apatite cat’s eye, gems

Describes cat’s-eye apatite reportedly originating from the Umba Valley of northeastern Tanzania and identifies the inclusions responsible for its chatoyancy. Several kilograms of cuttable fragments offered in Nairobi as feldspar ranged from yellowish green and green to greenish brown and reddish brown. Although some fragments were transparent, most contained parallel fibrous inclusions that produced sharply defined cat’s-eyes when suitably fashioned as cabochons. The resulting stones were considered comparable in appearance and colour with fine chrysoberyl cat’s-eyes. Uniaxial optical behaviour excluded feldspar, and X-ray examination established apatite. Measured refractive indices range from 1.632–1.636 for the extraordinary ray and 1.636–1.640 for the ordinary ray, with a consistent birefringence of approximately 0.004. Dichroism separates yellowish and greenish, brownish and greenish, or reddish and brown components, and occasional colour zoning is observed. Sharp absorption lines between approximately 570 and 590 nm, attributed to the rare-earth elements praseodymium and neodymium, provide an additional diagnostic feature. Reported densities range from 3.22 to 3.35 g/cm³, with variation considered partly related to the inclusions. Microscopy shows fine reddish to blackish needles aligned parallel to the crystallographic c-axis. Their relatively coarse development, compared with fibres in previously known Brazilian and Indian cat’s-eye apatites, permits electron-microprobe investigation. Qualitative examination of fifteen needles detects iron, while quantitative analyses of a few sufficiently thick examples indicate a composition corresponding to FeOOH. The authors assign these needles to goethite rather than its polymorph lepidocrocite on the basis of their acicular habit. Larger irregular iron-rich masses occur among the fibres; one grain suitable for quantitative analysis is identified as hematite. Some specimens also contain reddish-brown discs composed of radially arranged goethite fibres. Their location almost exclusively within fissures indicates secondary formation, interpreted as evidence that iron remained available after the earlier inclusions had developed. The study attributes the cat’s-eye effect to light scattered by the parallel goethite fibres and demonstrates how conventional gemmological testing, microscopy and microprobe analysis together resolve both the identity of a mislabelled gem material and the mineralogical cause of its optical phenomenon.