Caillaud et al.(2006)による〔『Weathering sequences of rock-forming minerals in a serpentinite: Influence of microsystems on clay mineralogy』(87p)から〕

『蛇紋岩中の造岩鉱物の風化系列』


Abstract
 Under closed geochemical conditions, the weathering of a serpentinite rock composed of serpentine (70-85%) and magnesian chlorite (10-15%) associated with magnetite and chromite leads to the complete replacement of serpentine and chlorite by 2:1 layer silicates and products new Fe oxides. The serpentine minerals crystallize under different habits issued from the serpentinization processes: mesh and hourglass pseudomorphic textures were formed from olivine, and thin-bladed pseudomorphic textures from pyroxene and amphibole crystals. Serpentine veins crosscut the whole rock with locally non-pseudomorphic interpenetrating and interlocking serpentines.
 Specific weathering microsystem habits with specific clay mineral crystallizations originate from these different habits: a poorly aluminous saponite in thin-bladed textures, two Fe-rich montmorillonites in mesh and hourglass (MH) textures, and in veins (V) which differentiate on Al, Mg and Fe contents. Magnesian chlorites, isolated from serpentine by hand-picking under a stereomicroscope, are found to weather to trioctahedral vermiculite. Magnetite and chromite extracted from the bulk samples are replaced by newly formed Fe oxides, maghemite, goethite and hematite, which give way to specific Fe accumulation habits in the regolith zone of the weathering profile.

Keywords: Iron; Regolith; Serpentine; Texture; Weathering』

要旨
 閉じた地球化学条件下で、磁鉄鉱とクロム鉄鉱を伴う蛇紋石(70〜85%)とマグネシウム質緑泥石(10〜15%)からなる蛇紋岩質岩の風化は、蛇紋石と緑泥石の2:1層珪酸塩による完全な交代と新しいFe酸化物の生成をもたらす。蛇紋石鉱物は蛇紋岩化作用の過程に由来した異なる晶相で結晶化する:網状および砂時計仮像組織はカンラン石から、そして薄い葉片状仮像組織は輝石と角閃石結晶から形成された。蛇紋石細脈は、全岩を横断し、部分的に非仮像の貫入し噛み合う蛇紋石を伴う。
 特定の粘土鉱物の結晶化作用に伴う特定の風化微小系の晶相は、これらの異なった晶相から生じる:薄い葉片状組織ではアルミニウム成分に乏しいサポーナイト、網状と砂時計(MH)組織、およびAl・Mg・Fe成分に差を生じている細脈(M)では2種類のFeに富むモンモリロナイトである。実体鏡下で手選により蛇紋石から分離したマグネシウム質緑泥石は、三八面体バーミキュライトに風化していることがわかった。全岩試料から抽出された磁鉄鉱とクロム鉄鉱は、新しく生成したFe酸化物・マグヘマイト・針鉄鉱・赤鉄鉱によって置き換わり、これは風化断面のレゴリス帯において特定のFeが集積した晶相をもたらす。』

Introduction
Materials and methods
 Materials
 Methods
Results
 Rock-forming minerals
  Serpentine minerals
  Chlorite
  Magnetite and chromite
 Weathering sequences
  Weathered serpentine minerals
  Weathering products of chlorites
  Weathered magnetite and chromite
  Fissural clays
Discussion
 Serpentine habits and crystal structures
 Weathering sequences of rock-forming minerals
 Relationship between weathering sequences at the microsite scale and bulk clay mineralogy
Conclusions
References


戻る