mineralogical and fabric characterization and classification of ...

More documents

Recommendations

Info

- 22 -of feldspars in intermediate stages of weathering. The increase in kaolin content in thediscoloured portions and in the moderately decomposed specimens is mostly derived from thealteration of plagioclases as the alkali feldspars do not show any significant petrographicchange.A trace amount of a possible smectite mineral was indicated in the coarse ash tuff fromAberdeen in addition to minor amounts of chlorite and vemiiculite or an intermediate mineralbetween the two (Table 9). The hydrothermal alteration of the rock might have resulted inthe formation of metastable smectite in the initial stages of feldspar alteration, as well asformation of chlorite and vermiculite from alteration of biotite (Man, 1994).The total amount of secondary minerals varies, up to 12% or even more in the slightlydecomposed tuffs. The proportion of microcracks and pores is very low, usually less than 1% by volume.A network of simple-branched and tight but mostly iron-oxide infilled or coatedmicrocracks is developed in the stained portions. The frequency of microfracturing increasestowards the joint surfaces. The boundaries of quartz and feldspar megacrysts are tight in thefresher cores but they may be coated with iron-oxides in the stained rims. The staining orcoating along grain contacts may indicate the development of intergranular fracturing or theformation of dissolution pits and channels around the exterior of the larger feldspar grains,as identified by Kwong (1985).9.4 Moderately DecomposedRapid reductions occur in Na + and Ca 2+ contents with the exception of the coarse ashtuff from Yuen Long (Tables 6 to 8), indicating a significant decomposition of plagioclase.The associated increases in clay mineral contents are reflected in LOI values which are almostdouble those of the slightly decomposed tuff. These changes are confirmed by XRD results,which show that the kaolin mineral content in this grade is increased significantly from a fewpercent to up to 20% or more (Tables 15 to 17). No consistent change is apparent in K +content indicating that the alkali feldspars are not yet significantly affected by weathering inthis grade.Although there is a sharp drop in Na + content as the rock weathers from the slightlydecomposed to the moderately decomposed state, the Ca" content is unaffected in coarse ashtuff from Yuen Long (Table 7). This might be due to the presence of calcite as indicated bythe high CO 2 content or absence of Ca- bearing feldspars in this rock type.Optical microscopy revealed that the majority of large plagioclase grains are decomposedto kaolin and micaceous minerals, with alkali feldspars only slightly affected but generallyhighly microfractured. The feldspars in the fine-grained to cryptocrystalline groundmass alsoappear to be affected by weathering in this grade. Numerous isotropic speckles were observedin many large feldspars under the microscope, similar to those identified in granites (Man,1996b). These are probably either the locations of solution pits and trenches observed byKwong (1985) and Dearman & Baynes (1978), or some intermediate alumina-silicate growthstransitional to kaolin minerals.
- 23 -The main kaolin mineral is a poorly crystalline kaolinite with a possible trace amountof halloysite only detected in the tuff from Tseung Kwan O. Parham (1969) also reported thegrowth of flame - shaped films and halloysite tubes on feldspars in some slightly tomoderately decomposed rhyolite- porphry specimens (probably a coarse ash tuff).In addition to kaolin minerals, small amounts of sesquioxides, vermiculite and chloritewere identified in some rocks from the XRD patterns. A trace amount of a smectite mineral,or an interstratified clay mineral of possible illite-smectite composition, was also detected inthe tuff from Aberdeen. The quantitative modal and mineralogical analyses show that the clayand micaceous mineral content can be up to 25% in this grade (Tables 15 to 17).The most significant change in the rock is the formation of an extensive network of tightto slightly open microcracks in this grade. As a result, a fairly rapid reduction in intactstrength occurs across the grade. Most large feldspars and some quartz grain boundaries arestained which indicate that the grain boundaries are opening up. Apart from the pores formedat the locations of dissolved calcite grains in some specimens, solution features have not beenobserved under the microscope.9.5 Highly DecomposedFairly rapid changes occur in the major oxides contents within the highly decomposedgrade (Tables 6 to 8), indicating more intense decomposition and more rapid variation of theclay mineral content of the volcanic rocks in this grade than in the moderately decomposedgrade. Both Na + and Ca 2+ contents are reduced to very small values in all rock types exceptthe coarse tuff from Yuen Long, with complete disappearance of plagioclase in the moreintensely weathered specimens as confirmed by XRD and optical microscopy. A noticeablereduction in K + content in all rock types except the tuff from Aberdeen indicates that alkalifeldspars are now being affected by weathering within this grade. The Si 4+ content also showsa noticeable reduction except in some hydrothermally altered specimens (Tables 7 and 8).Both feldspar types show variations in their degree and type of alteration. Plagioclasecontents are less than 3% in the fine ash tuffs, but can be up to 17% in the coarse ash tufffrom Yuen Long. Although plagioclases show near-complete alteration, the type of clayfabrics formed can be different in each rock. For example, the decomposed feldsparpseudomorphs are very porous in the coarse ash tuff from Aberdeen in comparison to thedense clay fabrics produced in the coarse ash tuff from Sai Kung. The altered feldspars inthe specimens affected by hydrothermal alteration have, in general, more clayey, densermicro fabrics than those affected only by weathering.Alkali feldspars are little altered in the tuff from Aberdeen, but they are intenselymicrofractured. In contrast, the tuffs from Sai Kung and Tseung Kwan 0 are highlydecomposed to clay minerals. The intensive solution of alkali felspars generally occurs in thecompletely decomposed grade (Plate 11). The decomposition of feldspars in the groundmassis more advanced in this grade than in moderately decomposed tuff, with specimensunaffected by hydrothermal alteration having more porous groundmass than those affected.The groundmass is particularly porous in the tuff from Aberdeen.Kaolin minerals are the dominant clay minerals, with small to accessory amounts of
Page 1: MINERALOGICAL ANDFABRIC CHARACTERIZ
Page 4 and 5: © The Government of the Hong Kong
Page 6 and 7: FOREWORDAs part of its research and
Page 8 and 9: - 6 -CONTENTSPageNo.Title Page 1PRE
Page 10 and 11: - 8 -PageNo.11.3 Degree of Decompos
Page 12 and 13: - 10 -2.2 Late-Stage Alteration and
Page 14 and 15: - 12 -characterizing soil zones of
Page 16 and 17: - 14 -(c) Chemical analyses, to det
Page 18 and 19: - 16 -7.5 Coarse Ash Tuff, Yuen Lon
Page 20 and 21: - 18 -The rock material is discolou
Page 22 and 23: - 20 -9.2 Fresh RockThe fine and co
Page 26 and 27: - 24 -micas and trace amounts of ch
Page 28 and 29: - 26 -(iv)Most halloysite may be of
Page 30 and 31: - 28 -9.9 Kaolin VeinsThe XRD analy
Page 32 and 33: - 30 -The alumina-to-calcium-sodium
Page 34 and 35: - 32 -Similar to granites, alkali f
Page 36 and 37: . 34 -Langford, R.L., Lai, K.W., Ar
Page 38: 36TableNo.PageNo.15 Results of Quan
Page 65 and 66: - 63 -Table 25 - Micropetrographic
Page 67: - 65 -FigureNo.PageNo.16 Variation
Page 70: - 68 -
Page 75 and 76:
- 73 -
Page 77 and 78:
- 75 -
Page 80:
- 78 -
Page 93:
- 91 -(Negative No. SP8907803 & 04)
Page 100 and 101:
- 98 -APPENDIX A : SAMPLE DESCRIPTI
Page 102 and 103:
100 -Locality : Access Road Cutting
Page 104 and 105:
- 102 -Locality : Landslip Site beh
Page 106 and 107:
104Locality : Sai Kung, Old Borrow
Page 108 and 109:
106 -APPENDIX BMETHODS OF PETROGRAP
Page 110 and 111:
- 108 -B.2.3Mineral Calculations fr
Page 112 and 113:
110extremely wet and before any ana
Page 114 and 115:
- 112 -B.4.6Comments on Mineral Ide
Page 116 and 117:
GEOTECHNICAL ENGINEERING OFFICE PUB
Page 118 and 119:
Report on the Shum Wan Road Landsli
Page 120 and 121:
Report on the Rainstorm of August 1
Page 122 and 123:
Conventional and CRS Rowe Cell Cons
Page 124 and 125:
Geotechnical Area Studies Programme
Page 126 and 127:
Hong Kong South and Lamma Island :
Page 128:
Siu Ho : Solid & Superficial Geolog
show all

mineralogical and fabric characterization and classification of ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?