Magnetic Properties of Minerals from the Red Sea Thermal Brines

Abstract

Studies of the magnetic properties of sediment samples recovered from the thermal brine area of the Red Sea have been made. The minerals which have distinctive magnetic signatures, particularly on heating, are goethite, lepidocrocite, siderite, manganosiderite, pyrite, hematite and small quantities of a ferrimagnetic mineral, which is probably maghemite.

The following reactions take place during heating in air:

• (a) goethite $\mathop \to \limits_{{{300}^ \circ }C}$ hematite

  The characteristic thermoremanent magnetization found in many goethites was not observed in these samples.

• (b) lepidocrocite $\mathop \to \limits_{{{250}^ \circ }C}$ maghemite $\mathop \to \limits_{{{450}^ \circ }C}$ hematite

  The development and breakdown of maghemite is very characteristic magnetically.

• (c) siderite $\mathop \to \limits_{{{500}^ \circ }C}$ magnetite

  On cooling, the magnetite acquires a strong magnetization at its Curie temperature (580°C).

• (d) manganosiderite $\mathop \to \limits_{{{500}^ \circ }C}$ jacobsite

  Jacobsite acquires a strong magnetization upon cooling. The Curie point is less than 580°C, depending upon the amount of substitution of manganese for iron in the magnetite structure.

• (e) pyrite $\mathop \to \limits_{{{400}^ \circ }C}$ maghemite $\mathop \to \limits_{{{450}^ \circ }C}$ hematite

  During heating a strong magnetic deflection is observed through the temperature range where maghemite is one of the mineral phases.

Hematite is present in two of the cores recovered from the Atlantis II Deep. The magnetic properties indicate that this material is coarse-grained (>0.5μd). By contrast the hematite formed chemically during the heating experiments is fine-grained (<0.5μd). A ferrimagnetic mineral, probably maghemite, is present in many of the samples examined. Most of this material is destroyed by heating above 500°C. The absence of an observable Neel temperature effect in the goethites at 120°C is attributed to the small grain sizes, making much of the material superparamagnetic at room temperature. Other data indicate that much of the goethite is in particles less than 100Å.

Woods Hole Oceanographic Institution Contribution No. 2200.