QUENCH TEXTURES, LITHIFICATION, AND MODES OF CRYSTALLIZATION IN A COOLED SLAG BLOCK FROM A SMELTER AT NORTHPORT, NORTHEASTERN WASHINGTON STATE

This investigation examines a block of slag approximately 35 cm square dumped out molten after passing through the smelting process to extract lead and zinc. It has cooled and solidified from the outside into the middle, and in that respect is analogous to a lava pillow – the edge cools very quickly and the deeper into the block the slower was the cooling. Most of the block is crystalline and scattered through it are small grains of sulfide minerals, some of which are Pyrite, some Chalcopyrite, a few consist of both minerals. And at least one grain included native copper and native silver. We have made the analyses of the rock based on a sequence of polished sections cut perpendicular to the surface and extending into the center. These sections were analyzed using a scanning electron microscope (SEM) which produced a series of backscattered electron (BSE) images and energy-dispersive (EDS) microanalyses. The outermost part of the rock consists of glass with a uniform texture and composition (predominately O, Fe, Si, Ca, Al, K, and Mg). Below the homogeneous glass zone the glass has separated into two main phases which differ in the Ca/Fe ratio. Further down these phases begin to form distinct crystal domains, one of which is coarser than the other. The larger domains have a larger Ca content and a more uniform overall composition. The smaller domains have a higher Fe composition and a more variable composition. Towards the center of the block the crystals become larger and the compositions more uniform, although the high Ca (lower Z) domains are still coarser than the high Fe (higher Z) ones. Also, the low-Z crystals are aggregated into large obvious pinnate leaf-like structures. Microanalysis indicates the coarser lower-Z crystals are basically calcium-iron-magnesium-aluminum silicates and the higher-Z crystals are iron-calcium-magnesium silicates. We conclude that the originally-molten slag has cooled, solidified, and crystallized into a rock consisting almost entirely of two minerals. The composition of these minerals and powder diffractometry of the rock indicate that they are both clinopyroxenes. We conclude that the high-Ca pyroxene is Hedenbergite and the high-Fe one is Augite.