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Ocean Jasper ~ Spherulitic Chalcedony

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Spherulitic Chalcedony of Madagascar

(“Sphärolithischer Chalcedon von Madagaskar”)

 

Werner Lieber, Heidelberg

Lapis, 28(9), p18-22, September 2003

(Translated from original German to English by Susanne Lomatch, February 2019)

 

A few years ago, on a mineral exchange, ground stones of 3-5 cm diameter were cut, which were also offered as so-called "flattering stones". On its surface, more or less numerous circle figures appeared. In a report by Neumeier & Weise (2000) there is a photograph of such a stone - there called "eye agate". The offering dealer described the material as "orbicular jasper". The search for larger slices and especially for larger pieces of raw material was initially unsuccessful until two sources could be found on the occasion of the Munich Mineral Days 2002.

 

On the Northwest Coast of Madagascar

 

The pieces ran under the name "Ocean Agate" and came from Madagascar. The occurrence is said to be on the northwest coast of the island and belong to the Mahajunga region. After Neumeier & Weise some stones from there were already offered in the fifties. The site is open on an area of ​​45 x 27 meters, but only accessible at low tide. The locality on the coast was occasion for the naming.

 

However, in May 2003 Dr. Federico Pezzotta (Milan) kindly informed us that the occurrence of the "diaspro orbiculare" stretches for several tens of square kilometers along the entire Analalava peninsula north of the provincial town of Mahajunga [sic]* (Majunga after Lacroix 1922). These are horizontal lenticular discordant zones (breccias?) in which the "jasper" hydrothermally deposited with finely divided green celadonite and yellow to brown Fe/Mn hydroxides during Tertiary volcanic activity**. However, exact geological and mineralogical studies of the occurrence are missing.

 

The beautiful appearance, the extraordinary texture and the varied textures of the stones suggested a closer description and an attempt to interpret their origin. The entirety of the material is chalcedony, with crypto-crystalline quartz as a base, in which different sized "eyes" are more or less numerously embedded. They were the first interest.

 

Chalcedony Beads - Thousands!

 

Very quickly it became apparent that all "eyes" represent sections through spherical structures. The balls - also made of chalcedony - occur in more or less large associations of hundreds, even thousands of individuals, with almost all members of the association have virtually the same diameter. Apparent deviations to smaller sizes are caused by the cutting positions by the balls. The further the cut away from the center, the smaller the diameter becomes. At dressings with the smallest balls one measures approximately 1 mm, at those with the largest approximately 8 mm diameter. Now and then the balls are not (more) firmly connected to the ground so that they break when breaking a disc or a blank, occasionally also when cutting or polishing. In smaller or larger areas of 5-15 cm in length, one can recognize associations of different sized beads, which are superimposed or drifted past each other and thereby it is at the margins somewhat mixed.

 

Cuts through the centers of the spheres show the most details. It should always be remembered that e.g. a thin circle around the center of the intersection of a thin layer, a shell, around the center of the sphere. Accordingly, this means

 

Figure 1:

Complex chalcedony spherulites surrounded by a variety of greenish globules only 0.6 mm in size. The large spherulite on the right seems to have been cut in the middle or the most part. Picture height 2 cm.

 

All photos of this article were made by Werner Lieber.

 

Author's address and references can be found on page 58

 

Translator Notes:

*Actual proper name is Mahajanga, a port city on the north coast of Madagascar.

**Tertiary volcanic period was ~65M-1.8M years ago, or in current terminology, Paleogene/Neogene period 66M-2.58M years ago.

 

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(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

a circular band of a more or less thick, spherical shell around the center. Neighboring spheres lie with their midpoint above or below the cutting plane; they were thus cut more or less far from the center, whereby the innermost core was no longer hit. The red dots are made of lepidocrocite (or hematite?) like the heliotrope. These baubles were present during the formation of the balls and are usually statistically distributed; rarely one sees spherical (in the average circular) enrichments.

 

Spherical crystal aggregates - spherulites (= globular stones) - are not uncommon in the mineral kingdom. They begin their formation at a central location.

 

Right: Polished chalcedony plate with spherulite bandage. Picture width 14 cm.

 

Below: Exposed chalcedony balls and cavities of fallen out balls (8 mm in diameter).

 

Above: This section gives a view into the interior of the preparation; the surfaces of lower-lying spheres show crystal peaks image width 1 cm.

 

"Dot", which may consist of one or many seeds, but also one or more tiny specks of dust. Chalcedony consists - as shown by X-ray analysis - of (low) quartz. By attaching material to the seed structure on all sides, a radial-spherical aggregate of finest chalcedony fibers develops, which are oriented perpendicular to the respective spherical surface. From the agate is known that it can be easily soaked with coloring solutions due to its microporosity. It is not surprising, then, that the chalcedony of this deposit - both the matrix

 

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(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

Left: Three intermeshed spheres, two of which were cut in the middle; red dots are lepidocrocite (?). Image width 7 mm. Middle: Countless small beads (~ 0.6 mm) "swim" in chalcedony - like sago in the soup! Picture width 1 cm. Right: Large spherulites of coarsely crystalline quartz and a very small nucleus; in movements due to flow processes, the bulky aggregates obstructed; while some crystal tips broke off. Picture width 2 cm.

 

Left: Greenish fibers as a covering of a spherulite turn into white and finally into coarser quartz crystals. Image width 6 mm. Right: Abrupt transitions from fine-grained spherulites (4 mm in diameter) to crystalline quartz with end faces. Image width 1.8 cm.

 

as well as the spherulite - have been dyed in a natural way.

 

Many-Colored Spherulite

 

The coloring substances are either randomly incorporated into the crystal lattice of the host crystals or incorporated as solid inclusions. Here the latter should have happened: Yellow, ocher, brownish. reddish and greenish colors indicate iron compounds: different color depth indicates concentration fluctuations. On the other hand, it is known that foreign particles can be pushed away from growing crystals: under appropriate circumstances, this creates color zones.

With the increasing magnification of the spheres a practically complete space filling occurred: They are free of visible spaces. For this small consideration were several spherical pieces and some plate cuts available, and there was shown that an incredible variety of colors and color combinations of chalcedony is given. The perfect spherical shape of the spherulites can be recognized on pieces whose matrix is ​​at least well-translucent or on fracture surfaces, which are not infrequently formed from the surfaces of juxtaposed spheres, but above all at individually broken-off spheres. In all cases, the adhesion between matrix and balls was not sufficient for breakage by the balls themselves.

 

From the Spherulite to the Quartz Crystal

 

For many spherulites their development is finished with the formation of dense spheres; in others, the transition from the cryptocrystalline (hidden crystalline) to the macrocrystalline state begins. Occasionally, a fine-fibrous shell forms around the spherulites, which gradually turns into radiant crystals.

 

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(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

"In the River"

 

Above: flow forms of lepidocrocite dots and linearly oriented spherulites. Picture width 3 cm.

 

Above: Chalcedony plate with several associations of various spherulites; These flowed over each other and mixed in the border reports. Picture width 7 cm.

 

Left: Thin section of a spherulite with a fibrous cover between crossed polarizers; in contrast to the granular core, the growths are crystalline and oriented. Picture width 5 cm.

 

Left: Flow figures and orientation of the balls in the flow direction. Image width 1.5 cm. Right: Damaged spherulite in a white chalcedony band indicating flow before the silica sol hardened. Picture width 6 cm.

 

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(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

From this, quartz crystals finally develop with end faces that are embedded in the matrix or grow into small open spaces. In the thin section you can follow the transition well. The quartz crystals show different interference colors between crossed polarizers depending on their thickness and orientation. The transition from the cryptocrystalline state to macrocrystalline crystals can also be abrupt. It may also be the shrinkage of the matrix as a result of loss of water and the associated formation of cavities plays a role. These, by the way, provide revealing insights into the surfaces of the spheres.

 

So far, the preferred spherulites have been considered. The matrix in which the ball bindings are contained is rarely colorless, but usually only in a thin layer translucent greenish, yellowish, gray or white. The color impression is determined by the spherulite clusters. This is not surprising, because they make up the overwhelming majority of all chalcedony, which is made up of siliceous solutions. The matrix, the matrix in which the spherulites "swim", often consists of very small, milky spheres of 0.6 mm in diameter. From their microscopic observation it can be seen that they consist of countless tiny, white granules, which are arranged only indistinctly and sporadically radially from the center to the outside. Between them red spots of - probably - lepidocrocite can be observed. Their statistical distribution gives the impression of a delicate pink color of the beads. Even with other spherulites can be seen under the microscope their granular texture. From a few samples thin sections were made, which show this even more clearly. Both between parallel and even very crossed Nicols***, the tested spherulites appear dark and do not show the radial ray texture, such as e.g. can be observed in agate (Landmesser 1987, Lieber & Kassautzki 1987).

 

Silicic acid tends to form colloidal solutions (Greek: kolla = glue). While in real solutions the solutes are considered as single ions. Atoms or molecules are present, the colloidal state is characterized by much larger particles, which arise by agglomeration of hundreds, even thousands of individual particles. Nevertheless, a colloidal solution remains transparent. With the smallness of the atoms and molecules, the thousand-fold means only particles of a hundredth of a thousandth of a millimeter. They are still so tiny that they can easily wander through the fine pores and channels of rocks.

 

Agate in Nodules

 

If colloidal particles aggregate to form even larger aggregates, or if they orient themselves in a certain way, then the colloidal solution, the sol, can form a gelatinous jelly (gel) which, although it is still mostly clear, has a tangible consistency. In the case of agates, it is observed that chalcedony spherulites prefer to grow on the walls of the agate nodules or at least settle there. Landmesser (1987) has impressively explained how the agate banding results from the neighborly assembly of spherulites. The fact that the spherical formations first appear on the walls of the nodules becomes obvious from the position of the crystallization centers. Only rarely are agates, whose interior is more or less full of space piled with complete spherulites. Schaefer (2002) has recently found some specimens in the Freisener area and has shown in its publication.

 

Spherulites on Rock Columns

 

The fact that exclusively complete spherulites appear in a chalcedony deposit is to the knowledge of the author a hitherto unobserved case. Unfortunately, a detailed description of the occurrence and geological situation at the site is missing. However, the available evidence indicates that it is one or more rock zones that are limited in their extent (see page 18). Obviously, there are no agates there (i.e., banded chalcedony filled former gas bubbles or rock cavities). Rather, silica-rich colloidal solutions (sols) with a certain viscosity appear to have moved through rock gaps. Many samples show distinct flow patterns on both the spherulite bandages and the red dot-like lepidocrocite particles. But there are other indications of flow processes, which are shown on the attached pictures.

 

It is easy to imagine that the solutions flowing on and in the rock are much more likely to pick up and transport small particles and soluble constituents than would be possible within a nodule. The variety of colors indicates changing circumstances, certainly also on several thrusts of solutions. Furthermore, it is not surprising that germs surrounded on all sides by "mother solution" grow more easily into spheres than those in a closed space (an agate-nodule) remaining in relative calm at one point. The resulting spherulites with "smooth" outer shells are certainly slightly displaced against each other, so easy to move with the solution. Only when a transition to the coarsely crystalline structure has occurred here and there, the quartz crystals will interlock and prevent further movements.

 

Another note on naming: As I said, it is not agate because of missing banding. Jasper is an opaque, intensely colored chalcedony (Klockmann-Ramdohr-Strunz 1978), which may contain large amounts of minor components (up to 20%, Webster 1970). Other varieties are not available. In order to take into account the peculiarity of the prevailing and complete spherulites, one can call the material "spherulitic chalcedony". All other terms are mineralogically incorrect, unnecessary and superfluous.

 

Translator Notes:

***Crossed polarizations.

 

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(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

References:

 

Landmesser, M. (1987): The Mystery Agate. Current Geo-Information, Trade Fair Catalog of the Munich Mineral Days (esp. P. 70).

 

Klockmann-Rambohr-Strunz (1978): Textbook of Mineralogy (16th ed.). Gerd. Enke Verlag Stuttgart, 876 pages (especially p. 526).

 

Lieber, W. (1987): Agate and its Relations. Current Geo-Info, Exhibition Catalog of the Munich Mineral Days (esp. P. 11).

 

Lieber, W. & Kassautzki, M. (1987): Traces in Agate. Current Geo-Info, Exhibition Catalog of the Munich Mineral Days (p. 13 + 14).

 

Neumeier & Weise, C. (2000): Report on the Tucson Show 2000. Lapis 25 (5) (page 41).

 

Schäfer, K. (2002): The agates of the Freisener Heights. Lapis 27 (6) pp. 13-21.

 

Webster, R. (1970): Gems. Butterworth & Co. Ltd. London, 836 pages (especially p. 176-185).

 

(© 2003 W. Lieber/Lapis, Translated from original German text by S. Lomatch, 2019)

 

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