1911 Encyclopædia Britannica/Göthite

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EB1911 Göthite.jpg

GÖTHITE, or Goethite, a mineral composed of an iron hydrate, Fe2O3·H2O, crystallizing in the orthorhombic system and isomorphous with diaspore and manganite (q.v.). It was first noticed in 1789, and in 1806 was named after the poet Goethe. Crystals are prismatic, acicular or scaly in habit; they have a perfect cleavage parallel to the brachypinacoid (M in the figure). Reniform and stalactitic masses with a radiated fibrous structure also occur. The colour varies from yellowish or reddish to blackish-brown, and by transmitted light it is often blood-red; the streak is brownish-yellow; hardness, 5; specific gravity, 4.3. The best crystals are the brilliant, blackish-brown prisms with terminal pyramidal planes (fig.) from the Restormel iron mines at Lostwithiel, and the Botallack mine at St Just in Cornwall. A variety occurring as thin red scales at Siegen in Westphalia is known as Rubinglimmer or pyrrhosiderite (from Gr. πυρρός, flame-coloured, and σίδηρος, iron): a scaly-fibrous variety from the same locality is called lepidocrocite (from λεπίς, scale, and κροκίς, fibre). Sammetblende or przibramite is a variety, from Przibram in Bohemia, consisting of delicate acicular or capillary crystals arranged in radiating groups with a velvety surface and yellow colour.

Göthite occurs with other iron oxides, especially limonite and hematite, and when found in sufficient quantity is mined with these as an ore of iron. It often occurs also as an enclosure in other minerals. Acicular crystals, resembling rutile in appearance, sometimes penetrate crystals of pale-coloured amethyst, for instance, at Wolf’s Island in Lake Onega in Russia: this form of the mineral has long been known as onegite, and the crystals enclosing it are cut for ornamental purposes under the name of “Cupid’s darts” (flèches d’amour). The metallic glitter of avanturine or sun-stone (q.v.) is due to the enclosed scales of göthite and certain other minerals. (L. J. S.)