Goethite (Brown Ochre)
Alternate Names: yellow ochre and brown ochre. Also spelled, ocher. Mars Yellow and Mars Brown are names given to the artificial substitutes for goethite. Obsolete Names: Brown Hematite, limonite Color: Brown Colour Index: Pigment Yellow 43 (77492) ASTM Lightfastness: Oil: I Acrylic: I Watercolor: I Density: 4.1 Hardness: 5 to 5.5 Chemical Formula: a-FeOOH
A yellow to brown mineral consisting of hydrated iron oxide, usually present in the oxidized portions of iron ore deposits, and the commonest constituent of many forms of iron oxide. Goethite was named for Johann Wolfgang von Goethe (1749-1832), a German philosopher and poet who also was a mineralogist. This widespread mineral, which is encountered practically everywhere on the earth's surface in the form of concretions, oolites (a form consisting of small round grains cemented together), reniform (kidney shapes) or botryoidal (form of bunches of grapes) accumulations, frequently is encountered in the swampy areas at the head of spring waters, and on the bottom of lakes and small creeks.
Properties: A well-known property of goethite is that it dehydrates and transforms to hematite when heated to 250-300°C, with a corresponding color change. While red earths are generally colored by hematite, most of the yellow or yellow-brown earths are colored primarily by goethite. Deposits of goethite mixed with other hydrated iron oxides were sometimes referred to as Brown Hematite or limonite. Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals, mostly consisting of goethite. Goethite with different quantities of hydrated iron oxide forms the range of yellow to brown hues found in natural mineral formations. In other words, goethite is the yellow-colored chemical compound for those natural pigments we call ochre, sienna and umber. At the beginning of the 20th century, V. I. Vernadskiy  defined ochres as clays rich in hydrates of trivalent iron oxide. Today, ochres are understood to be natural clay formations enriched by hydrated iron oxide, usually in the form of the mineral goethite, whose contents vary from 12 to 75%. Ochres are chiefly composed of different hydrated iron oxides in varying amounts with goethite forming the major constituent, together with clay, sand, chalk, gypsum, barites and occasionally silica.
History of Use: Goethite has been in continuous use as a pigment since prehistoric times. Evidence of goethite was found in paint pigment samples taken from the caves of Lascaux, France. In ancient Greece the term "ochre" was used for natural earth pigments of different yellow hues (ochre light, golden, dark, greenish, etc.).
Permanence and Compatibility: Authorities today consider ochre to be a permanent pigment. It was used on alkaline lime fresco walls. The natural mineral is said to be more stable than the manufactured mineral.
Oil Absorption and Grinding: Goethite absorbs a moderate amount of oil during dispersion and forms a good flexible film.
Toxicity: Goethite is not considered toxic but care should be used in handling the dry powder pigment so as not to inhale the dust.
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