Heavy Minerals

Introduction

Heavy minerals are a small but disproportionately informative fraction of sandstone framework grains. Despite their low abundance - typically well below 1 percent - they carry provenance information that the dominant quartz and feldspar grains cannot. Different source rocks produce different suites of heavy minerals, and some of those minerals survive multiple erosion-transport-deposition cycles while others do not. The contrast between what is present and what is absent in a heavy mineral suite tells the story of both the source terrain and the sedimentary history.

Definition and Micas

Minerals with an average abundance in sedimentary rocks below about 1-2 percent are classified as accessory minerals. This group includes the common coarse micas - muscovite (white mica) and biotite (dark mica) - and a large number of heavy minerals, which are denser than quartz. ^[1]

The average abundance of coarse micas in siliciclastic sedimentary rocks is less than about 0.5 percent, though some sandstones may contain 2-3 percent. Micas are distinguished from other minerals by their platy or flaky habit. Muscovite is chemically more stable than biotite and is commonly much more abundant in sandstones than biotite. Micas are derived particularly from metamorphic source rocks and from some plutonic igneous rocks. ^[1]

Heavy Minerals: Definition and Stability

Minerals with a specific gravity greater than about 2.9 are called heavy minerals. They include both chemically stable and unstable (labile) varieties. ^[1]

Stable heavy minerals such as zircon and rutile can survive multiple recycling episodes and are commonly rounded, indicating that the last source was sedimentary. Less stable minerals, such as magnetite, pyroxenes, and amphiboles, are less likely to survive recycling. They are commonly first-cycle sediments that reflect the composition of proximate source rocks. ^[1]

Heavy minerals are useful indicators of sediment source rocks because different types of source rocks yield different suites of heavy minerals. Heavy minerals are derived from a variety of igneous, metamorphic, and sedimentary rocks. ^[1]

The rounded, residually stable character of zircon and rutile in polycyclic sandstones deserves emphasis. Zircon, in particular, is extraordinarily resistant to both chemical weathering and mechanical abrasion. A single zircon grain can survive several complete erosion-transport-burial-uplift-re-erosion cycles. Its rounding therefore accumulates over geological time, and a very well-rounded zircon in a sandstone may have experienced three or four separate sedimentary cycles before reaching its final resting place. This makes rounded zircon not just a provenance indicator but a polycyclicity indicator.

Heavy-Liquid Separation Procedure

Because of their low abundance in sandstones, heavy minerals are commonly concentrated for study by separation from the light mineral fraction using heavy liquids such as bromoform or sodium polytungstate. In this procedure, disaggregated sediment is stirred into a heavy liquid in a funnel inside a fume hood. Many heavy liquids are toxic and must be handled with extreme care. The light minerals float on the surface of the heavy liquid; the heavy minerals sink into the funnel stem, where they can be drawn off. After washing off the heavy liquid in a suitable solvent, the heavy mineral concentrate is mounted on a glass microscope slide for petrographic study. ^[1]

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