Weathering of basalt and clay mineral formation in Leyte, Philippines

Weathering is the physical, chemical, and biological alteration of minerals in rocks, sediments, and soils at or near the Earth’s surface. It is an important link in the global rock cycle and is also an essential process for the formation of soils and landforms. Chemical weathering of silicate minerals which comprise over 90% of the Earth’s crust, removes CO₂ from the atmosphere so it helps regulate the Earth’s climate over long time scales. Basalts are among the more easily weathered crystalline rocks thus, weathering of these rocks acts as a major CO₂ sink. Chemical weathering of rocks likewise releases nutrient elements for use by the biota in the ecosystem and also produces clay minerals which are the central components of soils.

We studied the weathering of basalt by evaluating the gain and loss of elements, stream water composition, weathering indices, and clay mineral formation in the soil derived from basalt under the humid tropical conditions (average annual rainfall of 2700 mm and an average temperature of 28^oC) in Leyte, Philippines. The study site is located in the rain forest on the lower western slope of Mt. Pangasugan having an elevation of 100 m asl. The weathering profile studied is about 4 meters deep, heavy clay, acidic and yellowish red soil classified as Alisol (or Ultisol).

Results revealed that much of the basic cations Ca, Mg, K, Na, and part of Si have already been lost from the weathering product (saprolite and soil). This was however accompanied by the accumulation of Al, Fe, C, and H2O. The extent of weathering as indicated by the loss of elements based on the total elemental composition of fresh rock and of saprolite and soil was closely related to the cation composition of the stream water in the study site. Relative rates of loss of bases and silica revealed the sequence: Ca>K>Na>Mg>>Si for the soil, and Ca>Na ≥ Mg>K for the stream water. The ratio Na : (Na+Ca) of the stream water indicated that its major source of cations was rock weathering.

Results also showed that the intensive basalt weathering has resulted in the formation and abundance of kaolinite and halloysite clay minerals (see above TEM micrograph) as well as goethite in the highly weathered soil. The idea that weathering moves to a system composed of SiO₂, Al₂O₃, Fe₂O₃, and H₂O (residua hypothesis of Chesworth) appears to be supported by the results of this study.

Reference
Asio VB and R Jahn. 2007. Weathering of basalt and clay mineral formation in Leyte, Philippines. Philippine Agricultural Scientist 90 (3): 204-212.

Clay minerals in soil have antibacterial properties

Clay minerals are a major component of soils. They are an important source of negative charge which enable the soil to hold nutrients and pollutants. In recent years, the medicinal effect of clay minerals has gained increased interest among medical researchers.

In a recent paper in the Journal of Antimicrobial Chemotherapy, Haydel et al. (2008) reported:
"The capacity to properly address the worldwide incidence of infectious diseases lies in the ability to detect, prevent, and effectively treat these infections. Therefore, identifying and analyzing inhibitory agents are worthwhile endeavors in an era when few new classes of effective antimicrobials have been developed. The use of geological nanomaterials to heal skin infections has been evident since earliest recorded history, and specific clay minerals may prove valuable in the treatment of bacterial diseases."

The researchers found that specific clay mineral products have antibacterial properties which have potential to treat numerous human bacterial infections.

Reference
Haydel SE, CM Remenih, and LB Williams. 2008. broad-spectrum in vitro antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens. J. Antimicrob. Chemother. 61: 353-361.