Geology and Mineral Resources of Paraguay A Reconnaissance

46 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 36. Shonkinite (specimen P-54a) from Mbocayaty. Note the large rounded hydronephelite, h, crystals at left. At the center right are "graphic"remnants of nepheline or hydronepheline, g, that have been partly replaced by sanidine. Light-green monoelinic pyroxene, p, showing good cleavage,and dark-brown biotite, b, are abundant; olivine, o, in the lower right edge, is enclosed in biotite. Pale-bluish apatite and black iron oxide are accessoryminerals. X 15.strongly pleochroic brown biotite grains, sphene, and apatiteare also present.P 54c, shonkinite. Specimen P 54c is similar to specimenP-54a, described above. Numerous olivine crystals are seenin figure 38 (just upper left of center, just left of middle loweredge, and elsewhere). Large pyroxene crystals, showing goodcleavage, are also abundant. In the middle of the left half ofthe field is a large crystal of hydronephelite. The white areasare sanidinic feldspar.P-54d and P-S4e, shonkinite. Specimens P-54d and P-54eare similar. Both are rather coarse-grained medium-dark rockswith prominent tabular feldspars. They are variants of specimensP-54a and P-54c and differ essentially in having no olivine,and very little titaniferous amphibole. Titaniferous amphiboleshows normal absorption in deep brown parallel to c, unlike thekataphorite amphibole found in the other rocks from thislocality. Specimen P-54d has been analyzed (table 5).Figure 39 (specimen P-54d) shows the abundant tabular(laths in cross section) feldspar, and titaniferous augite. Undercrossed nicols, many of the clear feldspars show turbid cores,indicating perhaps replacement of an earlier generation offeldspar.Specimen P-54e contains some analcite, with clear feldsparapparently replacing nepheline. Many of the feldspar crystalshave turbid inner zones.P-64f and P-S4g, shonkinite. Specimens P-54f and P-54gare similar, and differ from the others somewhat. No definitelath-shaped feldspars are seen; instead there are abundant roundedwhitish areas, as much as 0.5 centimeter across, which arereplacements of nepheline. Figure 40 shows specimen P-54f, inwhich two such replacements (with more or less hexagonal outline)are seen in the upper half of the field. The smaller and clearerone is isotropic, and may be analcite; the other appears to behydrouephelite. In the center of the field, to the right of alarge augite, is more of the hydronephelite aggregate enclosingclear feldspar. There is an abundance of mafic minerals,including olivine, in this rock. Hornblende, though, is absent.Specimen P 54g, shown in figure 41, contains abundantwell-formed crystals of colorless fresh olivine, augite colorlessinside but usually with a thick outer green zone (aegirineaugite)fresh red-brown biotite, and some kataphorite amphi­bole. This mineral shows a strong and unusual pleochroism:X, pale yellow, Y, wine-red brown, Z, yellow; a small opticangle; strong dispersion, v>p; and the amphibole type ofcleavage, against which an extinction of about 15° can bemeasured.This amphibole resembles a kataphoritic amphibole namedmagnophorite by R. T. Prider (1939) from a leucite-bearing