Electromagnetic Minerals In Soil And Rocks
Nearly all soil and rocks contain a variety of minerals, the magnetic or electrically conductive properties of which interfere with the operation of metal detectors. The “big three” are magnetite, maghemite and sodium ions. But first, some definitions.
Susceptibility refers to a material’s ability to attract a magnetic field. In the context of metal detecting, it corresponds to the amount of magnetically active mineralization in the soil. It is often expressed as an equivalent percent by volume concentration of magnetite.
Tangent of loss is the ratio of magnetic energy absorbed by a material and dissipated as heat, divided by the magnetic energy which is attracted to the material and not dissipated. The tangent of loss is most commonly expressed in arctangent form as the loss angle. In the context of metal detecting, it corresponds to the ground balance point of the soil. In a general way it represents the type of mineralization present rather than the amount.
Magnetite (ferrous oxide) is a heavy black iron oxide mineral which exhibits high magnetic susceptibility and low magnetic loss angle. It is commonly found as “black sand” or as dense black rocks. It is strongly attracted to a magnet. It usually “balances” near the ferrite calibration point of the metal detector, which on most detectors is within the range of 80 to 95% of full scale. Many black colored rocks, especially igneous (volcanic & extrusive) and high-grade metamorphic rocks, contain appreciable amounts of magnetite. So do many rocks with a bluish or greenish cast, especially rocks in ultramafic greenstone belts. Magnetite in the soil is usually in the form of sand, because particles smaller than sand unprotected by rock matrix tend to oxidize to maghemite or to be dissolved by organic acids.
Maghemite (gamma ferric oxide) is an earthy iron oxide mineral found in most soils and some rocks. Red iron rust is a form of maghemite with which everyone is familiar. Maghemite is formed by the oxidation of lower oxidation state iron minerals such as magnetite, free iron and pyroxene. The oxidation commonly happens through weathering and exposure to fire. Maghemite is usually reddish brown or red in color, and even in low concentrations its color tends to dominate the material it’s in. Like magnetite, maghemite has high susceptibility. It differs from magnetite in having a substantial loss angle, causing it to ground balance in the range of 40 to 80% of full scale on most metal detectors and under most conditions.
Maghemite is often confused with the similar-appearing earthy hematite. However hematite has low magnetic susceptibility and therefore doesn’t usually affect metal detectors very much.
Sodium ions are produced when soil moisture dissolves salt or alkali, causing the soil to become electrically conductive. This effect is strong on ocean beaches but weak under most soil conditions. Usually just described by metal detector users as “salt”, soil electrical conductivity “balances” near 0% of full scale on most metal detectors.
Most iron-bearing minerals exhibit magnetic effects similar to that of magnetite (i.e., low loss angle), but with much lower magnetic susceptibility. Hematite, limonite and siderite are common examples. The black forms of limonite and hematite are often mistaken for magnetite by the untrained eye.
Soils high in maghemite tend to form most readily in warm humid climates, and by weathering of basalt. Fire tends to increase the maghemite content of rocks and soils by oxidizing magnetite and other ferrous minerals to maghemite which is a ferric compound. Subtropical and tropical laterite soils (oxisols) and laterite rock usually contain moderate to high concentrations of maghemite.
The “ground balance point” of a soil represents the weighted average of the ground balance points of the various minerals in the ground. As the relative concentration of these minerals changes from one spot to the next, the ground balance point will usually change also.