Analysis for Magnetite Utilizing Magnetic Susceptibility

Abstract

The analysis for magnetite or “magnetite equivalent” by means of magnetic susceptibility is more rapid, lower in cost, and in general, as precise as other methods of analysis. “Magnetite equivalent” analysis is affected by grain size if the individual particles are less than 40 microns in diameter, by the remnant magnetization if greater than about 12 percent of saturation magnetization, and by the presence of other magnetic minerals. The presence of other magnetic minerals does not reflect as great an error in the final result using the susceptibility method as in other methods. Davis tube separation as a method for magnetite analysis is affected by other magnetic minerals but is mainly dependent upon degree of mineral liberation by grinding. Chemical analysis with the subsequent calculation of magnetite content is in error due to presence of other iron‐bearing minerals whether magnetic or nonmagnetic and is particularly affected by the presence of metallic iron from sample preparation. Metallic iron apparently enters solution by the following reaction: [Formula: see text] Thus, one unit of metallic iron would be counted as approximately 12 units of magnetite in the calculation of magnetite content. Analysis for “magnetite equivalent” and chemical analysis for total soluble iron can provide an accurate evaluation of the amount of nonmagnetic iron in any particular ore, especially in mixed magnetite‐hematite ores. Equipment for direct readout of grams magnetite per 100 cc was designed using the differential transformer principle. Calibration is linear below 20 grams magnetite per 100 cc, and measurement of apparent density of sample readily converts the readings to percent magnetite by weight.

Magnetic susceptibilities of Westland-Nelson plutonic rocks: Discrimination of Paleozoic and Mesozoic granitoid suites

“…Results (averages of30 readings on each sample, in different orientations and 3 remixings) on <90llm, 90-500llm, and > 500 Ilm were 73, 67, and 59, respectively, suggesting that susceptibility measurements of rocks should not be used to quantitatively determine magnetite content unless the grain sizes are broadly comparable. This result is the opposite of that reported by Shandley & Bacon (1966) who recorded a marked decrease of susceptibility for grain sizes less than c. 40 Ilm. However, they achieved their range of grain sizes by crushing, which may have had a detrimental effect on the results due to transient high temperatures and oxidation.…”

Chapter 2: The problem of the magnetite/ilmenite boundary in southern and Baja California California

“…2) fall above 200 x 10 -5 SI (magnetite series), or below 25 x 10~5 SI (ilmenite series). The variation of magnetic susceptibility with grain size is not significant for grains larger than 40 /xm (Shandley and Bacon, 1966).…”

Provenance Investigations Using Magnetic Susceptibility

“…The volume percent of magnetite in rocks can be approximated from magnetic susceptibility readings, although susceptibility varies to a small extent depending on grain size and shape and the presence of other magnetic minerals. The variation of magnetic susceptibility with grain size is not significant for grains larger than 40 m (Shandley and Bacon 1966). Experiments by Gastil et al (1990) show that pure magnetite in grains 0.1-0.5 mm in diameter read approximately 1650 ϫ 10 Ϫ5 at 0.5% volume, 590 at 0.25%, and 300 at 0.125%.…”