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Goethite Mineral Facts:

Chemical Formula: FeO-OH

Colors: Yellowish brown to dark brown. Streak is Yellowish brown.

Hardness: 5.0 to 5.5

Density: 4.37

Cleavage: One perfect cleavage on (010), its fracture is uneven.

Crystallography: Orthorhombic
Prismatic, vertically striated. Often flattened parallel to brachypinacoid. In acicular crystals at times. Some forms almost metallic. In thin splinters it is often translucent with a blood-red color

Luster:. Adamantine to dull luster. Silky luster in certain fine scaly or fibrous varieties.

Optics: (Refractive Index):  about 2.5

Composition, Structure and Associated Minerals:
Goethite is one of the most common of minerals, and is formed under oxidizing conditions as a weathering product of iron bearing minerals. Goethite forms the gossan capping over the top of many metal bearing vein systems for silver ores and copper. This mineral, though it is occasionally found in blackish brown crystals, usually occurs in radiated globular and botryoidal masses. These massive, reniform, stalactitic forms have a radiating fibrous structure. Foliated. Rarely in distinct crystals. Goethite is usually associated with limonite and hematite as well as other ores of iron, especially in the upper portion of veins, where it is produced by weathering.

Identification and Diagnostics
Told chiefly by the color of its streak and distinguished from limonite by its tendency to crystallize and the smaller amount of water which it contains. The chemical reactions of the mineral are about the same as those of hematite, except that it yields water when heated in the closed tube. By this reaction it is easily distinguished from the fibrous varieties of hematite, as it is also by its streak.

Occurrence, Localities and Origins:
Some localities are Lostuithiel and Botallack in Cornwall, Saxony, Lake Onega in Russia, near Siegen in Nassau; near Bristol and Clifton, England, and in large, it occurs as fine crystals at Lostwithiel and other places in Cornwall.

In the United States it occurs in small quantity in connection with the Lake Superior hematite deposits at the Jackson Iron Mountain mine and the Lucie hematite mines in Negaunee, Mich.; at Salisbury, Conn.; at Easton, Penn., and at many other places. Uses. Goethite is used as an ore of iron, but in the trade it is classed with limonite as brown hematite.

Use: In some localities it is an important ore of iron.

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