The phosphorite is a pale yellow or buff-coloured rock, composed chiefly of well-rounded and strikingly polished grains of quartz, albite and minor glauconite set in a cement of crystalline francolite. Microscopically, however, it is seen that these minerals are associated with stained prochlorite, green hornblende, sphene, ferruginous epidote, clay, fragments of quartz-albite-epidote-muscovite-schist and some fossil material. The rounded grains of quartz and albite are associated with smaller, but less well rounded grains of the same minerals (text-fig. 2A); the average grain-size is 0·5–0·7 mm. Practically every granule of quartz shows very marked undulatory extinction and, in some cases, even incipient fracture.
The crystallography of the francolite has been described earlier, and it is only sufficient now to describe the microstructure. At least three modes of occurrence, each with transitional stages, are to be seen.
(1) As narrow crusts of prismatic crystals of francolite, up to 0·1 mm. in width, radially directed towards the plane of accumulation and surrounding minerals.
A. Grains of quartz, with some albite and glauconite set in a finely crystalline base of francolite. × 24.
B. A geode-like form of crystals of francolite radiating outward from quartz and albite. × 105.
C. A geode of francolite between quartz grains; also some areas completely filled by francolite adjacent to grains of glauconite. × 105.
(2) As geode-like structures due to radial growth of francolite crystals inwardly towards cavities between adjacent groups of quartz and feldspar grains (text-fig. 2B and C).
(3) As areas of dense aggregates of crystalline francolite formed by the complete filling up by phosphate of the geode-like structures mentioned above (text-fig. 2C).
This mode of occurrence of the phosphate mineral is closely similar to that described by Bushinsky (1935, p. 86) in the Cenomanian Phosphorites of the Briansko-Kurski district of Russia.
The heavy mineral residues with a specific gravity > 3 were separated from the phosphorite and the grain-count was found to be as follows: garnet (mostly pink) 12, epidote 34, clinozoisite 3, sphene 38, tourmaline 1, zircon 12. Microchemical tests made on three garnets gave positive reactions for manganese. A study of the main and accessory constituents of the Milburn phosphorite makes it clear that the Otago Central schists have supplied practically all the detritus for these sediments. Strained and fractured quartz, a sodic feldspar, in this case nearly pure albite, fragments of quartz-albite-epidote-muscovite-schists, and all of the heavy residue minerals are essentially common to the Otago schist area. The question of the origin of the phosphatic material has been dealt with by other workers.