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GeneralStudy and better definition of opalite

22nd Mar 2017 21:31 UTCDonald Kasper

Mindat states that opalite is called a manufactured glass type, and is also a variety of common opal. Variety means literally a color or textural variant of a mineral. But what mineral is common opal supposed to actually be? Common opal found in continental systems we would encounter is opal-CT exactly, and is not a slang term couched in “variety” and “common” terms.

Now let us look at opalite, and to do so, let us go to the Kramer, CA relict hydrothermal mound and dig some. This has been identified as opalite and jasper by rock and mineral collectors for over 60 years. Then let us run it through infrared spectroscopy, as other spectral methods are not able to identify opal mineral species easily or accurately. Then let us make some thin sections and study it with polarizing microscopy.

The Kramer mound sits on the Helendale fault, is full of stick and plant matter on a very small scale, often microscopic. So this was a low temperature mound, well under 100 C. So what opal do we find? Opal-CT?

No. We find an opal polymorph that does not exist in the science literature. Correct. This graph is not in our science. Well, things just got interesting. As the graph shows overlaid with an opal-CT, this has half the spectrum of quartz, and half the spectrum of opal of any polymorph type. When you have a pyramidal peak at 480 cm-1, and you lost the quartz doublet that occurs there otherwise for all silica polymorphs, you are looking at opal. But over at the Restrahalen Trough at 1159 cm-1 you have a quartz trough. This trough is conclusive and final evidence of quartz. Many other band assignments in this graph are also quartz such as the doublet under 400 cm-1 which never occurs with opal.

How do I reconcile this? We have a new opal polymorph, and I call it opal-Q. Opal-Q is an intermediate form that occurs in many jaspers, chalcedonies, agates and their host ignimbrite rocks worldwide. It is an intermediate between opal-CT and quartz. However, opal-Q makes opalite and jasper, dominantly granular quartz with residual silica banding and vugs on an extremely small scale. Yes, this is a boundary condition for agate formation. When you have to use a microscope to see two banding silica layers (isopachous quartz) and not 30,000 bands to the inch, you are looking at a boundary system that defines the limits of silica agate formation where the geochemical processes are shutting down.

Recognizing this helps us to constrain silica geologic systems where banded agates form, because there are more than one. It is not the model of agate formation, it is the models of agate formation. For example, did you ever notice there is no documentation that exists in the literature of tubes-of-entry for isopachous silica banding? Are the tubes supposed to show up later? Clearly not, and so we see that on a small scale, agates don’t have these structures. We on a fault gouge linear trend, and not in the granite and gabbro basement rock nearby which has zero opalite and jasper. So much for weathering there making agate and jasper 10 yards away.

In fact, if you study that Australian paper on silcretes (Milnes and Twidale, 1983) where they went out to the Great Artesia Basin bentonite and studied mesa tops for silcrete (we call this siliceous tuff in the Western U.S.), you will find they concluded just that. Weathering in Tertiary made their silcretes. With some opal in their silcrete, they have proof of weathering. Yet, what is just extraordinarily funny about this paper is something rather odd. Do you see it? Right. They wrote a blathering paper all about Tertiary weathering making silcrete from opaline mineral migration and forgot that was Tertiary, which means that modern weathering does make silcrete as they found no silcrete in modern soils. Oops. This is very funny, and shows the goofy traps people get into when talking about silica mineralization and weathering. They have mesa tops with tuff/silcrete, but not basin floors because weathering is only on mesa tops, we must presume.

We have mesa tops of this silcrete/siliceous tuff here in the Mojave Desert as well, commonly full of fossil reeds and palm. You first find them by looking for the fault up-thrust trends, as weathering did not make them either. This is what happens when you analyze rocks but not the full geology of their occurrence. As I have always said, if you want to sort out what silica is doing in volcanics, you absolutely must collect for yourself.

So in the condemnation of opalite as slang sits opal polymorph number 4. We might be tempted to resurrect the marine sediment alteration sequence of opal-A to C, to CT, to quartz or agate but these continental systems do not have opal-A. The Kramer mound has no opal-C. To find opal-A, you have to go to venting geyser systems that don’t have carbonate.

Is this opal-Q proof of a derivative from Opal-C or opal-CT? Well, if you look at the opal polymorph type in volcanic geode waterline structures of the world, they have quartz, opal-C and I propose opal-BC (beta-cristobalite), but exactly zero have opal-Q. So far, no opal-CT is found in them either. They are stuck half way from opal-A to agate according to some proposal pathways of alteration, and have been stuck there for up to Permian, 350 million years. That is a long time stuck transitioning, and probably means they simply aren’t doing so.

This opalite also contains bentonite from its source volcanic ash, and clinoptilolite, a zeolite.

Don Kasper 3/20/2017

24th Apr 2017 08:31 UTCRalph S Bottrill 🌟 Manager

Donald

I think opalite is generally used as a generic name for any form of opal, especially ones of interest to lapidarists.

Tuff is a consolidated volcanic ash, nothing to do with silcrete, though I guess it could get opalised. You may be thinking of tufa, another name for calcrete, a calcite rich analogue of silcrete. These are both groundwater-derived duricrusts forming in soil horizons and usually showing up as hard near-surface layers, and found as residual material on plateaux and plains where the softer soils have been eroded. During the Tertiary these parts of Austria were largely in a tropical climate, forming a lot more weathering, erosion and higher, more acid, groundwater levels than at present. In Tasmania we get similar Tertiary silcretes forming in permeable palaeo channels overlying deeply weathered kaolinite zones. The present climate is mostly dryer. I think that explains your problem with the Tertiary age and silcrete distribution ?

You really need to characterise this new form of opal by XRD, its possibly a mixture of different types.

24th Apr 2017 16:04 UTCGregg Little 🌟

Donald;

Weathering makes nothing as it is the destruction of minerals. Weathering is the process of disaggregation, dissolution, erosion and dispersal. It is the processes that follow weathering that create mineral deposits.

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