Hallo Miteinander,
Habe heute den gleichen Bereich noch einmal bei 16-fach im x-pol. Licht fotografiert, diesmal allerdings so, daß sich der Olivinkristall, in dem sich die betreffenden "
Fahnen" befinden, nicht in Auslöschung befindet. Ferner habe ich den mm-Maßstab hinzugefügt.
Ausserdem hier ein interessanter Auszug zu den "
alteration products" von Olivin aus Nesse. Dieser Auszug könnte vielleicht bei der Beantwortung unserer Fragen bezüglich Limonitisierung und/oder Serpentisierung weiterhelfen.
Bernd
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Olivine commonly alters to a number of distinctive products including
iddingsite and
chlorophaeite, which are combinations of various minerals and are usually subject to rigorous identification. Olivine may alter to
serpentine.
The alteration usually progresses from the periphery and along cracks inward.
Iddingsite is a name given to fine-grained reddish- or yellowish-brown material that consists of goethite, clay, chlorite, quartz, talc, and other minerals…
Chlorophaeite is an essentially isotropic material that is usually orangish or greenish. It also is a mixture and appears to be composed of limonite and chlorite or serpentine with other low birefringence silicates…
Magnesium-rich olivine often alters to serpentine, which may either be
chrysotile,
lizardite, or
antigorite.
Peridotites and other rocks rich in olivine and pyroxene may be partially or completely altered to serpentinite, which consists of
serpentine plus
carbonates,
Fe-Mg silicates, and usually some disseminated
magnetite.
Olivine from igneous rocks is often mantled by pyroxene or hornblende as the result of normal magmatic reactions between the olivine and melt.
NESSE W.D. (2004) Introduction to Optical Mineralogy (Third Edition, Oxford University Press, 348 pp. 242-243).
