『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