Thorium
A chemical element with symbol Th and atomic number 90. A radioactive actinide metal, thorium is one of only four radioactive elements that still occur in quantity in nature as a primordial element. In sedimentary rocks, thorium is essentially resistate in character, as its major host minerals (monazite and zircon) are highly resistant to both chemical and physical breakdown. Because it is radioactive, thorium is identified in gamma ray logging tools, useful for correlating with offset wells during drilling.
