Wednesday, January 26, 2011
What a Geologist Sees - Part 33 Heavy Minerals [Original Post Date 10/29/10]
Heavy minerals have several definitions;
1) Specific Gravity (density) greater than quartz (>2.65 times the density of water).
2) A relative resistance to chemical weathering, greater than that of most silicate “rock-forming” minerals (except quartz).
3) In their original host rocks, they usually constitute trace quantities.
Some “common” heavy minerals and their respective Specific Gravities (vs. Quartz 2.65) include:
Zircon 4.6 – 4.8
Monazite 4.6 – 5.4
Tourmaline 3.0 - 3.3
Rutile 4.2 – 4.3
Ilmenite 4.5 – 4.7
Magnetite 4.9 – 5.2
Staurolite 3.7 – 3.8
Kyanite 3.5 – 3.7
Garnet 3.56 – 4.32
Gold 15.6 – 19.3
Platinum 14 - 19
The process of releasing heavy minerals begins with the chemical and physical weathering of a given rock unit. In this Lithonia Gneiss saprolite, there is quartz, clays, altered minerals, and heavy minerals.
When the broken-down rock materials (clasts) are moved down-gradient - usually by water (erosion) and gravity - they are separated by density during high-energy water movement (floods).
The dark metallic colors of ilmenite, rutile, and magnetite help make placer concentrations of heavy minerals more easily seen.
Variances in water velocity help concentrate heavy minerals into "placer" deposits. Close to the source areas for mafic rocks, heavy minerals can include olivine, pyroxenes, and amphiboles, but these are unstable and will undergo degradation to clays during transport.
In the settings of a small to medium-sized creek, areas where the water velocity slows, such as the inside of a meander or on the upstream side of a boulder or other obstruction allow for the deposition of heavy minerals. If one is interested in panning for gold, the scoured area at the foot of a small waterfall might be a favorable site, also.
Another place where heavy minerals accumulate is in beach sands. In the southeastern portion of the United States, the white quartz sand beaches do contain small amounts of locally-concentrated heavy minerals in discrete stringers. In areas where there is little separation between the crystalline rocks and the shoreline (as in the northeastern United States), the heavy minerals present are much closer to their original source area.
The dark areas of this Jersey City, NJ urban beach are enriched with heavy minerals derived from the weathering of igneous rocks in the area and the erosion of glacial sediments. This heavy mineral portion of sands include a healthy quantity of garnet fragments. Other beach deposits in the NYC area - especially Montauk Point, Long Island - are even more garnet-enriched.