea of an enrichment of sulfur containing minerals in the sliding mud such as metal sulfides and pyrite. This allows us to dismiss the notion of the oxidation of pyrite within the slip zone instead; we consider that the formation of pyrite occurs in the sliding mud. The sulfate in percolated groundwater can be a source of reduced sulfur needed to form pyrite under reducing conditions. Such a style of pyrite formation is probably very similar to authigenic pyrite formation in aquatic systems (Berner, 1984, 1970). We have measured sulfur isotopic compositions for two pyrite crystals. Their δ³⁴S values were 5.1‰–5.2‰, which may support the above considerations. The pyrite may be formed, or at least enlarged, under reducing conditions within the slip zone along with landslide occurrence and development. On the other hand, abundant submicron-sized pyrite is distributed on the surface of bedrock, itself composed of mainly tuff and metamorphic tuffrous rocks. The above sliding mud adjacent to the bedrock contains fine cubic pyrite in much larger particles. These may again indicate the growth of pyrite along with landslide progression within the slip zone. Thus, we may use this parameter to obtain significant information about the formation and accumulation of sliding mud within slip zones along with landslide development.

As a popular component of landslides and a key responding factor to slip activity, sliding mud has attracted much attention (Shin, 1999; JLS, 1996). The neogenetic pyrite and iron speciation in the sliding mud have great potential to indicate the geochemical environment of a landslide slip zone. Geochemical conditions and reactions within a slip zone should be important indicators of landslide development but, unfortunately, only limited studies have been focused on these so far. Sometimes it is not so easy to link a slip zone to a recurrent or old landslide, particularly when the landslide has experienced several episodes of slipping, because various mud layers may co-exist. It is important to learn which layer is the most recently active one. Chemical speciation of iron and neogenetic pyrite will be potential indicators to help solve such problems.

The regional country rocks around the nakadaura landslide are dominated by tertiary granitite and marine volcanic sediments such as tuff and metamorphic rocks (toyama prefecture, 1992). these rocks are deeply weathered and the resulting slope wash deposits contain mainly small gravel, sands, and clay in red, orange, yellow, or more medium colors. in the study area where the nakadaura landslide is located, several layers of muddy sediments are distributed in the weathering profiles. among them, a specific layer of deep gray mud is observed adjacent to the weathered surface of the regional bedrock. landslide scientists and engineers have considered the mud as sliding mud (toyama prefecture, 1992), and the debris rocks as those that slipped downward along the mud layer at a rate of 1-2 mm/a, which was determined using several practical methods. numerous fractures occurred as typical features in the slip zone of the nakadaura landslide, where the mudstone was fractured into slices and developed as sliding mud. the layer of sliding mud was about 20 cm thick and matured with groundwater over about a year. The local precipitation is over 2 000 mm in a normal year and almost 100% of thick vegetation covers the continuously humid environment. In fact, the slip zone can be considered as a special underground aquatic system suitable for various geochemical reactions. A weak alkaline and strongly reducing condition is probably the typical feature of this environment. This kind of environment is probably favorable for sliding mud formation including smectite and pyrite. The formation and accumulation of sliding mud, in turn, will fill the fractures, which induce much more closed conditions and a stronger reducing environment within the slip zone. As the sliding mud expands towards both the bed rock and debris, then the stability of debris on the slope will decrease and a slipping action will follow.

To explain the above data and field observations, a special self-sealed closed system with the features of weak alkalinity and strong reduction should be considered as a particular situation under a landslide. Once slip zones occur, a series of water-rock interactions may take place along with landslide progression, particularly, the change in redox conditions becomes conducive to weathering of the debris and bed rocks, and thus favorable to the accumulation of newly formed sliding mud within the slip zone.

Landslides are one of a number of harmful geological phenomena to human beings and properties, and usually strike without warning (Kusky, 2008). However, it is not easy to understand the history and development of a landslide because of difficulties in finding suitable materials for direct dating of a landslide. To our knowledge, this is probably the first report on crystal pyrite as a newly formed mineral within a landslide slip zone, although several researchers mentioned the oxidation of pyrite related to slope failures and ground or foundation heave in marine sedimentary rocks (Chigira and Oyama, 2000; Steward and Cripps, 1983). The redox condition provides the essential linkage between the chemical reactions and engineering characteristics of a landslide; therefore, geochemical studies on landslide slip zones will be helpful to better understand landslide progression.

CONCLUSION

Neogenetic pyrite, iron speciation, and clay mineralization will all be significant topics for a better understanding of landslide progression.