Heavy mineral residue derived from sandstone (Pottsville Group, Middle Pennsylvanian; south of Bald Knob, Heath, Ohio, USA) : 無料・フリー素材/写真
Heavy mineral residue derived from sandstone (Pottsville Group, Middle Pennsylvanian; south of Bald Knob, Heath, Ohio, USA) / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Heavy mineral residue derived from the Pennsylvanian of Ohio, USA. (photo by James Cheshire)Heavy mineral analysis of sandstones is a traditional method for determining the nature of a sediment source. "Heavy minerals" are grains that have a density greater than 3 grams per cubic centimeter (3 g/cc). Sandstone samples are disaggregated and the loose sediments are density segregated by liquid separation. In general, most heavy mineral grains are zircons, tourmaline, and rutile - these minerals are stable, despite diagenesis and overprinting effects. The actual assemblage of heavy minerals is due to a combination of factors - provenance, diagenesis, and recycling - several signals are present.In the 1970s, researchers started focusing on provenance information from lightweight minerals in sandstones, such as quartz, feldspar, and rock fragments - these were traditionally discarded. Quartz-rich lights = cratonic setting. Feldspar-rich lights = volcanic arc setting. Diagenesis can be problematic for accurate interpretation. Diagenetic fluids alter feldspars and rock fragments differently than quartz (for example, small rock fragments are often broken down during diagenesis - this can result in the formation of a graywacke - a dirty or muddy sandstone; many graywackes turn out to be sandstones with a mud pseudomatrix - they weren't "dirty sandstones" at the time of deposition). It turns out that tectonic setting and diagenetic overprinting are important factors behind a sandstone's framework sand composition.The sample shown above is a relatively heavy mineral residue obtained by panning a hand-disaggregated, Pennsylvanian-aged sandstone from Heath, Ohio. Multiple minerals are present, including pinkish-colored zircons (click on the photo to zoom in a bit).Stratigraphy: float from a sandstone horizon above the Lower Mercer Limestone, Pottsville Group, lower Middle PennsylvanianLocality: loose piece from crest of hill along Blue Jay Road, south of Bald Knob, eastern side of the town of Heath, Licking County, Ohio, USA |
| 撮影日 | 2018-03-08 11:56:25 |
| 撮影者 | James St. John |
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