Fresh and old as-grown (001) surfaces of a-HgI, platelets were studied by optical microscopy. The presence of growth hillocks and the high value of the surface entropy factor are proofs of the spiral growth of platelets. In air surfaces became gradually dull. Consequently, the final micromorphology of HgI, is the result of both growth (primary) and post-growth (secondary) processes. For the revelation of defects a new, reliable etchant was used. 1. Introduction Tetragonal a-HgI, crystals are useful material for detection of X-ray and nuclear radiation. Different techniques of vapour and solution growth are used to obtain mercuric iodide bulk crystals or platelets (e.g. SCHIEBER et al.; WILKE, BOHM). Platelets seem to be especially useful for detection of low energy gamma radiation because no sawing and slicing is needed in fabrication of devices, and electrical contacts are deposited on as-grown or polished surfaces. The present paper describes the results of an investigation of the micromorphology of as-grown (001) faces of crystalline platelets by optical microscopy. The aim of the study is to obtain information on the crystallization and post-crystallization processes and on the distribution of defects. 2. Experimental Red, a-HgI, platelets were obtained from vapour in the presence of hydrocarbons, organic polymers or noble gases (LASKOWSKI et al. (a); PRZYLUSKI, LASKOWSKI). The best quality platelets of 1 cmz area and thickness up to 0.5 mm were grown at total pressure of 2- 10 Torr. The temperatures of the source and crystallization zones were about 150- 180 C and 118 - 127 C, respectively. Details of crystallization procedure Mere described previously (LASKOWSKI et al. (a)). The faces of the platelets were determined by Laue technique as {OOl) planes. The crystals were mercury-rich and the total concentration of 23 elemental impurities was less than 100 ppm (LASKOWSKI et al. (b)). As-grown and etched (001) surfaces of platelets were examined under an optical microscope in reflection. Inclusions were observed in transmission. For the revelation and examination of the distribution of defects in platelets a new etchant consisting of acetone, HAuCI, aq. (0.5 - 1% wt.) and MnCI, aq. (satd) was used. Dislocations are revealed after etching for a few seconds. 3. Results and discussion