FMF - Friends of Minerals Forum, discussion and message board

[Fiebre Verde]

Two generations of calcite, striated tabular form (common) and clear prismatic form (not so common)

[Fiebre Verde]

Group of beryl crystals in a calcite matrix.
Though smaller than the Muzo ones, the Chivor stones often exhibit more sparkle.

[Fiebre Verde]

Light-green beryl crystals on a marble-like calcite matrix mixed with patches of black shale.

[Fiebre Verde]

From Cunas,
Well crystallized dolomite, calcite, clear quartz.

[Fiebre Verde]

Host rock is black shale.

[Fiebre Verde]

Coscuez specimen with ~15 thin elongated beryl crystals on a calcite + dolomite matrix.

[Pierre Joubert]

I like this last specimen!

[Fiebre Verde]

[Fiebre Verde]

Laminar albite, pyrite, dolomite, on black shale.

[Fiebre Verde]

Translucent crystal from Palo Arañado.
Weathered pyrite and calcite on black shale.

[Fiebre Verde]

Elongated crystal from Chivor with a complex termination.

Crystal has an unusual hollow cavity down through the upper quarter (5mm).
The void inside the crystal looks like a real crystal - so this is clearly a negative crystal.

The cause of this negative-crystal cavity (crystal-shaped hollow cavity) is unclear to me.
Schmetzer & al. (2011), Chemical and growth zoning in trapiche tourmaline from Zambia – a re-evaluation, JoG, Vol 32, No.5-8
suggest 2 possibilities
1) Trapping of a solid inclusion and subsequent blocking of crystal growth with the formation of a growth induced, elongated void in the direction of crystal growth (arrow) subsequent to trapping of the small mineral grain
2) Trapping of a solid inclusion, movement of the trapped small mineral grain in the direction of the arrow and of the growing crystal face and formation of a growth induced, elongated void in the wake of the moving mineral grain, a mechanism repeated in several steps

Are any of you aware of any other possible explanation?

[Roger Warin]

Hi Fiebre Verde,
You said “The cause of this crystal-shaped hollow cavity is unclear to me.”

I think it would be a fluid inclusion, a liquid with maybe even a third gaseous phase (CO2 ?), which has locally prohibited growth (in hydrothermal conditions).
Under these conditions, the crystal grows by forming faces whose indices are consistent with inverted growth.
Subsequently, fluid inclusion escaped.
A solid inclusion would give foreign faces to beryl and would have dropped impurities, I believe.

[Fiebre Verde]

[Roger Warin]

OK, thank you Gerard.
Roger.

[Roger Warin]

Hi Gérard,
Summer nights may be good for insomnia.
Why not think about minerals?
I compared the presence of a cavity in your emerald to the texture of the beryl variety heliodor of Ukraine.
These negative facets are not growth figures, but on the contrary they result from an inverse process, namely dissolution in a hydrothermal medium at the end of growth.
The kinetics of this dissolution depends on the population density of the reticular planes.
These figures are not in relief, but in negative, as in an intaglio. These are hoppers (stairs cavities).

[Fiebre Verde]

[Fiebre Verde]

Beryl prism solidly attached to a calcite crystal.
Dissolution patterns can be seen on all faces.

[Fiebre Verde]

Lustrous dolomite, calcite, pyrite, on black shale.