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Lavendeljade (Jadeit - Metagranitoid), Türkei

»Lavender-Jade« (Jadeite - Metagranitoide), Turkey

Foto: K. Sieber, www.makrogalerie.de

The world-famous deposit for this lilac-colored rock is located near Harmancik, about 90 km south of Bursa in Turkey. The deposit covers an approx. 400m thick jadeite-bearing metamorphous rock embedded between mica schists and marbles. It was formed about 80 million years ago from an originally granitic rock (granitoid) by high-pressure metamorphism. The resulting metagranitoid rock consists of quartz, chlororitoid, lawsonite, glaucophane and phengite as well as up to 50% jadeite.

Jadeite occurs in this rock not only in the violet color of »Lavender Jade«. To a small extent you can find it there also in grey, white and green. The violet color variety developed during the metamorphosis process by the circulation of manganese-containing aqueous solutions.

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»Rainbow Basalt«, USA

»Rainbow Basalt«, USA

Foto: K. Sieber, www.makrogalerie.de

A dark basalt from Michigan (USA) with roundish cavities, filled with colorful minerals has been given the trade name »Rainbow Basalt«.

Basaltic rocks are formed wherever thin, low-silica magma emerges at the earth's surface in volcanically active zones and cools down relatively quickly to basalt lava. Beside very fine-grained plagioclase feldspars and pyroxene minerals, olivine and magnetite may also be present, as well as an uncrystallized glassy component. Sometimes basalt lavas are  quite rich in gaseous components. Therefore roundish cavities are frequently found.

The analysis of some thin sections of the »Rainbow Basalt« showed highly weathered minerals. The initial cavities are now filled with partially complex mineralizations.

Besides feldspar we found calcite and small quartz crystals. To some extent the feldspar is decomposed into clay minerals. In addition, zeolite minerals have been found.

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Oceanchalcedony, Madagascar

Round structures called spherulites are the typical attribute of »Oceanchalcedony«. If they are not present »Ocean chalcedony« can not be recognized as such

Foto: K. Sieber, www.makrogalerie.de

Sphaerolites in Oceanchalcedony

Radiating chalcedony cluster can also connect to agate-like structures

Foto: K. Sieber, www.makrogalerie.de

»Ocean chalcedony« has been known in Europe for a couple of years now. Sometimes the stone is eye-catchingly colorful. It originates in the Mahajanga Basin at the northwest coast of Madagascar. In the Cretaceous period (65 - 130 million years ago) this basin was still a shallow sea basin. About 100 million years ago a tectonic uplift combined with volcanic activities lead to the penetration of hydrothermal liquids rich in quartz into the calcareous and clay sediments. This led to the crystallisation of chalcedony in form of layers and veins.

The primary deposits close to the coast are largely depleted by now. Alternative material from the vicinity usually doesn't have the typical characteristics of »Ocean chalcedony« but contains more jasper and country rock.

The trade name for this variety of chalcedony has changed frequently in the past. Initially the material was offered as "Ocean jasper". This name is still in use in anglophone regions, but it is deprecated. Who ever examines the stone with the loupe, will recognize agate-like structures. In Europe the material was therefore marketed as »Ocean Agate«.

Dr. W. Lieber from Germany microscopically analyzed the stone by means of thin sections. He discovered that the material consists completely of chalcedony that has partially crystallized into radiating, spherulitic structures. Since such structures are called "Spherulites" in mineralogy Lieber named the material »spherulitic chalcedony«.

These round or radiating structures constitute the decisive feature of this kind of chalcedony. They are often embedded into a diversified structured matrix.

Once the mineralogical composition was clarified the trade name »Ocean Chalcedony« became more and more popular. The name »Ocean Agate« can still be used as there are areas with agate every now and then between the spherulitic structures. The name "Ocean Jasper", however, should be avoided. This gemrock does not have a lot in common with jasper. The most accurate way to reference this material is the description »Ocean chalcedony (Ocean agate)«.

For the recognition of a chalcedony as »Ocean Chalcedony (Ocean Agate)« it is essential, that the typical spherulitic structures are clearly present. Only material with spherulites should be labeled »Ocean chalcedony«.

 

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Shattuckite-Chrysocolla-Quartz, Kongo

Shattuckite-Chrysocolla-Quartz, Kongo

Foto: K. Sieber, www.makrogalerie.de

The copper deposit Tantara in Congo is known for its very nice specimens of plancheite and shattuckite with dioptase and white calcite. It also provides a rock composed of Shattuckite, Quartz, Chrysocolla, Azurite, Malachite, Bornite and other copper ores, sold simply as "Shattuckite".

The dark blue copper mineral shattuckite usually occurs as needles or fibrous crystals in radiating or massive mineral aggregats. It is formed as a secondary mineralization in the oxidation zone of copper deposits. With a Mohs hardness of 3½ it is too soft for jewelry.

However, embedded in quartz and together with other copper minerals like azure chrysocolla and/or grass-green malachite it makes up a mineral composition that takes a good polish and is not only pleasant to look at but also an attractive collector's stone.

 

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»Llanite Rhyolite«, Madagaskar

»Llanite Rhyolite«, Madagaskar

Foto: K. Sieber, www.makrogalerie.de

A colorful rock with roundish to oval domains of shimmering bluish colors, is marketed as »Llanite«, »Llianit« or »Llanoithe« or simply as "Porphyrite" and "Volcanite". The latter term gives an idea about its formation ("it is from volcanic origin") but it does not specify what kind of rock it is - there are many different volcanic rocks out there. The term "Porphyrite" is an outdated name for a class of volcanic rocks, called "rhyolite" in modern time. The term »Llanite« is based on the place of discovery in Llano County, Texas, USA. Known occurrences of rhyolithic rocks with this specific look are Brazil, Madagascar and the USA.

The samples we have examined in the EPI-Lab, were reported to come from Madagascar. We identified a slightly metamorphosed (alkalifeldspar-) rhyolite with exceptionally large crystals, embedded in a fine grained groundmass. Orthoclase feldspar crystals up to 10mm / 0.4in in size and large quartz and plagioclase feldspar crystals up to 5mm / 0.2in are visible to the naked eye. The analyses of various thin sections revealed a well crystallized groundmass. It consists of quartz, mica and feldspar very rich in inclusions, with no glassy component.