s
Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

GOETHITE, GROUP: GOETHITE, LEPIDOCROITE


GOETHITE group (Goethit—Goethite—ø) (Becher, 1789, in Lenz, J.G. 1806), after Johann Wolfgang von Goethe (1749-1832), a celebrated Germ. poet and scientist. Initially, goethite was called onegit (Andre, 1802), and goethite – lepidocrocite. Here also lepidocrocite is described, which forms an isomorphic row with goethite.

Composition & Properties. Hydroxide – α-FeOOH, orthorhombic system. Hardness 5-5.5. Density 4.3. Half-metallic luster to diamond one, in aggregates – silk or dull one. In crystals cleavage in two directions. Fragile. For goethite kidney-shaped aggregates are typical – brown glass head with lustrous surface and radial-fibrous structure. A similar structure is observed in stalactites, concretions, geodes and oolites of goethite. Porous cryptocrystalline masses of limonite are actually a complicated mixture of Iron hydroxides, including goethite, hydrogoethite and lepidocrocite.In kidney-shaped aggregates goethite is non-transparent, in fine plates it is translucent to transparent. Color of massive aggregates is yellowish-brown to brown, in plates it is reddish-brownish. On the surface of kidney-shaped aggregates, sometimes, golden or iridescent blueing is observed. Parallel-fibrous silification aggregates of goethite display the effect of cat’s-eye in cabochon. Its trade name is satin stone or owl’s-eye. Bluish crocidolitic hawk’s-eye receives brownish hue under oxidation to goethite and lepidocrocite, then, it is called crocidolite quartz, tiger’s-eye or lion’s-eye.In artificial and natural annealing of tiger’s-eye goethite transfers to hematite and receives reddish-brownish hue; then it is called ox-eye. Inclusions of parallel-fibrous aggregates of goethite in crystals of quartz are called in the U.S.A. binghamite. Regular oriented inclusions of flakes of goethite in crystals of apatite, beryl, quartz, feldspars, chalcedony lead to their reddish tone and sometimes produce aventurescence and the effect of cat’s-eye in cabochon.

Deposits. Goethite is formed as a product of weathering in the zones of oxidation of ore deposits and it is concentrated in so called gossans. It is found in the form of crystals in hydrothermal veins. Its deposits are numerous and often are of significant scale. In Russia, goethite was discovered at first in quartz geodes of the Volk-Ostrov Is., White Sea. Its red-brownish to black needle, small columnar and plate crystals are found as inclusions in crystals of amethyst. In the South Urals, at the Akkermanovskoye deposit, along cracks in iron ore, they find solid crusts of goethite of radial-rayed structure, up to 3 cm. thick. The specific of this goethite is its gem-quality, because of golden-yellow color of aggregates. At the and Bakal’skoye iron-ore deposits they find large geodes of brown iron-ore, which inner surface is covered with leakages of solid goethite. In front of the Mining Inst. in Yekaterinburg one such geode is established – about 2 m. high. In the Ukraine, goethite weathering crusts are developing on iron quartzites of Krivoy Rog. In pegmatites of Volhynia, they find sphaerolites of needle goethite, with individuals up to 12 cm. long, cemented with chalcedony. In their polished slabs such kidney-shaped formations, up to 12 cm. in cross-section, display the effect of cat’s-eye. In Kazakhstan, on the Mangyshlak Penin., at the Beskempir deposit, this effect is observed in polished slabs of kidney-shaped radial-rayed goethite, up to 10 cm. in cross-section. In Tadzhikistan, in the East Pamirs Mts., in the region of the Akbaytal Pass, they discovered the Goethitevoye deposit with kidney-shaped formations of ornamental goethite up to 20 cm. in cross-section, in a hydrothermal vein. Findings of goethite in all the diversity of its varieties are wide-spread throughout the world. In moonstone from India, inclusions of goethite produce red-brownish, greenish-brown to green color, and with regular orientation alas well asterism. In Australia, at the Broken Hill deposit in New South Wales, stalactites of goethite of radial-rayed structure, up to 30 cm. long, are used as an ornamental stone. In Guyana, on golden fields, they extracted oolites of goethite as additional material. They are of concentric-zonal structure and almost black with anthracite luster after polishing. Among imitations they are known as Guyana black pearls.

Synonyms. Aetite – a hollow secretion of goethite, from Gk. “aetos” – an eagle, after the discovery location in eagles’ nests, also Germ. adlerstein | Esmeraldit | Fullonite, after the Austrian mineralogist G.B. Fullon | Brown glass head, from Germ. “Glatzkopf”— a bald head, after the similarity in pronunciation Glatzkopf was transformed into Glaskopf (glass head) | Wood iron | Limonite, from Gk. “lemon” – a meadow, after the discovery location of marsh ores | Onegit, after the discovery location in Volk-Ostrov Is., by the Onega Bay of the White Sea | ~ Ore: bog ~, velvet iron ~, wood iron ~, needle ~ | Príbramite (przibramite), after the discovery location at the Príbram deposit, Czech Rep. | Samtblende, Germ., after the type of the surface of kidney-shaped aggregates | Eagle stone, geodes of goethite, Old Russ. | Brown iron stone.

Treatment. Massive fibrous goethite, aggregates of the type of cat’s-eye with plates of goethite as inclusions are ornamental raw material and are cut in cabochons. Goethite, as well as hematite as inclusions, is the main coloring components of jasper, perelift, quartzite, gypsum, bauxite, heliotrope, jedi opal, rock salt and others. On the base of goethite, they have produced paints since the deepest antiquity. In the Roman Empire, in the Umbria Prov., they found deposits of such natural paints: red umbra, near Tuscany – brown Siena. Red paint was called minium or Syrian stone, yellow pigments – ocher.

Lepidocrocite (Lepidokrokit—Lépidocrocite—) (Ullmann, J.C. 1813), from Gk. “lepidos” – scale and “krokis” – fiber or thread, alluding to its scaly or feathery habit. Syn. Ruby iron or Ruby mica. Hydroxide – γ-FeOOH, orthorhombic system. Hardness 4-5. Density 4. Half-metallic luster to diamond one, silk one in fibrous aggregates. Cleavage in two directions. Fragile. Lepidocrocite is usually found in the form of small plate crystals, but more often it forms massive radial-rayed aggregates of the type of glass head. Color: ruby-red to orange-brownish, tawny. Opposite to goethite, its crystals are transparent in fine plates, they display strong pleochroism from red to yellow. It is non-transparent in aggregates. It is formed at the last stage of hydrothermal mineral-forming, more often it is found together with goethite in the zones of oxidation of iron-ore deposits. Inclusions of lepidocrocite in clay bands of chalcedony amygdales produce reddish tone in agate. In solid iron ores it is difficult to distinguish it from goethite and hematite. In Russia, at East Transbaikalia, near Nerchinskiy Zavod, there is the Berezovskoye deposit, where lepidocrocite is deposited together with hematite. It is processed like goethite. In Czech Rep., lepidocrocite was described at first at the Zlaté Hory deposit.

Search