Feldspars in Sandstones

Introduction

Feldspars are the second most abundant mineral group in most sandstones, making up about 10-20 percent of framework grains in average sandstones. They are recognised in two broad groups - alkali (potassium) feldspars and plagioclase feldspars - and are distinguished from one another primarily by their optical properties under a petrographic microscope. ^[1]

Alkali Feldspars

Alkali feldspars form a mineral group whose chemical composition can range through a complete solid solution series from KAlSi₃O₈ through (K,Na)AlSi₃O₈ to NaAlSi₃O₈. Because potassium-rich members dominate, the alkali feldspars are commonly called potassium feldspars or K-spars. Common members of the potassium-feldspar group include orthoclase, microcline, and sanidine. ^[1]

Plagioclase Feldspars

Plagioclase feldspars form a complex solid solution series ranging in composition from NaAlSi₃O₈ (albite) through CaAl₂Si₂O₈ (anorthite). A general formula for the series is (Na,Ca)(Al,Si)Si₂O₈. ^[1]

Plagioclase feldspars can commonly be distinguished from potassium feldspars by their twinning, observed by petrographic microscopy. However, some K-feldspars (orthoclase and sanidine) and some plagioclase are untwinned, making them difficult to differentiate from one another and from quartz. ^[1]

Potassium feldspars are generally more abundant overall in sedimentary rocks than plagioclase feldspars; however, plagioclase is more abundant in sandstones derived from volcanic rocks. ^[1]

The volcanic exception is significant for provenance studies. A sandstone with abundant plagioclase relative to K-feldspar points toward a volcanic or andesitic source terrain - an arc setting, for instance - whereas K-feldspar dominance suggests a granitic source. Plagioclase-rich sandstones are also more chemically reactive during diagenesis, because calcium-rich plagioclase is less stable than sodium-rich plagioclase or K-feldspar.

Chemical Stability and Recycling

Feldspars are chemically less stable than quartz and more susceptible to chemical destruction during weathering and diagenesis. Because they are also less durable mechanically, feldspars become more readily rounded during transport. They are also somewhat more prone to mechanical shattering and breakup owing to their cleavage. ^[1]

Feldspars are less likely than quartz to survive multiple recycling episodes, although they can survive more than one cycle if weathering occurs in a moderately arid or cold climate. ^[1]

Because recycling is possible, the presence of a few feldspar grains in a sedimentary rock does not necessarily mean the rock was derived directly from crystalline igneous or metamorphic rocks. On the other hand, a high feldspar content - particularly on the order of 25 percent or more - probably does indicate derivation directly from crystalline source rocks. ^[1]

The 25 percent threshold is a practical rule of thumb. Below it, the feldspar may have survived one or two recycling episodes through sedimentary basins without indicating a direct crystalline provenance. Above it, the abundance is too high to be explained by recycled sedimentary sources - direct erosion of granite or gneiss is the only plausible explanation. This is the reason why high feldspar content is the defining criterion for the sandstone type known as arkose.

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