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[Roger Warin]

Hello,
I remain fascinated by the interpretations of twins and epitaxies, although I am only an amateur.
I also like quartz.
Silica SiO2 is the superior homolog of CO2. What a difference !

One of the most spectacular epitaxies comes from Brazil (Bahia) offering a design of rays of the Sun around a black center: acicular rutile radiating from a black center of tabular hematite.
I took an old specimen from Novo Horizonte, Bahia, Brazil (1980 < 300 BEF or ~ 7 USD) – Intermineral Liege.
A single quartz crystal coated all these rutile-hematite assemblages.

First of all, we perceive the {10.1} plane of quartz. It is the plan of a bad cleavage. Quartz has the structure of a tectosilicate. The tetrahedra (SiO4) are linked in 3 dimensions. The number of bonds per unit area indicates that there is a minimum cohesion across r and z planes, in comparison with the other planes m, c and a planes.

I wanted to photograph the end of an acicular rutile crystal. This is why I chose this specimen where the rutile crystal is immersed in the quartz crystal.
It is also noted that the rutile phase is well compatible with that of quartz.
We do not perceive any defect surrounding the rutile which seems to float in the air.

[Roger Warin]

Hello,
In the last photo, it can be believed that a rutile crystal induced a conchoidal fracture in the quartz crystal.

[alfredo]

Roger, I often saw such fractures in heavily included quartz crystals, and I wondered whether they could perhaps be caused by strain due to differential thermal expansion between the quartz and the rutile, pyrite or other included species? Just a guess...

[Roger Warin]

Alfredo,
Yes, you are right.
These fractures are quite frequent, even in pure quartz, without (visible) inclusion. They result from thermal shocks but above all from high pressures, but these conditions are often linked together.
Pyrite must be a more intolerable foreign body than TiO2, because of the sulfur. And as you say, it be caused by strain due to differential thermal expansion of FeS2.

Ionic ionic radius :
Rutile: TiO2 versus SiO2.
Si = 50 pm; Ti = 61 pm. And above all, the same O2- anions.
Pyrite: FeS2
S2- = 184 pm.