Explanation: Thorium is a naturally occurring radioactive element that is found in various minerals, with monozite being a significant source. Monozite is a rare earth mineral that contains thorium and other rare earth elements, such as cerium, lanthanum, and yttrium. It is typically found in beach sands and alluvial deposits, particularly in regions with a geological history that includes the weathering and erosion of granitic rocks.
In the context of Kerala, monozite sand is particularly rich in thorium. The state is known for its extensive deposits of monozite, which are primarily found along the coastal regions. The monozite sands in Kerala are a result of the weathering and erosion of granitic rocks, which have been transported and deposited along the coastline over millions of years. These sands are not only rich in thorium but also contain other valuable rare earth elements, making them a significant resource for the region.
The presence of thorium in monozite sand is due to the mineral's chemical composition. Monozite is a complex silicate mineral with the chemical formula (Ce,La,Y,Th)PO4, where Ce represents cerium, La represents lanthanum, Y represents yttrium, and Th represents thorium. The thorium content in monozite can vary, but it is typically around 5-10% by weight, making it a valuable source of this element.
It is important to note that while monozite sand is a significant source of thorium, it is not the only type of sand found in Kerala. Other types of sand, such as silica sand and ooid sand, are also present but do not contain significant amounts of thorium. Silica sand, primarily composed of silicon dioxide (SiO2), is commonly found in beaches and riverbeds but does not contain the rare earth elements like thorium that are found in monozite. Similarly, ooid sand, composed of small, spherical grains of calcium carbonate, is typically found in marine environments and does not contain thorium or other rare earth elements.
In summary, monozite sand is the correct answer because it is the type of sand in Kerala that contains significant amounts of thorium. The presence of thorium in monozite sand is due to the mineral's chemical composition and the geological history of the region, which has resulted in the deposition of monozite-rich sands along the coastline. This makes monozite sand a valuable resource for thorium and other rare earth elements in Kerala.