. Bulletin of the British Museum (Natural History), Mineralogy. VOLCANIC ROCKS FROM THE CAPE VERDE ISLANDS 53 [216], M. S. Pedro, 5 km. north of Praia, S. Tiago, is a moderately coarse rock with a seriate texture. The large pyroxene and olivine phenocrysts range up to 6-7 mm., the pyroxenes euhedral, the olivines rounded or subhedral. Magnetite is common in all sizes up to 1-5 mm. and accessories are apatite and occasional flakes of biotite. Optical data for the olivine and pyroxene are as follows: a y Fa Z Ac Olivine Titanaugite 1-665 1-708 1-704 1-736-9 Fa%i5 54-57° Olivine is chiefly concen
. Bulletin of the British Museum (Natural History), Mineralogy. VOLCANIC ROCKS FROM THE CAPE VERDE ISLANDS 53 [216], M. S. Pedro, 5 km. north of Praia, S. Tiago, is a moderately coarse rock with a seriate texture. The large pyroxene and olivine phenocrysts range up to 6-7 mm., the pyroxenes euhedral, the olivines rounded or subhedral. Magnetite is common in all sizes up to 1-5 mm. and accessories are apatite and occasional flakes of biotite. Optical data for the olivine and pyroxene are as follows: a y Fa Z Ac Olivine Titanaugite 1-665 1-708 1-704 1-736-9 Fa%i5 54-57° Olivine is chiefly concentrated in the phenocrysts, whilst the finer portions consist mainly of pyroxene and magnetite in a base composed of turbid analcime (Fig. 12a). The analysis (Table III, column 2) compares with that of a nepheline-ankaratrite from I km., S. 30° W. of M. Amargosa, S. Vicente (M. de Jesus in Bebiano, 1932: 255) [in which also Bebiano (op. cit.: 26) doubtfully records analcime in the groundmass. W. C. S.].. Fig. 12. (a) Olivine-nephelinite (analcitized). M. S. Pedro, Praia, S. Tiago [C. 3898] S. Tiago [216]. {b) Limburgite. [391], Rio Brava valley above Vila, S. Nicolao, is a scoriaceous lava rich in pheno- crysts of augite and olivine (0-25-1-o mm.). The olivine is entirely replaced by pseudomorphs in calcite and haematite. The groundmass, fine-grained and highly vesicular, contains much magnetite and haematite and numerous little augites in all sizes up to o-i mm. The base is analcime, but much of this is possibly secondary, as the shapes of many small areas suggest that idiomorphic nephelines were originally present. The augite has a greenish colour and high extinction of the order of 60°, and. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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