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[Daniel Garcia]

Dear all

I'm trying to document a silicification event in Oligocene evaporites near Narbonne (France)
The host rock is gypsiferous, with some diagenetic native sulfur. Silicification occurs sporadically, either as thin layers intercalated with clays and microcystalline gypsum (algal laminae ?) or as dispersed spherulites with a distinct zoning pattern. My question is how much cristobalite and how much chalcedony in these assemblages ?

[marco campos-venuti]

Cristobalite is high T mineral. All supposed cristobalite spherulites are other silica phase.

[Daniel Garcia]

[marco campos-venuti]

My impression is that it is all chalcedony. Chalcedony is a fibrous silica polymer so the structure you see in thin section looks very similar to it. Cristobalite is stable over 1470ºC, Therefore it is impossible to find it in a diagenetic environment. I have written several books about the equilibrium of microcrystalline silica phases that you can see on my website.

[Daniel Garcia]

[marco campos-venuti]

I don't see replacing after gypsum. The formation of your spherulites looks like the growth of a concretion. There is a granular gypsum with porosity. A Si-rich solution flows through the porosity. The solution dehydrates and polymerizes. Monomeric Si ions become dimers, then trimers, etc. When the ions reach 1 nm in diameter we have a colloidal solution that behaves like a hydrogel. When this dehydrates further, it begins to crystallize the chalcedony which is fibrous. Chalcedony is the stable phase in an acidic environment, if the conditions are basic we have opal. New space is created on the external surface of the chalcedony by dissolution. Little by little one band at a time the spherulite grows. Compared to the crystallization of quartz crystals in gypsum Keuper deposits, in your case the climate must have been hot and dry. Without a doubt we are in subaerial conditions because the hydrogel is water-soluble and does not form under water.

[Peter Megaw]

Robert Folk taught that length-slow chalcedony was indicative of formation under evaporitic or sulfate rich conditions. If you have a gyp plate you could check that out

[Roger Warin]

As I observed in LDG, cristobalite microcrystals readily aggregate into micro-ourses that occupy an intercalated phase (spherules) in another phase, as in semi-crystalline polymers.
I insist to my friend Herwig that I observe a Maltese cross in my case for Libyan glass (which has never been said before for LDG).

[Daniel Garcia]

Sorry to be unclear. What I see is that silica (chalcedony) takes the place of former gypsum lenses, growing from the faces inwards. I agree that the likely mechanisme is coupled dissolution-precipitation. Maybe replacement was not the proper term.

But my main question remains. Can we derive any inference from the treillis structure of this (chalcedony) intergrowth ? It cannot be true HT cristobalite, but at least it displays a symetry that is reminiscent of an isometric or pseudoisometric frame. Does this mean there was an isometric or amorphous precursor ?

Thanks anyway for replying

[Daniel Garcia]

[marco campos-venuti]

Hi Daniel,
From the hand sample the chalcedony is spherical and I don't see any faces. From the last photo you can see chalcedony filling a cavity inside a gypsum crystal, but I still don't see any faces.
If you are looking for a replacement, the original crystal must be in disequilibrium and in your case the gypsum is not in disequilibrium so you don't have a replacement of gypsum, but only filling a cavity in the gypsum. It could also be a primary cavity of the gypsum or a grain included in the gypsum and then dissolved.
However, an evaporite is an alkaline environment and therefore it would be good for CT-opal to precipitate which is amorphous and is often confused with cristobalite, as in the case of Roger's LDG.. Then the opal is replaced by chalcedony later when the chemistry becomes acidic.

[Daniel Garcia]

[alfredo]

Thank you, gentlemen, for this interesting discussion. I observed very similar looking millimeter-size spherules in gypsum in Archidona, Spain: https://www.mindat.org/loc-257672.html - and I had been wondering how they formed.

[marco campos-venuti]

Hi Alfredo,
perhaps you are referring to the gypsum of Mina Rica, Pulpì, Almeria, Spain, where gypsum crystals show inclusions of celestine needles and white spherulites. In my opinion, in that case they are bacterial colonies. I have found similar ones in some quartz (see my book "Biominerals"). Dissolution spherulites are common in glass (pitting) and in some microcrystalline silica, but I don't know if they are possible in crystalline material.

[Daniel Garcia]

Thanks everybody for feedback and suggestions.
Let me add some more pictures of chalcedony flowers growing inside gypsum cristals from the same evaporite formation.
Incidentally, Marco, it seems that growth from a face is not the most common case ; very often, a carbonate impurity, a wood fragment or another undetermined inclusion in the gypsum may serve as a nucleation center for the chalcedony flower.