sassolite (sas'-so-lite). A white or gray mineral consisting of native boric acid: H₃BO₃ . It usually occurs in small pearly scales as an incrustation or as tabular triclinic crystals around fumaroles or vents of sulfurous emanations. Also called: sassoline.

X-ray spectrum . The spectrum of X-rays emitted when a substance is bombarded with energetic particles or radiation. It consists of a "characteristic spectrum", resulting from specific electronic transitions within the atoms of the substance, superimposed on a "continuous spectrum" resulting from inelastic collisions of particles or incoherent scattering of the exciting radiation. The X-ray spectrum may be excited by electrons (cathode rays) as in standard X-ray tubes and electron microprobes, by other particles such as protons as in particle-induced X-ray emission, or by higher energy X-rays or gamma rays as in X-ray fluorescence.

carbon fixation . The process by which organisms (usually photosynthetic) convert components of the carbonate system, e.g. carbon dioxide or bicarbonate, to organic matter. Occurs most frequently in the upper sunlit portion of the ocean (euphotic zone) and is accompanied by the consumption of dissolved inorganic nutrients such as nitrate, ammonia, phosphate, and silicate and by the production of dissolved oxygen. Dissolved trace metals such as zinc, cadmium, and iron may be consumed along with inorganic nutrients.

solution fissure . In karst, one of a series of vertical open cracks, about 0.5 m wide, dissolved along joints and separating limestone pavement into clints. Syn: cutter [karst]; grike; kluftkarren. Cf: solution corridor.