Goethite
Goethite is a mineral that is common is really a popular species for mineral enthusiasts but is rarely thought of in the treasure trade. Goethite has, in the past, had a reputation for being a fairly uninteresting, dull, black mineral. However, Goethite commonly produces a very nice backdrop that is dark for vividly colored minerals such as for instance Vanadinite. There are also beautiful specimens that are massive show rainbows of iridescent colors. These specimens that are iridescent be cut into “cabochons” or just what I call “specigems”. An instance with this type of treasure is pictured at the top of this page. Goethite is additionally frequent as black crystal sprays that kind inside geodes on drusy Quartz. It normally forms a pseudomorph once different minerals, particularly Marcasite, Pyrite, Siderite, and Gypsum.
Sources for Goethite are widespread; some localities once and for all crystals include: from Siegen, North Rhine-Westphalia, and near Giessen, Hesse, Germany. At Príbram, Czech Republic. Exceptional crystals from the Restormel mine, Lanlivery; the Botallack mine, St. simply; and elsewhere in Cornwall, England. From Chaillac, Indre-et-Loire, France. In the united states, from the Pikes Peak district and Florissant, El Paso County, Colorado; an ore mineral in the Lake Superior district, as at the Jackson mine, Negaunee, while the Superior mine, Marquette, Marquette County, Michigan.
| Category: | oxide minerals, hydroxide subgroup |
| Chemical Formula: | α-Fe3+O(OH) |
| Iron Oxyhydroxide (with ferric iron) | |
| Molecular Weight: | 88.85 gm |
| Composition: | Iron | 62.85 % | Fe | 89.86 % | Fe2O3 |
| Hydrogen | 1.13 % | H | 10.14 % | H2O | |
| Oxygen | 36.01 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | As prismatic crystals, striated || [001], to 45 cm, or tabular on {010}. More commonly as aggregates of the capillary to acicular crystals, in divergent sprays, or reniform, botryoidal, or stalactitic masses with concentric or radial fibrous internal structure; nearly cryptocrystalline in “limonite”. |
| Twinning: | None |
| Cleavage: | Perfect on {010}; less perfect on {100} |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.0 – 5.5; VHN = 667 (100 g load) |
| Density: | 4.27 – 4.29 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Blackish Brown; Yellowish to Reddish Brown in massive aggregates, may be banded; shades of Yellow in thin section; Gray with Bluish tint in reflected light, with Yellow, Red, Brown internal reflections |
| Transparency: | Opaque, Transparent on thin edges |
| Luster: | Imperfect adamantine, metallic to dull earthy; silky when fibrous |
| Refractive Index: | 2.260 – 2.515 Biaxial ( – ); Uniaxial (–) for red light |
| Birefringence: | 0.133 – 0.134 |
| Dispersion: | Extreme; r > v |
| Pleochroism: | Strong; X = yellow to colorless; Y = yellow-brown, reddish orange; Z = yellow-orange, deep reddish orange |
| Anisotrophism: | Distinct from bluish greys. Gray with bluish tint with yellow, red, brown internal reflections. |
