Confirmation, par l'etude des inclusions intracristallines, de l'origine superficielle de quartz pyramides

Abstract

Quartz crystals have been found in colluvial materials overlying outcrops of ultrabasic rocks in New Caledonia. Two types of crystals were recognized on the basis of inclusions within their structures. These are designated here as type a and type b. Type a crystals are pyramidal. Crystals of type a have two kinds of liquid inclusions. One kind consists of fluid in primary cavities 5–30 μm in diameter. Because the fluid consists of a single liquid phase, it must have been entrapped at low temperature (⩽ 30°C) as the crystals were being formed. Microscopic examinations indicate that this liquid phase is fresh water rather than saline water. The second kind of liquid inclusions consists of fresh water that penetrated to the hearts of type a crystals by means of secondary fissures and was trapped in secondary cavities. Beside these liquids, solid inclusions are minute automorphic grains of quartz distributed throughout the crystals. In the rather scarce crystals of type b, it is difficult to distinguish primary from late inclusions, both of which can occur in quartz. However, the cavities contain two-phase inclusions: liquid phases and vapour bubbles. The liquid is saline water which was entrapped at a temperature of at least 120°C. Thus, the study of the liquid inclusions in type a and b quartz crystals demonstrates that: (1) type b crystals have crystallized in a warm and saline medium; consequently, they are detrital; (2) pyramidal quartz of type a is believed to have been formed in this pedologic environment. Within the colluvium in which they occur, weathering of peridotitic rock by meteoric water could provide the necessary silicon for crystal growth. It is postulated that minute “seeds” crystallized and sprinkled down periodically on the growing surfaces of the type a crystals and were then engulfed by further growth to become inclusions. The presence of minute quartz “seeds” as inclusions indicates that these large pyramidal crystals were formed within a soil void. The abundant secondary fissures containing fresh water are surprising in such well preserved mineral grains. The fissures and fluid inclusions would be expected to make the quartz grain more readily subject to weathering and dissolution than normal for the mineral. The presence of secondary water within quartz crystalz underscores the hazards of assigning all elements found through chemical analyses of minerals to their lattice structure. The study of fluid inclusions within crystals seems to offer an approach in pedology and surface geology for identification of environmental conditions under which soils were formed and sediments deposited.