Rock magnetism investigation of highly magnetic soil developed on calcareous rock in Yun-Gui Plateau, China: Evidence for pedogenic magnetic minerals

Detailed rock-magnetic and pedological analyses were conducted on the highly magnetic soils developed on calcareous rocks in the Yun-Gui Plateau of west-southern China in order to characterize its particular magnetic characteristics and pedogenesis of magnetic minerals. The magnetic concentration and mineralogy in the soils were determined by rock magnetism, powder X-ray diffraction and high resolution transmission electron microscope (HRTEM) with the energy dispersive X-ray analysis (EDX). The highly magnetic soils contained characteristics of highly weathered soils with a hue of 2.5YR, dominantly gibbsite-sesquioxide mineralogy, low organic matter content and high clay and free iron (Fed) content. Soil magnetic susceptibility (χlf) ranged from 2000 × 10− 8 to 6000 × 10− 8 m3 kg− 1; but in some layers it exceeded 6000 × 10− 8 or 6500 × 10− 8 m3 kg− 1, which was the highest magnetic soils so far found in the world. Magnetic measurements indicated that the frequency-dependent susceptibility (χlf − χhf) ranged from 210 × 10− 8 to 720 × 10− 8 m3 kg− 1, suggesting the presence of abundant ultrafine magnetic grains. High-temperature magnetization (M–T) identified the magnetic carriers as maghemite and magnetite with a Curie point (Tc) at about 230 °C and 580 °C, respectively. XRD patterns showed that the gibbsite and hematite were main clay minerals in these highly weathered soils. HRTEM/EDX analysis showed that the pedogenic nano-scale magnetite/maghemite were responsible for the high magnetic susceptibility value of the soil. These particles varied from 20 to 100 nm in size and exhibited well crystalline nanoparticles. Magnetic enhancement in the soil profile was due to increased concentration of pedogenic superparamagnetic (SP) ferrimagnetic minerals upon pedogenesis. This study provided a strong evidence for the evolution of pedogenic magnetic minerals in the soils formed on non-magnetic parent materials. The identification in magnetic properties and mineralogy of the highly magnetic soils has significance for pedoenvironmental implications.

► The magnetic soils formed on calcareous rocks are the highest magnetic soils so far found in the world.
► The formation of pedogenic SP/SD minerals upon pedogenesis cause magnetic enhancement of soil.
► The size of pedogenic magnetic nano-particles in the soil ranges from several nm to 100 nm.
► Soil magnetism helps to understand the soil-forming processes and pedogenesis intensity.