MORPHOLOGICAL OBSERVATION ON THE GROWTH OF SMITHSONITE

Smithsonite samples of various crystal habit are examined carefully under optical microscope and SEM to investigate the crystallization process and the relationship to its crystal morphology. Two major crystal habits are shown. The first one has strong development on pyramid faces {111} and {101} but insignificant development on the prism face {100}. The second one is more common among samples, it has a predominant growth along {100} and sheaths of needle shape crystal radiated out from a center to form a common botryoidal of mammilary texture.

Detail examination on the radiating growth of smithsonite revealed that crystals show different growth characters on the fan-shape cross section. Most crystals at the apex of the fan have c-axis lying at high angle to the radiating crystals on the body of the fan. As the crystals grew into the fanning part, they rotated gradually into its radiating orientation. The size of each crystal at the upper par of the fan is in the order of 1µ thick and 200µ long. It is significant to notice that as the needle-shape crystal grew vertically, there was also a development of horizontal layers that are parallel to the rim of the fan. The thickness of the horizontal layer increases with the thickness of the needle crystal. At the base of the fan where the needle crystal is about 10µ thick, the horizontal layer is about 1µ in thickness. The layered structure across the radiating growth may be related to the nature of the perfect {101} cleavage of the mineral. However, instead of being a cleavage, the layers have physical thickness. The layered structure gradually became invisible as the crystallization continued and turned into a straight {100} face.

Observation on smithsonite crystals suggests that its {101} cleavage has profound influence on the growth of smithsonite crystal. This is different from calcite, which is a more common mineral in the same structure group.