Goethite

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Goethite is a common mineral. It can be brownish yellow, reddish brown, or dark brown in color, depending on the size of the crystal in the specimen—small crystals appear lighter, and larger ones darker. It can occur as opaque black, prismatic and vertically striated crystals; velvety, radiating fibrous aggregates; flattened tablets or scales; and reniform or botryoidal masses. Goethite can also occur in stalactitic or massive forms and in tufts and drusy coatings. Goethite is an iron oxide hydroxide, although manganese can substitute for up to 5 percent of the iron. It forms as a weathering product in the oxidation zones of veins of iron minerals, such as pyrite, magnetite and siderite. Goethite may occur with these minerals in the gossan, or iron hat, which is the weathered capping of an iron ore deposit. It also occurs in a form called bog iron ore, which can be produced by living organisms.

Name: Honors the German poet, dramatist, and philosopher, Johann Wolfgang von Goethe (1749–1832).

Polymorphism & Series: Trimorphous with feroxyhyte and lepidocrocite.

Association: Lepidocrocite, hematite, pyrite, siderite, pyrolusite, manganite, many other ironand manganese-bearing species.

Chemical Properties of Goethite

Chemical Classification	Oxide minerals hydroxide subgroup
Chemical Composition	FeO(OH)

Physical Properties of Goethite

Color	Yellowish to reddish to dark brown or black
Streak	Brown, brownish yellow to orange yellow
Luster	Adamantine to dull
Cleavage	{010}, perfect; {100}, less perfect.
Diaphaneity	Opaque
Mohs Hardness	5–5.5
Specific Gravity	3.3 – 4.3
Diagnostic Properties	Goethite was shown to be the most common diagenetic iron oxyhydroxide in both marine and lake sediments
Crystal System	Orthorhombic
Tenacity	Brittle
Fracture	Irregular/Uneven
Density	4.27 – 4.29 g/cm3 (Measured)    4.18 g/cm3 (Calculated)

Optical Properties of Goethite

Type	Anisotropic
Anisotropism	Distinct; in bluish grays.
Color / Pleochroism	Visible
Optic Sign	Biaxial (-)
Birefringence	δ = 0.133 – 0.134
Relief	Very High

Occurrence

A common weathering product derived from numerous iron-bearing minerals in oxygenated environments; an important component of ore in weathered iron deposits. Also a primary precipitate in hydrothermal, marine, and bog environments upon oxidation of reduced iron-bearing waters.

Formation

The formation of goethite is marked by way of the oxidation nation change of Fe2+ (ferrous) to Fe3+ (ferric), which lets in for goethite to exist at floor conditions. Because of this oxidation nation change, goethite is normally seen as a pseudomorph. As iron-bearing minerals are delivered to the region of oxidation within the soil, the iron turns from iron(II) to iron(III), whilst the authentic form of the parent mineral is retained. Examples of not unusual goethite pseudomorphs are: goethites after pyrite, goethite, siderite, and marcasite, although any iron(II)-bearing mineral should become a goethite pseudomorph if right situations are met. It will also be brought on by way of groundwater or in different sedimentary conditions, or shape as a primary mineral in hydrothermal deposits. Goethite has additionally been observed to be produced through the excretion procedurs of positive micro organism kinds.

Uses Area

• Its main use is iron ore. Most important after hematite
• There are some uses as clay soil pigment.
• Banded or iridescent Banded or iridescent varieties are cut and polished into cabochons for jewelry making. varieties
• One of the most prominent applications is the elimination of cadmium, which is the chemical element from the contaminated waters present in industrial liquid wastes.

Distribution

Widespread; some localities for good crystals include:

• from Siegen, North Rhine-Westphalia, and near Giessen, Hesse, Germany. AtPrıbram, Czech Republic.
• Exceptional crystals from the Restormel mine, Lanlivery; the Botallack mine, St. Just; and elsewhere in Cornwall, England.
• From Chaillac, Indre-et-Loire, France.
• In the USA, from the Pikes Peak district and Florissant, El Paso Co., Colorado; an ore mineral in the Lake Superior district, as at the Jackson mine, Negaunee, and the Superior mine, Marquette, Marquette Co., Michigan.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
• Mindat.org. (2019). Goethite: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/min-727.html [Accessed 4 Mar. 2019].