mineralogical and fabric characterization and classification of ...

- 27 -In addition to predominant kaolinite and trace amounts of halloysite, small amounts ofmicaceous minerals (illite and muscovite), trace chlorite and, in some soils, vermiculite andpossibly interstratified clay minerals of illite-smectite composition are present. Smallquantities of gibbsite was reported by Lumb & Lee (1975) in near-surface volcanic soils andby Irfan (1996b) in residual granitic soils. Gibbsite has not been identified in any of thevolcanic soils analysed here. In some soils, small amounts of CO 2 were detected. This isattributed to the presence of organic matter in the specimens. There is a small increase in thetotal iron-oxide content, which forms up to 8% of the soil. The dominant iron mineralpresent in the volcanic soils was identified by thermogravimetric analysis as poorly crystallinegoethite with haematite in some specimens.The original rock fabric gradually disappears within the transition soil grade and theresidual soil comprises some silt-to sand-size quartz grains (usually less than 20% of the soil)in a structureless clay mineral-iron oxide mineral matrix. The coating and cementation byiron minerals are so intense, locally, that the details of soil fabric may be completelyobliterated. Occasional large pores, some of which are ant holes and decayed root locations,are present. Microcracks are much reduced in amount and most are secondary features, e.g.desiccation cracks. There may be small aggregations or 'pebbles 4 of reddish brown iron-oxideminerals throughout the soil fabric and these may be taken as an indication of commencementof laterization at some sites.9.8 Iron-oxide Minerals in Volcanic Saprolites and Residual SoilsThere are variations in the iron-oxide contents of the saprolitic and residual specimensanalysed, as reflected in their Fe 2 O 3 contents, depending on the degree of weathering and thelocation of the specimen within the weathering profile. Generally, the iron-oxide content isless than 10%. However, it may be higher in areas of iron-oxide concentration, e.g. in thegroundwater fluctuation zone, as was the case with the Aberdeen landslide site (Irfan, 1994).In thin section, iron minerals generally appear as poorly crystalline aggregationsscattered throughout the groundmass, but also as coatings to partially to completelydecomposed minerals, or as microcrack infillings. Locally, weak cementation is also present.Detailed studies on granitic rocks indicated that goethite is the main iron-oxide mineralformed via ferrihydrite or directly from the decomposition of biotite in the intermediate andadvanced stages of weathering (Irfan, 1996a). This is probably the case for the coarse ashtuffs and most other volcanic rocks in Hong Kong. However, the lack of any weight lossbetween 200° and 400°C in the thermogravimetric analyses of specimens from Sai Kung andother volcanic rock sites (except Aberdeen) were interpreted as these rocks not containing anydetectable goethite or gibbsite by one of the laboratories (British Geological Survey).Haematite was considered to be the main iron mineral present in the weathered tuffs fromthese sites. The tuffs from Sai Kung contain some magnetite. Diffuse magnetite boundariesmay indicate the commencement of alteration to haematite in the moderately to highlydecomposed stage. Thermogravimetric analyses carried out for Aberdeen specimens indicatedthe presence of poorly crystalline goethite. In granites, Irfan (1996a) reported that thecrystallinity of goethite increased with weathering. This might also be the case for volcanicrocks.