FMF - Friends of Minerals Forum, discussion and message board

[gk68]

Hi Forum,

May I ask

is this a rhyolite matrix?
are these small druzy crystals, smoky quartz?
are these lines around the tiny crystals, calcites? (note, under UV, they have a bright pink color, much brighter than it shows in the picture)

Concluding, what is this?

Item broke during an older Athens earthquake, it had been glued and you can see on UV the repair green straight line.
Origin: Brazil most likely but not certain

Size: ~ 125 X 98 X 25 mm
weight: 505 gr.


As always any help much appreciated !

[Roger Warin]

Hello,
Here is an attempt at an answer.
Rhyolite is an extrusive igneous rock with a very high silica content.
Your nodule is an anomaly of detrital sedimentary rocks.
Your rock is a siliceous nodule extracted very slowly by water under high temperatures and pressures of various silicate minerals including quartz, forming detrital rocks.
This nodule forms because it is incompatible with all other detrital rocks. The overall texture is a sort of ellipsoid.
Quartz, a stable form of SiO2, is the most difficult mineral to study because it is a tectosilicate sensitive to the presence of water capable of interacting chemically with H2O. When it is pure, the appearance of twins is very frequent to the point that manufacturers have had to carry out its synthesis to find its piezo- or pyroelectric properties.
These are polar molecules which attract each other with the formation of various derivatives including chalcedony. Your specimen is one of them, but the minerals that stand out there are not of great aesthetic quality.

[Volkmar Stingl]

For me, it looks like a kind of septarian nodule that is filled by agate and small quartz crystals. I cannot see any hint to rhyolithic rock.

Volkmar