Hallo,
eben gefunden :): (http://www.minsocam.org/msa/collectors_corner/faq/faqminspec.htm)
What causes the banding in malachite?
There is a significant color change possible for malachite when one looks at different crystal sizes. The lighter bands tend to be fibrous in nature, while the darker areas are crystals of larger size. I have a chunk of malachite from Zaire and the lighter areas are fibrous, while the darker areas consist of crystals that are much larger. The streak of malachite is light green ( the color of small pieces of the material) while crystals are usually dark green to almost black. The specific gravity of the fibrous habits range down to 3.6 while crystals are 4.0. There is not much evidence for any major substitutions in the chemistry of malachite. There can be changes in the growing conditions in the veins which would result in the inclusion of foreign minerals, or even inclusions in the crystals. The changes in chemical conditions in the veins could also cause the nucleation of additional crystals giving rise to the fibrous habit. J. S. White found that the variation in color is due to changes in grain size, with the dark bands consisting of large crystals, while the light bands were composed of smaller grains. White, J. S. 1991. Chemistry of crystal size. Lapidary Journal 45:32-36,38.
Das ist eine nachvollziehbare Erklärung. Durch Änderung der Wachstumsgeschwindigkeit werden mal mehr oder weniger Fremdmineralien eingelagert und als Hauptursache durch die Wachstumsgeschwindigkeit bedingt unterschiedliche Kristallitgrößen.
Gruß
Berthold
PS: Hat jemand auf das Lapidary Journal Zugriff - und könnte mir den Aufsatz ggf. einscannen - wäre super.