FMF - Friends of Minerals Forum, discussion and message board

[Pete Richards]

[marco campos-venuti]

In any case, I learned a lot during this discussion and I hope more peoples add to the thread (double meaning joke!!).
I'm still convinced that quartz faden crystals grows over a preexisting thread, that is the second part of your two step model. So we can say that we are 50% in agreement.
I have no idea what is the preexisting thread and I don't have my own theory, but I'm sure more investigation is necessary to clarify the mechanism, also in the case you are right.
In your thin section cross-polarized picture it is possible to observe 2 trends in the thread. One is many short lines perpendicular to the thread. The second is perpendicular to the first and consists in few lines along the thread direction. While the first are the supposed fractures, the second can be a relict of the preexisting thread. Veil inclusions in crystals composed of many little voids with water are a common feature and are formed in a variety of ways, between others fractures, growing defeats or pseudomorphose after some preexisting material. Between all, the only one I think can not be finely repeated is the case of a fracture.
A pseudomorphose could be an alternative.
Just as an example I want to show you something. The picture 1 show a quartz with a strange inclusion. It is not a real thread, but it remember one. And is made by repeated lines. Observing a big number of specimens I can related the millefeuille structure (as I called it) with the pseudomorphose of an included tourmaline crystal. Picture 2 is a less advanced case and picture 3 is a quartz with a true tourmaline inside.
In the diagram we have 1) tourmaline crystallizes in a pegmatitic environment of high temperature and high pressure. 2) tourmaline is altered in hydrothermal high temperature environment. 3) tourmaline is the feeding crystal for the overgrowth of an epitactic quartz (the hypothesis is about an epitaxy with constant angle of around 23,5°). 4) included tourmaline is transformed into chlorite or Muscovite (a typical alteration process of the tourmaline) and then into quartz, giving the millefeuille structure.
I'm not sure about the physics of all the steps of the process, but there are many evidences that the sequence is as described.

[Roger Warin]

Hi Pete, Marco,
Excuse me for participating in a discussion of such a level, because I am not competent but only curious. However, I wonder if certain factors are not forgotten, like the variation of coefficients of dilation with temperature. They sometimes change sign. This would occur in an anisotropic crystal.
Could this not cause cracks?
These voids could purify the growing crystal by capturing internal fluids in the hot crystal, as if it was an exsolution.
Can we not remember the helical dislocations of beryl?

[Roger Warin]

The origin of the Faden quartz.

In 1990, I was happy to read Rykart's interpretation. Over time, doubt has been insinuated especially after observing an increasing number of specimens of Faden quartz. Gwindels themselves can have a Faden.
I greatly appreciated the theses defended by Pete and Marco, but I do not think they adequately describe the initial situation that will give rise to the Faden quartz.
Here is how I conceive the phenomenon.
All the Fadenquarz do appear in a vug, which amounts to saying that they crystallize in an almost closed system, as in a box. I have difficulty in conceiving the growth of an initiating filament of the thread in the void of the crevice, or even in a solution. I suggest that this linear ribbon can be sustained in the initial medium. This environment would be a supersaturated gel of non-crystalline silica (in fact polymerized Si(OH)4). This silica gel can withstand temperatures up to 300 ° C without losing water.
It is this silica gel environment that I choose as phase that will crystallize.
This factor which breaks the metastable equilibrium of the supersaturated gel could be correlated with a sudden as well as brutal retraction initiated by a shock.
Under the effect of an adiabatic expansion (rupture of the box), a silica thread does appear suddenly, as if a lightning bolt had crossed the silica gel. This triggering of nano-crystals would be done in a straight line, although curvature effects are possible.

This factor which breaks the metastable equilibrium of the supersaturated gel could be correlated with a sudden as well as brutal retraction initiated by a shock.
The velocity of this primordial crystallization is great, incorporating bubbles of fluids in the blank of small crystal domains. In practice, one would have a series of lattices substantially aligned, since the flash of crystallization would not have allowed a position of equilibrium between all these domains.
Then, the normal growth of the crystal (or the crystal sequence) can resume normally, the initial stresses having disappeared.
The value of this assumption lies in the fact that the linear ribbon germ is supported by the gel and does not float in a vacuum.

This environmental context being proposed, we can think of other mechanisms of initiation of crystallization. Thus, the creation of a hydrodynamic cavitation makes it possible to cause nucleation and the direct production of crystals.

This cavitation could be caused by the sudden drop in pressure, by a wave or by any impurity which strikes the supersaturated silica gel. Let us not forget also that the collapse of a vacuum bubble in a liquid creates a strong localized shock wave. The Faden abandons a thread of small cavitation bubbles. Even ultrasounds are likely to initiate crystallization but I do not think they exist in this context.
In fact my main hypothesis lies in the nature of the initial medium enclosed in a pocket that opens suddenly.

[marco campos-venuti]

Hi Roger,
the gel context is exactly what I was thinking when I supposed a chalcedony thread. The polymerization starts from the wall rock in the form of curved chains. Is like an agate, but in a different geological context.

[Harjo]

Just as an illustration...here is a nice curved fadenquartz from Bierghes, Belgium.
I found many curved fadenquartzes in one specific cleft, hardly any in many other nearby clefts. The curvature of the faden most likely depended on the direction in which the specific cleft opened but it puzzles me why the curved fadens were restricted to merely one cleft.
We collected them from a cleft in a series if typical Scherklüfte, schearing clefts, which tend to form in rows of similarly orientated clefts.

[Harjo]

Another quite interesting fadenquartz from the same locality

[alfredo]

I think it is mistaken to think of a long "thread" growing across a cavity, because it starts out as an extremely short thread, being just the "healing" process between the 2 parts of a cracked quartz grain in which, of course, the two halves of the broken grain have the same crystallographic orientation. As the crack continues to widen, the healing process continues and the thread gets longer. All that is necessary is for the crack to widen at a slower rate than the quartz can reheal, which would be not unexpected in an alpine tectonic setting.

[Harjo]

Exactly, and while expanding the fluids present cause quartz crystals to grow on the row of quartz grains, just up to the point of saturation. The orientation of the faden towards the c-axis as illustrated in the above drawings can be explained by the orientation of the grain (its crystal lattice)
But explain why only one cleft in a series of concordant shearing clefts produced the bent faden ;-)
Here's a link to a video showing another Bierghes faden peculiarity. It shows a fadenquartz which is ordinary on one side, but the single faden "sprouted" two differently orientated parallel rows of crystals on the other side. I show the video because the peculiarity doesn't come across on a photo: https://www.youtube.com/watch?v=wnNuvfh_bV8

[Roger Warin]

Hi Alfredo,
I persist in the error ... I do not believe in healing process, but in an almost linear flash in a phase of saturated silica gel, set between the two walls of the cavity. The importance of silica gels in geology is underestimated

[Roger Warin]

Hi,

John Kashuba (Oregon) sent me a lot of photos of Faden quartz in thin sections. To have a certain depth, he chose a thickness of 100 μm instead of 30 μm normally. Faden TS are observed in oblique incident light.

[Roger Warin]

In conclusion, John Kashuba said to me:

“The inclusions appear to be in flat planes across the entire Faden and at right angle to the length of the Faden so it appears we cannot invoke an "inclusions during episodic skeletal crystallization" mechanism. Further, I have found cross-cutting planes of inclusions that beg to be seen as off-axis breaks.”

Many thanks John.
Note: only the inclusions are visible, not the crystal
The growth seems discontinuous creating multiple microcrystals

[marco campos-venuti]

[Pierre Joubert]

The last photograph and one of the others, looks like microscopic negative crystals with water.

[Pete Richards]