In geology, a heavy mineral is a mineral with a density greater than 2.9 g/cm³, most commonly referring to the dense components of siliciclastic sediments. The contrasting light minerals have lower densities and include quartz, feldspar and calcite. Heavy minerals typically form only a small fraction of a sedimentary rock, but their assemblage records the source rocks, transport history and post-depositional alteration of the sediment.
A heavy mineral suite is the relative percentage of heavy minerals in a sample. Heavy mineral suites are used to determine the provenance and history of sedimentary rocks, because different source terrains produce characteristic heavy mineral associations and because minerals with different stabilities are lost at different rates during weathering, transport and diagenesis.[1]
Common heavy minerals include zircon, tourmaline, rutile, garnet, apatite, monazite, chromite, magnetite, ilmenite, hematite, goethite, hornblende, pyroxene, epidote, allanite, staurolite, kyanite, topaz, cassiterite, scheelite, gold and platinum-group minerals. Most have densities between about 3.0 and 5.5 g/cm³, well above quartz at about 2.65 g/cm³ and feldspar at about 2.55 to 2.76 g/cm³.
Heavy minerals differ greatly in stability. Zircon, rutile, tourmaline, kyanite and staurolite are chemically resistant and hard, so they survive long weathering, transport and recycling histories and are called stable heavy minerals. Apatite, hornblende, pyroxene, epidote and olivine are less stable and dissolve or alter during chemical weathering, acidic groundwater flow or diagenesis. The ratio of stable to unstable heavy minerals therefore indicates weathering intensity and recycling history.
Because heavy minerals are a minor constituent of most sedimentary rocks, they must be separated from large samples before study. Heavy mineral separation generally uses a dense liquid in a separatory funnel or centrifuge. Liquids used include bromoform, tetrabromoethane, tribromoethane, methylene iodide and polytungstate solutions.[1]
Dense-liquid separation exploits the density contrast: grains denser than the liquid sink, while lighter grains float. Bromoform and tribromoethane have densities near 2.89 to 2.96 g/cm³ and are suitable for separating minerals around the 2.9 g/cm³ threshold. Methylene iodide, with a density near 3.33 g/cm³, can resolve denser fractions, while sodium polytungstate solutions have adjustable density and lower toxicity and are increasingly used. Separation is usually preceded by crushing, sieving, washing and acid etching to remove clay, carbonate cement and iron coatings.
Magnetic and electromagnetic separation, Wilfley tables and panning can also concentrate heavy minerals. Magnetite and ilmenite are strongly magnetic and can be removed with a magnet, while hornblende, pyroxene and epidote can be further divided electromagnetically. Separated grains are commonly mounted as grain mounts and identified under a polarising microscope, or quantified with an electron microprobe, scanning electron microscope or automated mineral-analysis system.
Heavy mineral suites are an important provenance tool. Different source rocks produce different assemblages: granite terrains commonly yield zircon, apatite, tourmaline and monazite, metamorphic terrains yield garnet, staurolite, kyanite, sillimanite and epidote, and mafic volcanic terrains yield pyroxene, hornblende, chromite and magnetite. Comparing the relative abundances of these minerals allows the source terrain to be inferred.
Heavy mineral suites also record transport and depositional processes. Hydrodynamic sorting concentrates denser and coarser grains in nearshore or high-energy settings, while finer heavy minerals are carried farther offshore. Chemical weathering and diagenesis selectively dissolve unstable minerals, raising the proportion of stable heavy minerals, so changes in the assemblage reflect sediment maturity and recycling. Recycled sandstones commonly contain much less heavy mineral material than first-cycle sandstones because unstable components have been consumed during earlier weathering cycles.
Some heavy mineral sands are economically important. Ilmenite, rutile, zircon and monazite concentrate in beach placer deposits and are major sources of titanium, zirconium, rare earth elements and thorium. Cassiterite, gold, platinum-group minerals and diamond can also form workable placers in river or beach settings. Placer formation depends on source mineralisation, prolonged weathering, hydraulic sorting and preservation of stable minerals, so heavy mineral analysis is both an academic tool and a basis for placer exploration.
Radioactive heavy minerals such as monazite and zircon can also raise environmental and health issues. Monazite contains thorium and uranium and requires radiation protection during mining and processing. Zircon generally has lower uranium and thorium contents, but exposure must still be managed during high-temperature processing or chemical dissolution. Tailings management and land use are therefore important environmental issues in heavy mineral sand development.
The heavy mineral concept is closely linked to heavy mineral analysis, heavy mineral suites, heavy mineral indices and heavy mineral placers. Heavy mineral is a descriptive density-based term rather than a mineralogical classification, and the same mineral can behave as a heavy or light component depending on grain size and deposit type. Heavy mineral studies therefore combine grain size, morphology, surface features and associated minerals rather than relying on density or a single mineral species.
References