Simple and Efficient Separation of Magnetic Minerals From Speleothems and Other Carbonates

Abstract

Trace concentrations of iron oxide minerals in carbonate sediments can preserve fine details about Earth processes, from high-resolution recordings of the Earth's ancient magnetic field to microscopic remnants of extraterrestrial impacts. This paper presents a novel flask extraction method which uses a neodymium magnet and an orbital shaker for simple and efficient separation of magnetic minerals from carbonate sediments. A mineral assemblage of magnetic standards (titanomagnetite, magnetite, goethite, and hematite) combined with other mineral standards (kaolinite, quartz, and nanoscale TiO 2 ) was subjected to the extraction procedure and compared to a natural speleothem sample. Exposure of the magnetic standards to a mildly acidic acetate buffer (pH , 4) did not cause physical or chemical alteration. The strongly magnetic minerals were reproducibly extracted, with greater than 90% efficiency (by mass), from mixtures of the mineral standards. XRD and low-temperature magnetic characterization demonstrated phase purity of the extracts. Quantitative comparison with two commonly used literature methods showed that the flask extraction method was more reproducible and efficient. The addition of surfactant (Na(PO 3 ) 6 ) did not significantly improve extraction efficiency. Sequential dissolution and flask extraction of a simulated speleothem containing magnetic particles resulted in consistent extraction efficiencies for samples containing large (. 1 mm) strongly magnetic grains, but a reduction in efficiency was observed for smaller (, 1 mm) grain sizes. No method successfully extracted the weakly magnetic goethite and hematite. However, unprecedented, representative characterization of these minerals was possible through quantitative analysis of the remainder after the collection of the magnetic extract. This approach may facilitate detailed characterization of a wide range of carbonates, such as pelagic limestone, dolostone, unlithified carbonate ooze, speleothems, and freshwater and pedogenic carbonates. Such mineral extractions can lead to new insights into paleoenvironmental processes as well as an improved understanding of the recording of the Earth's magnetic field.

Linking speleothem and soil magnetism in the Pau d'Alho cave (central South America)

“…[], and Strehlau et al . []. Magnetite grains have remanent coercivities varying between 14 and 17 mT (from ARM acquisition curves and FORC diagrams) and grain sizes ~1–2 µm as determined with SEM observations.…”

“…Room‐temperature saturation isothermal remanent magnetization (RTSIRM) curves were obtained in a field of 2.5 T at room temperature, then samples were continuously measured in zero field while cooling down to 10 K and warming up back to 300 K. Field‐Cooling (FC) curves were performed by applying a constant field of 2.5 T down to 10 K, and then measuring the remanence at 5 K intervals as the specimen was warmed back to 300 K. Zero‐Field Cooling (ZFC) curves were conducted in zero field down to 10 K, whereupon the specimen was given a 2.5 T IRM, and then remanence was measured at 5 K intervals as the specimen was warmed back to 300 K. In order to get a clearer view of the magnetic mineralogy throughout the speleothem, a magnetic extract was collected from one sample using the protocol of Strehlau et al . []. This extraction process dissolves the carbonate in the speleothem and then separates the resulting residue into a magnetic “extract” and a weaklier magnetic “remainder.” In practice, the extract tends to contain strongly magnetic minerals such as magnetite and maghemite, while the remainder tends to provide a clearer view of any weakly magnetic minerals (e.g., hematite and goethite) or extremely fine grained magnetite/maghemite that may be present.…”

Holocene ENSO-related cyclic storms recorded by magnetic minerals in speleothems of central China

“…The acetate buffer solution [4:1 (vol/vol) of 2 M acetic acid and 1 M sodium acetate] recommended by Perkins (16) was used for stalagmite dissolution. The extracting procedure followed that of Strehlau et al (49).…”

Diagenetic Fate of Biogenic Soft and Hard Magnetite in Chemically Stratified Sedimentary Environments of Mamanguá Ría, Brazil

“…For transmission electron microscope (TEM) imaging, magnetic nanocrystals from Core 4 (at 0‐ to 30‐, 45‐ to 60‐, and 75‐ to 90‐cm depths) and Core 9M (at 0‐ to 15‐, 15‐ to 30‐, 45‐ to 60‐, and 90‐ to 105‐cm depths) were extracted following Strehlau et al (). After concentrating nanocrystals with a magnet, the material was deposited over formvar‐coated 300 mesh copper grids and air dried.…”