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Sodalite-Syenite

Sodalite-Syenite, Brasil

Image: K. Sieber, www.makrogalerie.de

This blue and white speckled stone, which is very popular as a gemrock, has been mined in Brazil since 1965. The name "Azul Bahia", which is commonly used in the stonemasonry industry, refers to this origin.

Petrologically, it is a syenite rock with approx. 50% microclinic feldspar, 30% sodalite, 10% cancrinite, 5% plagioclase as well as 5% aegirine and other dark components. Syenites are magmatic alkaline rocks low in quartz, but often with an exotic mineral composition. Their main mineral composition usually comprises alkali feldspars, amphibole- and/or pyroxenes-minerals or plagioclase feldspars and biotite. In addition, rare minerals such as astrophyllite, eudialyte or (as in in the case of sodalite syenite) sodalite occur in larger quantities.

The mineral Sodalite is closely related to an other blue mineral: lasurite, one of the coloring minerals of Lapis Lazuli rock. Contact with (even weak) acids should be avoited, as it can turn grey under their influence.

Sodalite-syenite is used as a decorative stone (gemrock) in the jewellery industry, where it is also known as "Sodalite" abbreviated.

 

 

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»Sonora Sunrise«, Mexico

»Sonora Sunrise«, Mexico

Foto: K. Sieber, www.makrogalerie.de

This colorful stone comes from the Milpillas Copper Mine west of the small town Cananea in the North Mexican Sonoran Desert. The mine is small and produces not very much material. As a consequence the stones achieve high prices ever since their market launch in 2006.

Depending on the mineral composition the color varies from brownish red with olive-green spots, to black areas with a metallic luster, together with blue and green areas, grown closely together like a patchwork. Using X-ray diffraction analysis (EDX) the green-blue zones could be identified as an intergrowth of  brochantit (a copper sulfide) with the green copper silicate chrysocolla. The brownish to orange-red areas consist of the copper oxide cuprite grown together with another copper oxide: tenorite.

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Mariposite-Dolomite

Mariposite-Dolomite

Foto: K. Sieber, www.makrogalerie.de

»Mariposite dolomite« is a white stone with green striationss. It occurs in different places in the Sierra Nevada in California (USA), for example around Coulterville in Mariposa County. Benjamin Silliman Jr. named the stone after this region in 1868. Its occurrence is linked to a contact zone of argillaceous schists and ultrabasic rocks.  Therefore dolomite and quartz coexist together with green phyllosilicates. These silicates are in a large part mariposite (a chrome-muscovite mica) but sometimes also of nickeliferous chlorite.

Minerals of the mica group are classified in white micas and dark micas depending on their appearance. Muscovite is a common member of the white mica group. The pure mineral is colorless and transparent. Variations in its chemical composition modify its physical properties (e.g. flexibility, color). Such varieties bear their own names like »Fuchsite« for a chrome mica or »Phengite« for SiO2 rich mica and »Mariposite« which is rich in chrome and SiO2. Sometimes Mariposite is also called »Chromium phengite« because of it's high content of chrome and SiO2.

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Fluorite-Opal-Jasper (»Tiffany Stone«)

finely dispersed fluorite gives this Fluorite-Opal-Jasper (»Tiffany Stone«) it's purple color

Foto: K. Sieber, www.makrogalerie.de

At the beginning of the millenium many tumbled stones we sold under the trade names »Tiffany Stone« and "Fluorite Opal".

This beige rock with violet veins, which is known in the USA as "Tiffany Stone" is provided by a large number of small deposits in southwestern Utha (USA), which came to be known as "Spore Mountain Lavender Mines". In Europe tumbled stones were sold under the name "Fluorite Opal".

The rock owes its formation to the activity of volcanoes, which deposited in several eruptions large quantities of silica-rich ash layers in an area dominated by limestones. Between the sporadic eruptions quartz-rich sediments were gradually solidified and transformed. Fluorine-containing gases led to an impregnation of the sediments with fluorite. In an older period mainly quartz, quartzite, jasper, agate and flint were formed along the contact to the underlying limestone. Manganese ores associated with the limestone, were incorporated and form brown to black, often metallically shiny veins in the rock.

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Maw-Sit-Sit, Burma

The macro photography (size 3 x 4 cm) of Maw-Sit-Sit shows the finely matted structure of this jadeite-containing rock.

Foto: K. Sieber, www.makrogalerie.de

Maw-sit-site is a rock that consists of six main components that can vary significantly in quantity. The appearance of individual specimens can vary noticeably. This name is derived from a local name of its provenance near the town of Tawmaw in Burma (Myanmar) [Gübelin, 1978].

The rock consists of a groundmass of light-colored aggregates (mainly albite, but also zeolites and chlorite) in composition with numerous dark minerals, especially minerals of the jadeite-cosmochlor solid solution series of intense green color. These jadeite minerals can form single crystals or a blocky mosaic. In between, there are always black nests of fine felted cosmochlore (NaCrSi206), a chromium analogue of jadeite (NaAlSi206).