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Copyright by Nysha Chaderton 2009 - The University of Texas at ...

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Authigenic Minerals<br />

Authigenic minerals occur as both pore filling and grain replacements. Various<br />

types are described below:<br />

Quartz overgrowth – occurs as a pore-filling authigenic m<strong>at</strong>erial nucle<strong>at</strong>ed as<br />

overgrowths in optical continuity with the detrital grain. This type <strong>of</strong> cement is best<br />

identified when there is a dust rim present th<strong>at</strong> deline<strong>at</strong>es the original boundary <strong>of</strong> the<br />

quartz grain.<br />

Kaolinite – also occurs as pore-filling m<strong>at</strong>erial th<strong>at</strong> appears grey or light brown in<br />

plane polarized light. A vermicular appearance (wormlike kaolinite “books”) is visible in<br />

cross polarized light.<br />

Chlorite – a pore-filling cement th<strong>at</strong> is pale green in plane polarized light.<br />

Grain Replacement<br />

<strong>The</strong> most common grain replacement cements in these samples is kaolinite. Its<br />

vermicular appearance (wormlike kaolinite “books”) is visible in cross polarized light. In<br />

plane polarized light, it can have a bluish grey appearance. Kaolinite commonly takes on<br />

the shape <strong>of</strong> the grain it is replacing, most typically feldspar but also other minerals.<br />

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