Pyrochlore group minerals have the general formula A16–xB16O48Z8–y·nH2O, with A mainly Na, Ca, Sr, REE, Th, U; B mainly Nb, Ta, Ti and Z being O, F, OH. In this study, pyrochlore specimens were examined from carbonatites at Argor, Carillon Dam, Chilwa Island, Fen, Lueshe, Oka, Mbeya, Meech Lake and Verity. Primary features include a background with little compositional variation, from core to rim, upon which are commonly superimposed narrow oscillatory zones, parallel to {111}. These zones are usually characterized by high Ta, in many cases coupled with U (here Argor, Meech Lake and Verity specimens), but Chilwa Island and Fen pyrochlores have little Ta and zonation is mainly by enrichment in Ce and Nb. Primary zonation may persist through high-temperature metamorphism (Meech Lake and Verity) and metamictization (Meech Lake). Oscillatory zones were generated by a disequilibrium system that cooled under tranquil conditions, signalling absence of magma turbulence and, in many cases, the end of crystal growth. Some fresh crystals (Oka, Fen) have no oscillatory zones, possibly the product of magma turbulence in space or time. Low-temperature effects may mimic those of primary high temperature and are especially characterized by replacement rims, pyrochlore-inpyrochlore veinlets and low A-ion totals (Carillon Dam, Lueshe, Myeba).

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A thin (100 m) cover of fiat-lying, Recent, calcite-rich tuff at Ndale near Fort Portal, Uganda, unconformably overlies steeply dipping Precambrian metamorphic rocks. It is locally radioactive owing to uranium-rich pyrochlore minerals and lesser amounts of zircon, monazite, titanite, and an unidentified thorium phosphate. In one concentrate, four grains of uranpyrochlore and one grain of uranoan pyrochlore showed a positive linear correlation of Ti with U, and negative linear correlations of Ti with Na, F and Sr. Ta remained high and relatively constant [11 anal., ave. 14.5 (0.6)% Ta2Os]. In the same concentrate the composition of a separate grain of uranoan pyrochlore did not plot on these lines and Ta was comparatively low [2 anal., ave. 4.5 (0.3)% Ta2Qs]. The data suggest two separate paths of differentiation. However, zoned grains were not 0bse~ed. Unit cells were cubic with a = 10.351 + 0.002,~ for a grain with 12.9% UOt~ ~ and 10.333 + 0.002~ for a grain with 26.6% UO[ ~ On heating in air the cell size decreased, possibly due to Qxidation of U 4+, The crystalline nature of these minerals can be attributed to a very young (4000-5000 yr) geological age.

A stock of fine grained potassic granite (Meech Lake aplite) has been investigated using a stochastic model for 'ideal' granite developed at the Laboratory of Mathematical Geology in Leningrad. From observations of the contacts of the stock with its country rocks, it can be assumed that the stock originated from a melt and that crystallization took place in a magma chamber. A hypothesis on the absence of significant metasomatic alteration in the stock was tested and confirmed through the investigation of about 140 specimens collected from the stock. This conclusion does not contradict traditional petrographic analysis. Because the stochastic-process model of a magmatic, relatively unaltered, ideal granite fits the observations well, the parameters of this model were investigated in detail. These parameters can be indicative of the viscosity of the magma. The pattern of parameter values for the stock shows that different parts of the stock were generated from the same magma but with differences in viscosity and in content of volatile components.