FMF - Friends of Minerals Forum, discussion and message board

[marco campos-venuti]

Years ago I cut a beautiful clear opal cabochon with a beautiful well-centered black dendrite. The best quality of that cabochon was that it was from Cabo de Gata, Almeria, Spain a little-known location for gemological materials. After 12 years something happened to my dendrite. Can anyone suggest what happened? The photos are taken with the same camera and in the same microstudio without any processing.

[Roy Starkey]

Hi Marco
Interesting question. It looks as if the iron (?) oxide / hydroxides may have undergone some form of slow hydration through uptake of moisture from the atmosphere and diffused through the microstructure of the opal.
Best wishes
Roy

[Matt_Zukowski]

I can think of a couple of unlikely possibilities:

1) Aqueous mobilization - It seems hard to me to mobilize through dissolution these Fe/Mn oxides in such a static environment because i can't imagine it becoming much more reduced/acidic.

2) Some solid-state phenomena - 12 years seems too short a time for solid-state phenomena, especially at these PT conditions.

3) Mobilization of solid particles in a moving fluid. Small particles do move through porous media, but it is hard to believe that the pores here are large enough and the movement of water large enough to spread solid particles out.

4) Some way that the Fe/Mn oxides have become less opaque or that the opal covering them has become more opaque.

[Bob Carnein]

I'm curious to know whether the cabochon has been regularly or continuously exposed to light. Also, it's interesting that no change occurred to the more opaque inclusions to the upper right. It might be useful, also, to see a photo of the back of the cabochon.

[Pete Richards]

This is an amazing change!

Does any part of the dendrite intersect the surface of the cabochon, including the "stem"?

[marco campos-venuti]

I show you the back of the cabochon made of white opal. The dendrite is all inside. This is the drawer where it has been for 13 years. The only change was the addition of the azurite and malachite samples. The malachite is unpolished and when I bought it it still smelled of kerosene. Can be kerosene fumes responsible? Many samples of dendritic agates share the same drawer.

[James Catmur]

The malachite looks polished to me?

[Riccardo Modanesi]

Hi to everybody!
There could be another possibility: the dendrites are made of manganese and iron oxides and hydroxides. As the biggest majority of you knows, iron oxidates from Fe++ to Fe+++ with the time,and manganese increases its oxidation number as well. That could be a reason of the colour changement of the dendrites in your specimen.
Greetings from Italy by Riccardo.