J'ai trouvé cet article qui explique le sujet : HOW SPELEOTHEMS GROW: AN INTRODUCTION TO THE ONTOGENY OF CAVE MINERALS
CHARLES A. SELF & CAROL A. HILL
dont la partie la plus significative pour notre sujet me parait être ce passage :
"(2) SECOND LEVEL: Mineral Aggregates. Mineral individuals very seldom occur singly; they grow multiply over a
substrate surface as mineral aggregates. Aggregates are much more than simply a group of individuals of the same mineral
species growing together: interaction between individuals directly affects and limits the growth of each crystal. During such
"group" or "common" growth, there is competition between the mineral individuals constituting the aggregate. Most
speleothems are mineral aggregates.
Most aggregates form where growing individuals compete for space by physically contacting one another. In such a
situation, contact faces develop between neighboring individuals, leaving a group growth front comprised of the
crystallographic terminations of many individuals. However, aggregates do not necessarily have to be in direct physical
contact for competition to occur. An example of indirect competition for the supply solution is when growth is in a plastic
substrate such as porous clay, where interaction between crystals is due to the closure of feeding pores in the clay as a result
of crystallization pressure. When growth is in a capillary film environment, there is competition for the loss of solvent
molecules and interaction is by convection of water vapor and CO2 between individuals. The mineral individuals
constituting an aggregate have contact faces when they are in direct competition, but display true crystal faces when they are
in indirect competition.
Competitive growth on a substrate surface normally leads to a reduction in the number of individuals constituting the
aggregate, a situation called selection. The most influential process during the early stages of crystal growth is geometric
selection. The crucial elements of geometric selection are (Fig. 4): (1) initiation of separate centers of crystallite growth, (2)
the beginning of competition of these crystal individuals for growth space, (3) selection and a reduction in the number of
competing individuals according to a geometric rule, and (4) continued growth with no further selection. There are several
geometric rules for selection, but perpendicularity to the substrate is the most common. This rule applies to most mineral
veins and to many common varieties of speleothems (e.g., dripstone, flowstone, pool spar)."
La "règle" géométrique de survie des cristaux semble être la perpendicularité au substrat.
Connait on la raison de cette règle qui semble être vérifiée assez souvent dans la nature ?
Un autre point qui me questionne, c'est qu'au début on a généralement une croissance cryptocristallinne du type calcédoine, puis la croissance des quartz. Hors d'après ce que j'ai compris : la calcédoine est elle-même composée de très fines fibres de quartz . Ces fibres croissent à partir d’une dislocation et d’un empilement en spirale des micro-grains de quartz . Comment s'effectue alors la transition calcédoine - quartz "visible" ?