In mineralogy and geology, goethite is considered a fairly common mineral that usually occurs with a black, brownish-yellow, reddish, and even reddish-brown hue, depending on the size of the crystals, the color varies in such a way that the largest ones usually be dark reddish, while the smaller crystals are usually light reddish and yellowish.
Goethite is basically a mineral that belongs to the group of hydroxides of iron that has been formed by the oxidation of minerals that contain iron in their composition due to weathering processes at low temperatures or ambient conditions.
Therefore, it is found forming oxidation patinas that mainly affect the pyrite, magnetite, marcasite, and the siderite.
Properties and characteristics of goethite
It is a very varied mineral in its habit, in fact it can appear as opaque, prismatic and even striated black crystals. It also forms as fibrous radial or acicular growths, as superimposed flattened scales and often appears as botryoidal masses.
Although it is less common in cavernous environments with a high percentage of humidity and water, it usually appears in stalactitic or drustiform forms.
| Chemical formula | FeO(OH) |
| Composition | Iron hydroxide, often with some manganese |
| variable formula | (Fe,Mn)O(OH) |
| Color | Black, brown, yellowish-brown, reddish-brown, yellow. May be iridescent with a multicolored, rainbow-like display of colors of darker shades. Rarely banded. |
| Stripe | brownish yellow to yellow |
| Mohs hardness | 5 – 5.5 |
| crystal system | orthorhombic |
| Transparency | Opaque |
| specific gravity | 3.3 – 4.3 |
| Gloss | Submetallic, silky, opaque |
| cleavage | 1.1 |
| Fracture | chipped,uneven |
| Tenacity | Fragile |
| Cluster | Oxides; Hydroxides |
| Environment | As a secondary mineral in replacement hydrothermal deposits, in igneous pegmatite and basalt, and in sandstone, limestone and clay |
Geological environment
Goethite is generally a secondary mineral that can appear in any type of rock and environment, replacing minerals that contain iron in its composition. It can be important when it replaces minerals in hydrothermal deposits in veins that possibly have indications of containing precious metals.
It is also a primary precipitate in hydrothermal, marine, and swamp environments after oxidation of waters containing reduced iron.
Habit
Individual crystals occur in small, flattened sheets and plates, or finely acicular with a velvety appearance. Most often in botryoidal, reniform, or stalactitic aggregates of radiant crystals or ball-shaped crystals. Also granular, veined, concretionary, oolitic, and in earthy masses. Goethite often assumes the forms of other minerals forming a pseudomorph instead of or as a layer on top of the original mineral.
Genesis or formation
The genesis of goethite is related to the oxidation change of Fe2 + (ferrous) ions to Fe3 + (ferric), which allows this mineral to exist at ambient conditions at ground level. Thanks to this variation in oxidation, the goethite is normally seen as a pseudomorph.
Examples of goethite pseudomorphs are: goethites formed due to oxidation and replacement of pyrite, siderite and marcasite, although any iron(II)-bearing mineral should become a goethite pseudomorph if the right situations are met.
It will also come through groundwater or different sedimentary conditions, or form as a primary mineral in hydrothermal deposits.
Optical properties of goethite
The following shows the optical properties of identification in the microscope of this mineral.
| Guy | Anisotropic |
| anisotropism | Distinct;in bluish greys. |
| Color / Pleochroism | Visible |
| optical signal | Biaxial (-) |
| Birefringence | δ = 0.133 – 0.134 |
| Relief | Very high |
Deposits or distribution
Goethite is found throughout the world, although specimens worthy of collection are less common. The minerals with stalactite shapes and large botryoidal masses, often associated with the brightly contrasting Vanadinite, come from the Four Men Mine, Taouz, in Morocco.
Regarding the variety of acicular crystals with ornamental stone quality, they are mainly mined in the United States in the areas of Crystal Peak and Lake George, Park and Teller Cos., Colorado.
Varieties
Botryoidal goethite
It appears in the form of small rounded accumulations in layers, often associated with malachite and azurite in iron caps and in oxidation zones very close to copper-rich deposits.
Iridescent goethite
It occurs in botryoidal, prismatic and scaly habits, but it is characterized by the fact that its faces present a great variety of shades. The varied colors will depend on small impurities in the oxidation of the iron.
Acicular or radial goethite
This variety has an elongated acicular and prismatic habit, it is quite common in hydrothermal deposits associated with veins of intermediate temperature that exceeds 120°C.
Importance of goethite in mineral exploration
Despite the fact that this mineral is quite common, the truth is that it becomes important when it appears in large quantities, confirming important structures related to the occurrence of hydrothermal minerals or with the important accumulation of iron minerals or in iron hats.
Thus, for example, goethite can appear in large quantities in hydrothermal veins rich in pyrite and magnetite veinlets, which give indications of hydrothermal activities that may contain precious metals such as gold and silver.
Likewise, when it is discovered that it has replaced large amounts of magnetite, it may indicate that there may be a copper porphyry system nearby.
Uses and applications of goethite
various uses
- Its main use is the mineral from which the metal iron, however let us remember that the most important mineral to extract iron is the hematite.
- Sometimes it can be used as a pigment, especially when it is presented in clay size.
- One of the most outstanding applications is the elimination of cadmium, which is the chemical element of contaminated water present in industrial liquid waste.
Uses in jewelry and costume jewelery
Goethite can be used to create costume jewelery when it comes in the variety of iridescent goethite and with botryoidal habit. This variety is characterized because on its faces it presents a variety of colors in reddish, green, yellowish, black and brown tones.
In this case, the stone must also be kept quite fresh so that it reaches its real Mohs hardness, which is 5, and for the gem to have resistance over time.
Uses as ornamental stone
Goethite is characterized because it can occur in many forms or habits and replaces a wide variety of minerals. This makes very rare or exotic shaped stones very attractive to mineral collectors who pay excellent prices to have these stones.
These collectors carry them to adorn spaces in their homes, offices or museums.