An amphibole occurring in the Mbozi syenite–gabbro complex in SW. Tanganyika approaches very closely to the theoretical end-member Na2Ca;it has α 1·705, pale to light yellow, β 1·713, deep blue-green, γ 1·715, deep blue to green to black, with γ ∥ [010] and β : [001] about 7 to 14°; absorption γ > β ≫ α, with the maximum absorption on (010) at 9 to 12° to [001]. The mineral shows anomalous bright orange-red interference colours and incomplete extinction in sections parallel to (010) even in monochromatic light. X-ray powder data are given. A very similar amphibole occurs in the Darkainle nepheline-syenite complex, Borama district, Somali Republic.

A chemical and modal analysis of the marble, together with chemical analyses of the coexisting spinel and the chondrodite-humite mixture are given. The spinel, a 8·101 ± 0·001 Å, n 1·731 ± 0·003, is much richer in iron (Niggli's mg 0·90), than the humite mixture (mg 0·99), which agrees with the only previous analysis of a coexisting spinel and humite mineral. It was not possible to separate the two humite minerals from each other and analyse them, but the analysis of the mixture shows less iron than any previously analysed chondrodite or humite; the composition is very near to that of humite although the mixture contains at least 50 % chondrodite. It is pointed out that there are serious discrepancies between the chemical analyses of some of the humite minerals and their accepted structural formulae. The possibility that some humite minerals contain irregular numbers of olivine and Mg(OH,F)2-TiO2 layers would fit the available chemical analyses of the humite minerals very well but more structural work is needed as the present scanty X-ray results do not confirm this view. It is suggested that Ti substitution in the humite minerals is primarily controlled by the availability of F in the rock.

Four new isotopic U/Pb ages from Cornwall are reported. The Pb206/U238 and Pb207/U235 ages for each specimen are concordant. The Pb206/U238 ages are : South Terras mine 225 ± 5 million years (Myr.), Redruth Area 124 ± 4 Myr., Wheal Owles mine 58 ± 3 Myr., and South Terras Area 62 ± 3 Myr. Cell sizes, calculated from X-ray powder photographs, decrease with decreasing ages of the specimens, indicating a progressive increase in the oxidation states of the uranium present. It is concluded tentatively that at least two periods of mineralization affected south-west England at about 290 Myr. and 225 Myr., and that a final event occurred at about 60 Myr. Whether this final event involved the remobilization of older uranium with loss of lead, or the introduction of additional uranium, or a combination of both effects cannot be established from this evidence. No unique explanation has been made for the 124 Myr. age.

Detrital grains, noted during the mineralogical examination of certain Snowdonian soils, have been identified as varieties of the mineral pumpellyite : the source of one variety has been located in dolerites exposed in the area, and the identity of this variety confirmed by X-ray analysis.

An attempt is made to systematize the nomenclature of the calciferous and alkali amphiboles. The compositions of end-members can be plotted on a simple diagram, which can also be used to display the affinities of the naturally occurring minerals after deriving a basic atomic formula according to the rules given. The composition of an amphibole can in some cases be expressed quantitatively in terms of solid solution of two or three pure end-members.

An Irish locality for morenosite is recorded. A chemical analysis shows that a small amount of magnesium is present and this agrees with the composition as indicated by refractive index measurements. X-ray powder data confirm the identification and the genesis of the mineral is briefly discussed.

Only two, or at most three, variables can be considered on a two-dimensional composition diagram; six variables must be fixed to completely define a variety of the chlorite or septechlorite groups. The positions of tetrahedral coordination may be described by a single variable and positions of sixfold coordination require three independent variables for complete definition. The number of OH− ions is probably very near the theoretical value of eight, however, it may vary somewhat from this value, and, in so doing, influences the ratio of trivalent and bivalent cations in octahedral coordination.

The normal 14 Å chlorites and 7 Å septechlorites have traditionally been associated together and, although they represent two distinct structural groups, are considered together for classification purposes, since they share many identical variety compositions; a sixth variable denotes structural type, as determined by X-ray powder diffraction or D.T.A.

Each chlorite or septechlorite variety can be described by a numerical notation (chlorite number) consisting of six units and, by dividing the chlorite numbers into units according to accepted nomenclature, each possible chlorite composition can be assigned a variety name, and all variety names can be assigned to a definite composition range.