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.

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).

Due to the replacement of Al by Cr, maw-sit-sit contains wide sequences of a solid solution between cosmochlore and jadeite. In the centre of the cosmochlore nests metallically shiny chromite can be found in form of small residual grains. In addition, coarser crystals of grey to green colored amphibole or chrome-amphibole were formed. In the sixties and seventies of the last century this rock was also called "Jadealbit". However, since maw-sit-sit has been increasingly offered, hardly any albite can be found. As a consequence this appellation is considered obsolete.

Piemontite Quartzite rock, Norway

Image: K. Sieber, www.makrogalerie.de

The best known deposit of »Thulite«, the pink variety of the mineral Zoisite, lies in Norway in the region of Lom. Lom is also the location of piemontite-quartzite which looks very similar to »Thulite« and can easily be mistaken for it.

»Thulite« and Piemontite are Calcium Aluminium Silicates with the same chemical formula Ca₂Al₃(SiO₄)₃(OH) (+ Mn, Fe), which crystallize in different crystal systems: »Thulite« (zoisite) in the orthorhombic crystal system, piemontite (klinozoisite) in the monoclin crystal system. When aluminium (Al) is substituted by manganese (Mn), both modifications show pale reddish to strong pink colors. 

The deposit is located in a zone of gneisses of sedimentary origin (paragneisses) of the so-called "Western Gneiss Region", in which numerous inhomogeneities such as quartz lenses and amphibolites occur. Embedded in this is a layer of about 3 x 250 m of manganese-rich minerals such as piemontite and thulite, with varying proportions of quartz, muscovite mica, tremolite and calcite.

Fig. 1: »Pinolite« - The resemblance to pine nuts gave this magnesite rock its name.

Photo: K. Sieber, www.makrogalerie.de

The Sunk/Hohentauern deposit in Styria (Austria) is known worldwide for a rock forming magnesite with a typical coarsely crystalline growth structure which resemble pine nuts or ice flowers in shape. Because of this special texture the rock was named »Pinolite« (pignolia = pine nut) or "ice flower magnesite". Large, bright magnesite crystals are embedded in a gray, fine-grained matrix rich in inorganic pigment.

Until 1963 the rock was mined as decorative rock and marketed under the name "Sunk" (the name of the finding place). Today it is found almost exclusively in arts and crafts objects or as tumbled stones.

»Rainbow Basalt«, USA

Foto: K. Sieber, www.makrogalerie.de

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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.

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.

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 with a very low content of quartz, but often enriched with exotic minerals. 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.

»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.