Limonite

Authors: Rayna, Mairee, and Nolan

Mineral Name: Limonite

Limonite doesn't fulfill the qualifications to be considered a mineral and is actually an Amorphous mineraloid.

Chemical Composition: α-FeO(OH)

Color: LImonite is yellow to yellowish brown; reddish brown to brown

Streak: Yellowish brown

Hardness: 4-5.5

Cleavage/Fracture: Fracture looks splintery and uneven, showing elongated, sharp points as it breaks.

Crystal Form: Limonite can pseudomorph. This means that crystals you may see of limonite are minerals that have decomposed into limonite but have not lost their crystalline shape. The shape is maintained due to limonite coating the crystals form and assuming its shape. Limonite can also form from accumulative drips from cave ceilings, which are called stalactites, or create reniform samples, which appear as kidney-shaped, or botryoidal, which appears lumpy like grapes (2).

Luster: Earthy, dull, vitreous, silky

Special Features: Limonite is not a true mineral, instead it is a mineraloid because it lacks a distinct chemical composition and crystalline lattice (2). Limonite is known for being an amorphous mineraloid because it is able to pseudomorph. As one mineral, such as pyrite, weathers, limonite replaces it and takes on the shape of the original pyrite crystal (5). Because of the nature of limonite, any “crystal” of limonite is not truly a limonite crystal but a pseudomorph of the crystal it replaced (2). Another interesting thing about limonite is that once it has been heated it becomes magnetic due to its iron content (2).

Varieties: Adlerstein contains nodular concretions of iron oxides/hydroxides around a core of clay minerals (3). Alumolimonite is aluminum-bearing limonite. Auriferous limonite is a gold-bearing variety. Avasite is a variety of limonite that is probably siliceous (3). Bean Ore is a name for a variety that shows small bean-shaped pieces spread throughout the sample. Chromiferous limonite is chromium bearing variety. Exotic limonite is a variety that is precipitated in rock that didn’t previously contain iron sulfides (3). Indigenous limonite is limonite derived from sulfides and stays where it forms. Liminitogelite s a type of limnite is a recently-formed variety that contains a lot of water, called bog ore (3). Stilpnosiderite is a colloidal variety, meaning it contains microscopically dispersed limnite in a matrix of other material (3).

Mineral Group: Limonite belongs to the oxide group because it contains a metal (Fe) combined with oxygen, water, or hydroxyl (OH-). And because it can contain water or hydroxyl, it is also placed in the hydroxides subgrup.


Environment: Limonite is a secondary material and often accumulates due to its own resistance to weathering. Limonite is formed as a result of iron-rich materials like hematite or magnetite weathering (1). Limonite commonly forms as a precipitate around mine entrances, springs, bogs, and other shallow marine areas (1). Stratified deposits of limonite can be a sign that acid mining took place because they occur when the iron-rich water comes into contact with the oxygenated surface water and sink to the floor of the water body(1). Limonite is often the colorant in soil that is iron-rich and resists weathering very well (1).

Associated Rock types: Limonite can be found in iron ore. It can also be found in itron sediments like iron sediment like gravel, sand and mud as well as limonite sediment such as gravel, sand and mud (3). Limonite can also be found in sedimentary rocks such as limonite-stone, iron-mudstone, limonite-mudstone, and iron-wackestone to name a few (3). Limonite can also be found in the metamorphic rock birbirite (3).

Occurrence in North America: Limonite is found all across North America. Limonite is heavily mined in Pennsylvania as well as Utah (2).

Economic Uses: Limonite is most commonly used today as pigmentation in paints and dyes. First, the limonite must be ground down into a powder and refined for consistency (6). Sometimes limonite is pre-treated with heat to remove any water so that a powder-form is more easily achieved and color is improved (1). Then the powdered pigments are mixed with oil and/or together with other powdered pigments to create the desired color and added to paint bases or gum-arabic (1,6). Limonite is essential to creating colors that are classified as “ochre” (1).

Industrial Uses: Limonite is used as a low quality iron ore mined in areas where hematite and magnetite aren’t present. The iron is retrieved through iron processing, which uses a smelting process that turns the iron ore into a form that can be utilized in product creation (7).

First Notable Identification: Limonite pigments have been used since prehistoric times including in neolithic pictographs (1). Limonite is based on the Greek word for meadow, named in 1813 by Johann Friedrich Ludwig Hausmann because of its common appearance in boggy areas (3).

How We Identified It: We found our limonite samples to appear similar to the rock scoria due to its rough and vesicular appearance. It is easily differentiated from scoria because it has a lower hardness and has a yellow hue to it in places.

Don’t Confuse It With: Wad which is darker in color and doesn’t become magnetic when heated (2).

Bibliography:

  1. King, Hobart M. “Limonite.” Geology.com, geology.com/minerals/limonite.shtml.

  2. “Limonite.” Minerals.net, www.minerals.net/mineral/limonite.aspx.

  3. “Limonite.” Mindat.org, www.mindat.org/min-2402.html.

  4. “Limonite.” ScienceDirect, www.sciencedirect.com/topics/earth-and-planetary-sciences/limonite.

  5. Powell, Wayne. “Making Paints from Minerals.” Pedagogy in Action, 7 May 2018, serc.carleton.edu/sp/library/urban/examples/mineral_properties_paint.html.

  6. “THE MINERAL LIMONITE.” The Mineral Gallery, www.galleries.com/Limonite.

  7. Walker, Robert Donald. “Iron Processing.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 27 Jan. 2017, www.britannica.com/technology/iron-processing.