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Bronzite-Pyroxenite

»Bronzite-Pyroxenite« rock with grey magnetite inclusions

Picture: K. Sieber, www.makrogalerie.de

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»Bronzite-Pyroxenite«

In modern mineralogy, "bronzite" is no longer regarded as a mineral in its own right, but as an iron-bearing variety of the mineral enstatite (so-called "Ferro-Enstatite", abbreviated: "Fe-Enstatite", (Mg,Fe)2[Si2O6],). Fe-Enstatite of the variety Bronzite contains 5 - 15% iron (in relation to magnesium) and belongs to the mineral group of pyroxenes. This group comprises single chain silicates with the basic component [Si2O6].

In most cases Fe-Enstatite is a rather unimpressive mineral. In its earthy, brownish to almost black colors it is neither rare nor valuable. But as soon as it appears as shiny golden flakes and bars, it attracts the interest of mineral dealers and collectors.

The golden-yellow shining "bronzite", which is available as tumbled stones, does not consist solely of the mineral Ferro-Enstatite, but also of other minerals. It is a rock in which Fe-Enstatite is the main component in company with black magnetite and colorless quartz. For this reason, the hardness of this rock varies between 5 to 6 (bronzite) and 7 (quartz). The density of the rock is about 3.2 - 3.4.

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The blue and pink varieties of the carbonate minerals calcite and aragonite are often mistaken as the silicate mineral hemimorphite. They are offered as rough or tumbled stones and polished strands. With a few simple tests it is possible to separate genuine Hemimorphite and its simulants.

Hemimorphite:

Genuine Hemimorphite is a relatively rare mineral. It forms in the oxidation zone of sulfidic lead-zinc deposits. On the basis of its physical and chemical properties Hemimorphite can be distinguished from its imitations without difficulty.

Chemical composition:
Hemimorphite is a water-bearing, alkaline zinc silicate with the chemical formula Zn4[(OH)2/Si2O7] · H2O. It does not react with acids unlike its most common imitations from the Calcite group. Dabbing it with 10% hydrochloric acid does not cause any gas bubbles.

Appearance:
Typical aggregates are grained, crusty, kidney-shaped, fibrous and radiating. Crystals which have different surfaces at one end than at the other end (called "Hemimorphie") are rare.

Color(s):
Hemimorphite usually occurs in white or brownish, rarely in light blue or pink colors.

Density: 3,30 - 3,50

Mohs hardness: 5

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The best known deposit of »Thulite«, the pink variety of the mineral Zoisite, is 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. 

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Lemon Magnesite

»Lemon Magnesite« with a small quartz vein

Foto: K. Sieber, www.makrogalerie.de

A special variation of nickelous Magnesite is traded under the deceptive name "Lemon Chrysoprase". The material comes from the Eastern Goldfields area of Western Australia. The rock is found as nodules and veins in a strongly weathered serpentinite in a belt of ultramafic rocks that stretch from  Norseman in the south to near Wiluna in the north. For this reason, the raw pieces often show an outer edge of heavily corroded, brownish serpentine that has weathered to laterite. Now and then narrow, green, partly translucent areas of chrysoprase or green opal are next to it. However, the largest part of this rock consists of a greenish-yellow, opaque magnesite, which is at best interspersed with translucent, colorless veins of quartz. Studies conducted by the Gemmological Institutes of America (GIA) showed that these veins of quartz are indeed colorless and can not be named "Chrysoprase".