Tiny bubble theory of Lake Superior agate formation (Part 2): Results from other laboratories and inclusions in Lake Superior agates

Wayne Sukow (USA)

In Part 1 of this article, I showed how agate begins to form. Starting with tiny bubbles on the surface of a cavity in basalt, a husk forms showing their imprints. These bubbles are implicated in the creation of amorphous silica aggregates. There is also a steady supply of silica monomers percolating through the micro fissures of the surrounding basalt, entering the cavity to feed the growing amorphous silica aggregates until they ripen into quartz granules. This forms the first layer of a fortification band of the agate. With the continuing supply of silica monomers, new bands continue to form.

In this second part, I will discuss research from the laboratories of other scientists that supports the above conclusions. Plumes and other inclusions will be discussed along with new ideas about their formation in agate. I will suggest that these new ideas fit in with the tiny bubble theory presented here and extend it. I apologise if sometimes the text is a bit abstruse. However, I have assumed that readers of Deposits are sufficiently intelligent and motivated to try to understand (and the article gets easier the further you get). I have also included a glossary of terms at the end of this part. The first use of a defined term appears in the text in bold and italics.

Even in a dark, agate-forming cavity deep in basalt, the phase changes leave behind a mixture of varieties of quartz. Lee (2007) has reviewed research focused on identifying and understanding the fortification bands in agates, from the nanometre scale to the millimetre scale. Using measurement techniques, including one called ‘Electron Back-Scatter Diffraction’ or EBSD, he identified three distinct quartz species in agate bands, based on their different crystal morphology. In his own words, Dr David Lee distinguishes the three in the following way:

(1) Isolated equi-axial crystals ~2 micrometers in diameter in zero-solution matrix… This component is representative of cryptocrystalline silica. This component is the outermost component of the band e.g. nearest the husk.

(2) Parallel aligned fibrous elongate crystals grading from 25 – 100 micrometers length with axial ratios approximately 1:5 though showing a gradual increase in width toward to tip of the individual crystallites in the direction of growth. The fibres are elongated parallel to the growth direction of the bands; indicative of chalcedony.

(3) Larger polygonal euhedral quartz crystals up to 50 micrometers diameter. Crystals which do show a degree of elongation on a particular axis do not exhibit the strong parallel alignments evident in the chalcedony, though a number of crystals do share elongate axes sub-perpendicular to the band orientation. A number of crystals show internal changes in crystal orientation of 60º sub parallel to the banding direction; these are interpreted as dauphiné twins. The crystals exhibit curved crystal edges; 6 particularly on grain boundaries adjacent to similar larger crystals.” (Lee, 2007)

Studies by other scientists support Lee’s conclusions. Goetze, using techniques called EPR (electron paramagnetic resonance) and CL (cathodoluminescence) found that there was internal structure and zoning in agates, which differ from those seen using conventional polarising microscopy (Goetze, J. et al, 1999). In agates, this suggests a rapid growth of silica from a strongly, supersaturated solution from a non-crystalline precursor.

In another study of agates, Goetze, J. et.al (2001) measured the relative amounts of the trace element and stable isotope, oxygen-18, in agate, as compared to the amounts of this isotope in earth surface solutions. The report concluded that fluids are responsible for circulating the trace elements of the agates and the parent volcanic matrix, during and after volcanic alteration of the volcanic rock walls of the agate cavity. The oxygen-18 study supported the conclusion that the circulation of ¹⁸O enriched hydrothermal fluids originated from heated meteoric water and/or residual magmatic fluids.

That is, trace elements enter the agate cavity during its formation and in the aqueous solutions that enter later. The radioactive O¹⁸ is produced by cosmic rays entering the earth’s atmosphere and is incorporated into rain or meteoric water and, subsequently, in the silica molecules. By measuring the residual radioactivity of the O¹⁸ in the SiO₄ in the agate and comparing it to current earth surface levels, it is possible to get a measure of how long ago the SiO₄ in the agate was formed. It also provides evidence that meteoric waters percolating through the host rock carried the trace elements into the cavity rather than just being included at the time of cavity formation.

Using scanning electron microscopes, Moxon (2002) and Moxon and Reed (2006) found that the older the host rock, the bigger the size of the agate crystals and the greater the density of the agate. This implies a condensation reaction that results in the elimination of water molecules and means that the formation of wall-banded agate occurred at the same time or shortly after the formation of the host rock.

In connection with band formation in agates, Lee (2007) reviewed a number of studies of possible mechanisms and concluded that:

The crystallographic, textural and compositional relationships of silica minerals present within a banded agate have been examined using EBSD, BSE, CL and FT-IR to reveal the following:

1. The boundaries between individual bands within the agate are sharply contrasting, reflected in crystallography (degree of crystallinity, morphology), and composition (trace elements, water). This indicates formation from discrete siliceous fluid influxes.
2. Crystallographic information reveals that the degree of crystallinity and morphology of the silica minerals within the individual bands changes in a systematic fashion analogous to a diagenetic cycle. Initial cryptocrystalline silica progressively grades to fibrous chalcedonic crystals which, in turn, evolve to form larger equiaxial quartz crystals.
3. CL and BSE revealed gradual changes in luminescence and signal response across the individual bands. These probably reflect decreases in trace element (i.e. iron) concentration during band growth. These changes are coincident with changing silica mineral polymorphs present indicating a trace element control on crystal 17 formation and morphology within the host solution. This is significant in the creation and evolution of chalcedony fibres.
4. In the article some data analysis was presented as “Pole” figures from EBSD, which revealed there is a strong crystallographic control throughout all the bands in the agate irrespective of crystallinity or micro texture. This suggests crystal orientation in agate is dictated by a process independent of the silica mineral polymorph present.” (Lee, 2007)

This brief discussion on what scientists have learned about beautiful, multi-coloured, fortification agate, with all their poking, prying and prodding, shows that we are at a fascinating stage. However, researchers do not seem to have studied things that are not chalcedony. This is odd. I suspect that research on trace elements, and how they may cause the high density of defects in chalcedony’s twisted, quartz fibres, may be important. So, what do scientists say about the non-silica crystals in agates? And, what about the plume and moss-like inclusions?

From an agate’s perspective, having trace elements included in the chalcedony and having beautiful frond-like plumes or squiggly moss, which are easily seen by the unaided eye, represent two extremes of a continuum of inclusions. It is like having a continuum of quartz granules from a nanometre scale up to the familiar, six-sided crystals visible to the unaided eye. The same is true for the non-silica crystal inclusions or casts. There is a continuum of different size crystals from the microscopic, which few collectors ever see (or even look for), to those easily seen by the unaided eye, such as the gorgeous, sagenitic sprays.

So, let’s go back to the beginning and to our guiding concept:

During the formation of Lake Superior agates, the determining condition is that each agate has its own unique soup, mixture and concentration of secondary minerals, which reflects the distant and local physical and chemical environment outside the cavity and determines what mineral plumes or moss are formed in the agate cavity.

We also know that some minerals, such as feldspar, can crystallise from magma at very high temperatures, so I suggest that some agate-to-be-cavities have such crystals intruding into them when the cavity is formed. The agate then grows around them and, when the agates are freed from their host matrix, the surrounding environment quickly changes, often leading to the dissolution of the crystal, leaving a cast. (Fig. 6a shows an agate with a very large crystal cast, indicated by an arrow.) I also hypothesise that tiny bubbles can form on some crystals in the early period of agate formation, leading to fortification bands spreading outward from them (see Fig. 6a again).

So, how do the sagenitic sprays of needle-like crystals or single crystals come about? Sometimes, the unique soup and mixture of secondary minerals in the solution-filled agate cavity may be very concentrated while the concentration of silica monomer is quite low. However, the solution still contains dissolved gases, such that tiny gas bubbles begin to form on all the walls of the cavity. Fig. 6b. shows evidence for such bubbles. In the valleys between bubbles, silica concentration goes up, but it is still quite dilute. However, the concentration of secondary, non-silica minerals increases to the point where there is nucleation of what is to become a crystal.

Again, the supporting data is the photograph itself (such as Fig. 6b) and other agates like it. Note that the tiny bubbles are on the surface of a much larger, rounded surface, which provides the geometry necessary for the sagenitic spray geometry. Agate collectors are well aware that a common feature of agates with sagenitic sprays of needle-like crystals is that they burst outward from a rounded pit. Often, these crystal sprays have no banded fortification sheaths enclosing them, which suggests their formation is syngenic with the agate.

Again, the mixture and concentration of secondary minerals determines the exact species of the crystal produced. While these non-silica crystals are growing in the solution-filled, agate-to-be-cavity, the concentration of the mineral is decreasing because molecules of it are not being continually supplied by the host rock outside. I suggest that the temperature and alkalinity needed to make such minerals capable of being dissolved is no longer present. The copper-agates in Figs. 8.a to 8.c, which are from glacial till, provide the data to support this assertion. On the other hand, the concentration of silica monomer continues to increase through the pore solution, leading to the formation of the amorphous silica aggregates and the process of patterned chalcedony formation.

Plumes and moss-like structures

As noted earlier, on the one hand, there is a rich scientific literature reflecting scientific research into the mechanisms and processes of agate formation, while on the other, there is a remarkably paucity regarding the formation of plumes and moss in agates. The simplest, and a common explanation, is that chalcedony amygdules, with plumes and moss made of non-silica minerals, are not agates. However, I believe plume agates are a variety of agate and the data on them make a convincing case, including well-formed and unfractured fortification bands that are a distinguishing feature of nodular agates and are often found encompassed by plumes.

If the plumes are not densely packed, there is a zone of milky chalcedony showing depletion of mineral inclusions, which extends beyond the tips of the plumes. This indicates that, to grow, the plumes effectively leach mineral building blocks from the surrounding medium through diffusion, as in Fig. 9c.

I would suggest that plume formation in agates is on the cusp of being understood. To begin, look at a large plume, its structure and branching. Then, focus on a smaller section of plume and, again, examine its structure and branching – it appears to be duplicating the larger plume’s pattern of structure and branching. Now focus on an even smaller section of plume and again examine its structure and branching – it still appears to be duplicating the larger plume’s pattern of structure and branching. Carry this through to a microscopic level and this continues to be the case. This phenomenon is called fractals and is well understood mathematically.

However, if the plumes are densely packed, no fortification bands appear to be present, as seen in Figs. 9d and 9e. The densely packed plumes are embedded in a translucent, milky chalcedony (as in Fig. 9e) and no fortification bands appear to be generated.

Crystals, such as ice and frost, are common and beautiful examples of fractals in nature. I suggest that plume agate patterns are fractal patterns. Critical variables, which determine the plume pattern, are likely to include the following.

• The diffusion length from the mineral reservoir to an attachment site on the developing plume.
• The chemical species that is diffusing.
• The viscous forces that act on the chemical species.
• The charge carried and temperature.

This idea is backed up by several studies, for instance:

In an article in the magazine, Physics Today (Wilson, 2009), stated

Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaf like sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how?”

Similarly, Juan Manuel García-Ruiz and his post-doctoral student, Emilio Melero-García (both at the University of Granada), and Stephen Hyde (Australian National University) propose a mechanism for the morphogenesis:

As the carbonate crystallizes, it lowers the pH of the local environment and triggers the precipitation of silica. The silica precipitation, in turn, raises the local pH, which prompts another round of carbonate formation. The sensitivity of silica and carbonate species to opposite trends in pH fluctuation creates a chemical feedback that eventually produces rod-like, carbonate particles, each coated with silica. Freed from the hexagonal symmetry restrictions imposed by carbonate growth, the silica-coated nanoparticles form clusters that can adopt various shapes. For reasons unexplained by their mechanism, the clusters align themselves on the micron scale and grow as two-dimensional sheets. The edges of those sheets can then curl like a scroll to create the sort of curved and twisted filaments captured here by optical microscopy.” (Garcia-Ruiz, et al. 2009)

It is clear that research focused on understanding self-organizing patterns and structures is a hot research topic in the fields of chemistry, physics, biology and the study of evolution.

The following is a quote from Garcia-Ruiz:

The precipitation of barium or strontium carbonates in alkalinesilica-rich environments leads to crystalline aggregates that have been named silica/carbonate biomorphs because their morphologyresembles that of primitive organisms. These aggregates areself-assembled materials of purely inorganic origin, with anamorphous phase of silica intimately intertwined with a carbonatenanocrystalline phase. We propose a mechanism that explainsall the morphologies described for biomorphs. Chemically coupledcoprecipitation of carbonate and silica leads to fibrillationof the growing front and to laminar structures that experiencecurling at their growing rim. These curls propagate in a surf-likeway along the rim of the laminas. We show that all observedmorphologies with smoothly varying positive or negative Gaussiancurvatures can be explained by the combined growth of counter-propagatingcurls and growing laminas.” (Garcia-Ruiz, 2009)

For our purposes the key ideas are:

1. the chemically coupled co-precipitation of carbonate and silica, which sets up an oscillatory pattern;
2. the self-alignment of the nano-particles by an, as yet, unknown mechanism.

One might hypothesise that there is a net charge separation on the rod-like nano-particle, giving it an electric dipole nature. The patterns of dipole electric and magnetic (EM) fields are well understood and can be described mathematically with great precision. Based on the above and similar works by several other authors, I suggest that plumes and moss in agates are a result of the formation of nano-scale aggregates of coloured minerals in the silica solution. Again, following Juan Manuel García-Ruiz’s arguments above, I believe that in the developing agate, the aggregates become encased in a thin silica skin. When the concentration of these aggregates reaches a critical point, they begin to self-organise. I further hypothesise that, in any single agate system, the nano-aggregates are all of the same geometric shape: nano-rods, nano-spheres, nano-discs and so on.

In an assemblage, these different shapes have different symmetry restrictions for close packing, for example, hexagonal sheets for spheres, hexagonal sheets or columnar stacks for discs, and so on. Then, in larger arrays, the fringing electric fields may cause the fractal patterns. In this model, the pattern of self-organisation is determined by the geometric shape of the nano-aggregates and their associated EM fields, which gives it broad application.

Based on the discussion above and related work by other scientists, I suggest that, not only are plumes and moss structures in agate a result of nano-scale aggregates of coloured minerals with a very thin silica sheath, but also that silica sheaths provide a bridge to the chalcedony of the agate. Going one step further, I suggest that the EM fields of the various shaped, nano-particle aggregates become the building blocks for forming the banded agate’s fortification. The symmetry restrictions for different geometrically shaped nano-aggregates and the fringing electric fields at the ever-changing edges of arrays of aggregates may help set the patterns of the numerous arcs and vertices seen in fortifications bands of agate.

In Part 3, I will focus on non-silica minerals and colours in agates. The non-silica minerals include iron minerals, copper minerals, native copper, native silver, calcium minerals and others, such as titanium, mostly in trace amounts. Also considered will be the ‘iris effect’ in agate, which provides direct information on the regularity of the structural nature of the fibrous chalcedony, with consequences for theories of agate formation.

Glossary
Amorphous silica aggregates are considered to be a polymerisation of Si(OH)4 molecules to form clumps of SiO2 (silica) along with water that is both present from the initial solution and from the condensation reaction that transforms the Si(OH)4 to SiO2.
Banded fortification sheaths are the successive fortification bands that are seen surrounding some needle-like inclusions in agates.
Botryoida: refers to the cauliflower shaped, three-dimensional texture seen in some agates.
Chalcedony is a variety of quartz that is microcrystalline. Some chalcedony in agate is fibrous. This is caused when the tiny quartz fragments are stacked up, which gives them a thread-like appearance.
Chalcedony amygdule is the name given to a cavity in hardened lava, which has been filled with the variety of quartz called chalcedony.
Condensation reaction is a chemical reaction in which molecular water is produced, often by splitting off hydroxyl groups from the parent molecule.
Diffusion, in our context, is the process whereby molecules of a substance move in a random path from a region of high concentration to a region of lower concentration. Think about walking across a street, which is filled by people jostling about – you don’t walk a straight line to get across the street and you are also jostled about.
Electric dipole nature is the strength of the electric dipole and the geometric shape of the electric field it produces. An ideal electric dipole is a composed of a positive charge (+q) and an equal size negative charge (–q) separated by some distance ‘d’. The electric field caused by such a combination of electric charge has a particular geometric pattern, which is the same as that seen around a bar magnet. It can also be described mathematically.
Fortification band is the common pattern in agates, which consists of concentric bands of varying colour and width that reminds people of crenulations in a fortress wall. Such a band has three structurally different parts. The first layer formed consists of tiny quartz fragments or crystallites (nanometre to tens of nanometres in size) where the crystal axes are essentially indistinguishable. The second layer is the chalcedony layer composed of stacks of quartz crystal fragments, from hundreds of nanometres to micrometer in size, where the crystal axes are distinguishable. The final layer of a fortification band is the coarse or euhedral quartz, which is tens of micrometers to centimetres in size and have the “c-crystal” axis roughly perpendicular to the fortification band.
Inclusions in an agate are aggregates of a non-silica compound. They include non-silica crystals that are usually needle-like or miniature fern-like tendrils, or resemble moss that is twisted, looped and thread-like.
Meteoric water is water in the ground or percolating through the ground, which originates from precipitation – rain, snow, sleet and hail. Nano-aggregate is a collection of tiny, nanometre-size particles, loosely clumped together.
Nano-particle is a particle, such as a molecule or group of molecules, which is about one nanometer in the longest dimension.
Net charge separation is the condition where electrical charges appear or, as physicists say, ‘are induced’ on an uncharged object, because of a nearby charged object. The net charge on the object is still zero. A rod-like aggregate with net charge separation arising through induction acts like an electric dipole.
Nucleation, particularly heterogeneous nucleation, is the process where a foreign particle or structure acts as a scaffold for a crystal to grow on. It eliminates need to create a new surface for crystal growth and the associated surface energy requirements.
Patterned chalcedony is chalcedony that forms in repeating bands, arc, swirls, and lines of various colours in agates. While the colours are understood to result from different non-silica molecules such as iron oxide, the process for the formation of repeating bands, arc, swirls and lines is not understood. Plumes in agates refer to the tree-like or fern-like growths of branched non-silica minerals surrounded by the chalcedony of the agate.
Residual magmatic fluids refers to that portion of the solution and gas in an agate-forming cavity that is a leftover or a residue from the original exposure of the magma to earth surface or ocean bottom conditions.
Rutile is titanium dioxide, or TiO2, which is a naturally occurring mineral. In agates, it is often found as needle-like crystals.
Sagenitic sprays refer to the needle-like or acicular inclusions in agate, which are often in a radiating pattern.
Silica monomers are the tetra-hedral molecules described by the equation Si(OH)4.
Syngenic means two geological processes or events occurring at the same time.
Wall-banded agate refers to agate in which the fortification bands repeatedly mimic the shape of the cavity in which the agate forms. If the cavity is spherical, the wall-banding fortifications in three-dimensions are a series of concentric shells of chalcedony, or in cross-section, a series of concentric circles.

Other articles in this series:
Tiny bubble theory of Lake Superior agate formation (Part 1): Tiny bubbles, the husk and silica accumulation in the cavity
Tiny bubble theory of Lake Superior agate formation (Part 2): Results from the laboratories of other scientists and inclusions in Lake Superior agates such as crystals and plumes
Tiny bubble theory of Lake Superior agate formation (Part 3): Non-silica minerals and colours in agates
Tiny bubble theory of Lake Superior agate formation (Part 4): Tiny bubbles and reflections on theories of agate formation