IN-SITU STUDY OF THE THERMAL-BEHAVIOR OF CRYPTOMELANE BY HIGH-VOLTAGEAND ANALYTICAL ELECTRON-MICROSCOPY

Scanning electron microscope (SEM) and transmission electron microscop e (TEM) studies show that cryptomelane and Cu-rich cryptomelane crysta ls from weathering profiles in Brazil range from 10 to 200 nm in diame ter and have aspect ratios of 1:10 to 1:100 between the short and long dimensions. Acicular crystals identified in hand specimen and in the scanning electron microscope are often composed of bundles of fibers e longated along the c axis (tetragonal) or the b axis (monoclinic). In- situ heating with a high-voltage transmission electron microscope (HVE M) and an analytical electron microscope (AEM) indicates that upon hea ting in vacuum cryptomelane crystals begin to transform into a mixed h ausmannite and manganosite phase at 648-degrees-C. Mineral transformat ions are followed by the loss of K from the cryptomelane tunnel sites. Cu-rich cryptomelane transforms into hausmannite between 330 and 515- degrees-C, and manganosite begins to form at 620-degrees-C. Cu-rich cr yptomelane also loses K and exsolves native copper upon heating, sugge sting that Cu occupies the same site (the A site) as K. Similar minera l transformations are observed when the same samples are heated in air , although the transformations occur at higher temperatures than those observed in vacuum. The variation in the thermal stability of holland ite-group minerals suggests that the size of A-site occupants may cont rol the mobility of cations along the tunnel direction. Relative grain sizes and the degree of crystallinity of supergene cryptomelane may a lso influence the thermal stability of hollandite-group minerals. The applicability of cryptomelane and other hollandite-group minerals to K -Ar and Ar-40/Ar-39 dating is discussed in light of the electron micro scope investigation results. Ar release from the cryptomelane samples during Ar-40/Ar-39 vacuum laser heating is also discussed in terms of the phase transformations observed by electron microscopy.