The phosphate mineral occurs in stumpy hexagonal, or pseudohexagonal prisms, usually terminated at one extremity only by basal plane and low pyramids. The crystals rarely exceed 0.6 mm. in length. They are colourless, but show clouds of minute dust-like inclusions of haematite or limonite. The inclusions are commonly regularly arranged so that a central hexagonal zone is heavily charged, leaving the periphery quite clear. The refractive index of the central portion appears to be very slightly less than that of the peripheral zone, although no other differences in optical properties were noticeable.
In a hexagonal section, γ of the refractive index ellipsoid is observed to be parallel with the hexagonal edge in each triangular sector of the twinned crystal, and not at an angle to it as found by Deans (1938, p. 136). Thus the Milburn francolite appears to be similar to that described by Lacroix (1910, p. 558) from the Vosges, and by Sandell, Hey and McConnell (1939, p. 397) from the mine Wheal Franco, Tavistock, Devon. In other cases basal sections do not show any evidence of repeated twinning and, therefore, are isotropic or extinguish uniformly. The refractive indices are closely comparable with previous data, some of which are given in Table I.
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|2.||1.624||—||1.629||0.005||Sandell, Hey and McConnell, 1939|
|4.||1.614–||1.627||1.627−||0.013||de Villiers, 1942.|
The mineral is negative with an optic axial angle ranging from 0°–25° (±2°). Measurement of the angle on the universal stage
was a matter of some difficulty because of the fine grain-size and low birefringence. However, it was clearly seen that the optic axial angle varied from grain to grain, and sometimes within individual grains. The acute bisectrix is normal to the basal plane, hence prismatic crystals have negative elongation.