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Posted: Dec 30, 2016 17:53 Post subject: Re: Faden Quartz Crystals - (26)
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.
Posted: Dec 30, 2016 17:56 Post subject: Re: Faden Quartz Crystals - (26)
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.
Posted: Dec 30, 2016 18:09 Post subject: Re: Faden Quartz Crystals - (26)
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
Posted: Jan 08, 2017 03:05 Post subject: Re: Faden Quartz Crystals - (26)
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
Posted: Jan 08, 2017 03:54 Post subject: Re: Faden Quartz Crystals - (26)
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.
Posted: Jan 08, 2017 04:01 Post subject: Re: Faden Quartz Crystals - (26)
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
004-Quartz faden running left-right, curtains of defects, FOV=0.44mm wide P1200x900.jpg
Joined: 29 Dec 2008
Location: Northeast Ohio
Posted: Jan 08, 2017 14:26 Post subject: Re: Faden Quartz Crystals - (26)
Pierre Joubert wrote:
Pete Richards wrote:
Pierre Joubert wrote:
The last photograph and one of the others, looks like microscopic negative crystals with water.
That's exactly what they are, water with a gas bubble.
Yes, but understandable too. When fluid is trapped in a growing crystal, it is enclosed in a space in the crystal which might have any shape to start with. The bubble, being trapped, can neither grow nor shrink, but its boundary can change shape. The same crystallographic forces that cause the external shape of the crystal cause the bubble cavity to evolve toward a crystal shape by dissolving in some places and depositing in others. If there is enough time, this process reaches an end point - an equilibrium - which looks like the crystal itself.
The fluid trapped in the crystal is initially a single phase. As it cools, the liquid part of the fluid and the gas part separate, very approximately like gas evolving from a carbonated beverage, because less gas can dissolve in the liquid at lower temperatures. Since all the fluid inclusions in a local region form at the same time and trap the same fluid, they release the same amount of gas in cooling to room temperature.
So this explains why the inclusions generally have the same shape (and orientation) - a negative crystal - and all have bubbles of about the same size.
One can get an idea of the temperature at the time the fluids were trapped by heating the crystal until the gas completely dissolves into the liquid again. _________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy
Posted: Jan 12, 2017 07:44 Post subject: Re: Faden Quartz Crystals - (26)
Definitely not a Gwindel. This could be a faden crystal, but it looks more like a double terminated "rehealed" crystal. A crystal comes loose from the pocket wall and subsequently crystallisation continues (when the conditions are still favourable for crystallisation) or resumes (when the conditions become favourable again) on the fracture.
Joined: 07 Aug 2006
Posted: Mar 27, 2017 13:51 Post subject: Faden Quartz Crystals - (26) / The Roger Warin contribution
Is a great pleasure see discussions from FMF "externalized" in so interesting printed papers like the A.G.A.B. bulletin.
The A.G.A.B. is the prestigious association of the Belgium collectors/geologists and as you can see in the images below, there Mr. Roger Warin took what was discussed in this thread and improved it with their own thoughts.
Mr. Roger Warin and the A.G.A.B. were so kind to allows me reproduce this article here as well as kind of digest from Mr. Warin written in English language (the bulletin is published in French language) in order to make their thoughts more comprehensible for the English speakers.
I believe is a great synergy what Mr. Warin did and I encourage all the FMFers to continue this scientific and so interesting discussion.
I love science! ;-)
FADEN QUARTZ - A personal opinion
Differently inspired and in opposition to the notion of a pre-existing chalcedony soul in a hydrothermal liquid and disappointed by the classical hypothesis, while remaining sensitive to the Pete Richards’s remarks of especially on the very low frequency of the phenomenon generating the Faden quartz, I suggest that if the process of the origin of these Faden crystals is so difficult to specify, it is that one observes only the crystal. One forgets to consider the initial medium which gives birth to these “quartz à âme” (in French). As noted by Pete Richards, these environmental conditions are not frequent, and therefore the environment must be special.
In what natural laboratory are these Faden quartz synthesized? Of course, in an alpine environment, or assimilated. That is to say, where tectonics induces high temperatures and pressures conditions in which very hot hydrothermal solutions circulate. Under these parameters of environment, we must forget the notion that we have a solvent such "liquid water" at room temperature. This insipid liquid, indispensable to life, becomes a chemical reagent at high temperatures. The notions of pH differ (the corresponding theoretical notions are known) and silica SiO2 can dissolve in this liquid, which seems unlikely at ordinary temperature (for example, sand in sea). Formally, by "adding" H2O to a molecule of SiO2, H2SiO3 and H4SiO4 are obtained. The molecule H4SiO4 or Si(OH)4 is silicic acid. In a hydrothermal phase this product crosslinks, that is to say it polymerizes in all three directions to an amorphous phase of silica gel. It is bulk crystallization or a mass polymerization is carried out. This phase is more stable than imagined, so it does not lose its water below 300 °C under pressure. On the other hand, it can crystallize by abandoning crystals of a polymorphic phase of quartz, as a function of the temperature which controls the crystallization. For temperatures below 450 ° C, we obtain the quartz (alpha-quartz, which we know). Much higher temperature values than those of alpine environments would give rise to cristobalite, for example.
The composition of silica gel is based on a distribution of associated Si(OH)4 molecules and water to form [Si(OH)4]x. n H2O. This assembly is crosslinked forming a condensation product (bulk polymerization) in all three directions. Its structure is composed of SiO4 units joined by internal siloxane bridges (Si-O-Si). It is an amorphous phase. Siloxane bridges have been created by condensation of two intermediate silanol groups with loss of one molecule of water. Dry, the material is hard and porous. It is thus normally used as desiccant.
All Fadenquarz (in German) appear in an Alpine vug. This narrow cavity differs from a fault which it supposes in addition a transverse displacement of the walls. These crystals originate in an almost closed system where high temperatures and pressures prevail. This cavity is filled with a silica gel, in hydrothermal solution.
Under the effect of a large impact, for example an adiabatic expansion induced by the spacing of the walls of the diaclasis (the rupture of the box), a substantially linear ribbon appears between the two walls, consisting of individual nano-crystals (minute-crystals) of quartz. John Kashuba (Oregon) ask to cut thin sections of a specimen of Faden quartz, at the level of the core, according to his instructions (Figs 5-8). John Kashuba chose a thin section thickness of 100 μm instead of 30 μm to achieve deep microscopic vision. The observation is made in incident light to observe only the inclusions and not the crystal itself. John Kashuba concludes by saying that he sees a simple quartz crystal with a confusing network of transverse non-axial planes, filled with fluid inclusions / defects / negative crystals.
All the conditions being identical during this expansion, these microcrystals align substantially with each other (FIGS. 4, 5, 10, 15). Here we find the concept of chalcedony microcrystals proposed by Marco Campos-Venuti. A curvature will only appear if the shock wave is deflected within the silica gel. At this point, nothing favors the rapid crystallization of the gel, which has lost its internal tensions, but the shock wave is damped, the core no longer develops laterally. It is only after the amorphous mass of the rich (water saturated) silica gel enveloping the aligned nuclei that the core of silica gel continues to feed, for reasons of thermodynamic stability, the growth of several macroscopic monocrystals, which are often very clear. The existence of isolated Faden quartz, biterminated, proves that the attachment of the core at both ends is not necessary. This process is relatively rare because the feeder gel must be enclosed in a narrow jail, a sort of flattened box set between the two walls of the rock. This explanation also agrees with the parallel aggregates of Faden Quartz aligned in the vugs (or small crevices) (Figs 9 and 10).
This primordial crystallization rate is great, incorporating bubbles of fluids in the opening up of small crystal domains. In fact, these fluid inclusions would themselves originate from the crystallizing gel zone, rather than an exchange with the outside. Water and gas are released from the gel during crystallization. In practice, there would be a series of crystal lattices of substantially aligned quartz, since the flash of crystallization would not have allowed a position of equilibrium between all these micro-domains. Then, the growth of the crystal (or the crystal sequence) could resume normally, the initial stresses having disappeared. The value of this hypothesis lies in the fact that the linear germ ribbon is supported by the gel (viscous) and does not float in an aqueous (liquid) solution.
When we observe the set of substantially parallel crystal chains and perpendicular to the walls, it is also understood that these aggregates are derived from the crystallization of a mass of silica gel. A "fracture-migration" crystallization sequence does not seem consistent, whereas shocked silica gel can induce several columns of parallel Fadens in the mass. Even the texture of these aggregates is an argument in favor of our hypothesis.
In this proposed environmental context, we can think of other mechanisms of initiation of crystallization. Thus, the creation of a hydrodynamic cavitation would make it possible to induce 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 that strikes the silica gel. Let us not forget that the collapse of a vacuum bubble in a liquid creates a strong localized shock wave (corrosion of boat propellers).
In conclusion, we propose this new hypothesis of the origin of Faden quartz with a core based on the nature of the initial medium enclosed in a rocky pocket that receives a shock wave, either by the sudden opening or spacing of the original cavity by Tectonics, causing a sudden drop in pressure, or by another process. This medium is a silica gel, multi-crosslinked by bridges of the siloxane type and saturated with water under high temperatures and pressures.
A.G.A.B. Mini Bul--March 2017--50 Year--Number 3 - Cover.jpg
Front cover of the A.G.A.B. Mini Bul--March 2017--50 Year--Number 3
"L'Origine des Faden Quartz" by Roger Warin published on the A.G.A.B. Mini Bul--March 2017--50 Year--Number 3
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