Evidence of an authigenic origin for palygorskite in a Middle Eocene carbonate sequence from North Qatar.

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Evidence of an authigenic origin for palygorskite in a Middle Eocene carbonate sequence from North Qatar.

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Title: Evidence of an authigenic origin for palygorskite in a Middle Eocene carbonate sequence from North Qatar.
Author: Holail, Hanafy [حنفي محمود هليل]
Abstract: The "Simisma Member" from the Ras Laffan area in North Qatar is mainly composed of dolomites of shallow marine origin. Clay minerals contained in the dolomite (average 4%) are mainly palygorskites. There is no indication of the presence of any transitional forms between palygorskite and other clay minerals. Delicate dense aggregates of the palygroskite fibres form matted films covered fine-grained dolomites. The long palygorskite fibres are also bridged and fill pore spaces between the coarse dolomite crystals. The presence of well preserved long fibres indicates that the palygorskite could not have been transported; they are authigenic and must have been formed directly by precipitation from solutions rich in Al, Si and Mg. The elemental compositions of the studied palygorskite are similar to previously published data. However, the high concentrations of Ca and Mg indicate that the palygorskite samples are mixed with carbonate minerals. The relatively high ^0 values (average + 18.7) of the palygorskites support the hypothesis that the palygorskite has been formed from solutions of high salinity. Meanwhile, the ^°0 contents of dolomites (range from -i-0.3 to +2.7%o PDB) and late sparry calcite crystals (range from -5.7 to -11.8%o PDB) argue against the formation of dolomite, calcite and palygorskite from the identical fluid.يتكون عفهعو السيمسما بمنطقة رأس لفان بشمال قطر أساساً من صخور الدولوميت التي تكونت في بيئة بحرية ضحلة ، ومعادن الطين المتواجدة مع الدولوميت بنهمبة تصل الى 4 % وتكون أسماساً من معدن باليجوسكيت والمتوانَجد في شكل تجمعات ليفية كثيفة على هيئة أفلام رقيقة تغطي حبيبات الدولوميت الدقيقة . كما أن التجمعات الليفية الطويلة لهذا المعدن . تصل بين سبيبات الدولوميت الخشنة وتملأ الفراغات الموجودة بينها ، وهذا يدل على أن معدن الباليجوسكيت لم يتعرض لعملية نقل وأنه ذو نشاة مكانية نتيجة ترسيب مباشر من محاليل غنية بعناصر الألومنيوم ، السليكون والماغنسيوم .
URI: http://hdl.handle.net/10576/10130
Date: 1997

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