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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
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Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another due to alteration, or chemical substitution, dissolution and refilling, structural changes or incrustation.[1] The name literally means "false form". Terminology for pseudomorphs is "replacer after original", as in brookite after rutile. The term pseudomorphoses was initially used by Haüy.[2]
A substitution pseudomorph is a pseudomorph in which one mineral or other material is replaced by another. The original shape of the mineral remains unchanged, but chemical composition, color, hardness, and other properties change to those of the replacing mineral.
This happens typically when a mineral of one composition changes by chemical reaction to another of similar composition, retaining the original crystalline shape. It can occur due to the loss of water or through the action of atmospheric agents, such as oxidation, hydration, or carbonation. An example is a change from galena (lead sulfide) to anglesite (lead sulfate) by oxidation. Pyrite crystals transformed into limonite, a compact mixture of iron oxides where goethite generally predominates, are common. In some cases, only partial replacement occurs. The resulting pseudomorph may contain an unaltered core of galena surrounded by anglesite that has the cubic crystal shape of galena.
An example of this process is the replacement of wood by silica (quartz or opal) to form petrified wood in which the substitution may be so perfect as to retain the original cellular structure of the wood. An example of mineral-to-mineral substitution is replacement of aragonite twin crystals by native copper, as occurs at the Corocoro United Copper Mines of Coro Coro, Bolivia.
A paramorph (also called allomorph) is a mineral changed on the molecular level only when the structure of a mineral transitions to a more stable polymorph. It has the same chemical composition, but with a different structure. For example, the cubic form of silver sulfide, argentite, does not actually exist below 173°C, and all are pseudomorphized into the monoclinic mineral acanthite. When the structure is very different, as in the aragonite and calcite, the unstable mineral can remains indefinitely in a metastable form, although transformation can occur under some conditions. Usually, the mineral looks identical to the original, unaltered form.
An incrustation pseudomorph, also called epimorph, results from a process by which a mineral is coated by another and the encased mineral dissolves. The encasing mineral remains, and retains the shape of the original mineral or material. Alternatively, another mineral may fill the space (the mold) previously occupied by some other mineral or material. Examples of quartz epimorph after calcite are found in the La Viesca Mine, Siero (Asturias), Spain.
Pseudomorphs are also common in paleontology. Fossils are often formed by pseudomorphic replacement of the remains by mineral matter. Examples include petrified wood and pyritized gastropod shells.
In biology, a pseudomorph is a cloud of mucus-rich ink released by many species of cephalopod. The name refers to the similarity in appearance between the cephalopod that released it and the cloud itself, in this context meaning literally "false body". This behaviour often allows the cephalopod to escape from predation unharmed, and is often performed as part of what is known as the blanch-ink-jet maneuver.
In philosophy, the concept of pseudomorphosis was used by the German philosopher Oswald Spengler to describe how the forms of an older, more widely dispersed culture affect the expression of forms of a younger, emerging culture. The latter develop into forms that are fundamentally alien to the culture's own world-feeling and thereby prevent it from fully developing its own self-consciousness.[3]
In archaeology, organic pseudomorphs are impressions of organic material that can accumulate on the surface of metal artifacts as they corrode. They may occur when metal artifacts are buried in contact with organics under damp soil.[4]
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another due to alteration, or chemical substitution, dissolution and refilling, structural changes or incrustation. The name literally means "false form". Terminology for pseudomorphs is "replacer after original", as in brookite after rutile. The term pseudomorphoses was initially used by Haüy.
Hematite ( HE(E)M-ə-tyte), also spelled as haematite, is a common iron oxide compound with the formula Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of Fe2O3. It has the same crystal structure as corundum (Al2O3) and ilmenite (FeTiO3). With this crystal structure geometry it forms a complete solid solution at temperatures above 950 °C (1,740 °F). Hematite occurs naturally in black to steel or silver-gray, brown to reddish-brown, or red colors. It is mined as an important ore mineral of iron. It is electrically conductive. Hematite varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While these forms vary, they all have a rust-red streak. Hematite is not only harder than pure iron, but also much more brittle. The term kidney ore may be broadly used to describe botryoidal, mammillary, or reniform hematite. Maghemite is a polymorph of hematite (γ-Fe2O3) with the same chemical formula, but with a spinel structure like magnetite. Large deposits of hematite are found in banded iron formations. Gray hematite is typically found in places that have still, standing water, or mineral hot springs, such as those in Yellowstone National Park in North America. The mineral may precipitate in the water and collect in layers at the bottom of the lake, spring, or other standing water. Hematite can also occur in the absence of water, usually as the result of volcanic activity. Clay-sized hematite crystals also may occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, which is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.
Shattuckite is a copper silicate hydroxide mineral with formula Cu5(SiO3)4(OH)2. It crystallizes in the orthorhombic – dipyramidal crystal system and usually occurs in a granular massive form and also as fibrous acicular crystals. It is closely allied to plancheite in structure and appearance. Shattuckite is a relatively rare copper silicate mineral. It was first discovered in 1915 in the copper mines of Bisbee, Arizona, specifically the Shattuck Mine (hence the name). It is a secondary mineral that forms from the alteration of other secondary minerals. At the Shattuck Mine, it forms pseudomorphs after malachite. A pseudomorph is an atom by atom replacement of a crystal structure by another crystal structure, but with little alteration of the outward shape of the original crystal. It is sometimes used as a gemstone.
Metatorbernite (or meta-torbernite) is a radioactive phosphate mineral, and is a dehydration pseudomorph of torbernite. Chemically, it is a copper uranyl phosphate and usually occurs in the form of green platy deposits. It can form by direct deposition from a supersaturated solution, which produces true crystalline metatorbernite, with a dark green colour, translucent diaphaneity, and vitreous lustre. However, more commonly, it is formed by the dehydration of torbernite, which causes internal stress and breakage within the crystal lattice, resulting in crystals composed of microscopic powder held together using electrostatic force, and having a lighter green colour, opaque diaphaneity, and a relatively dull lustre. As with torbernite, it is named after the Swedish chemist Torbern Bergman. It is especially closely associated with torbernite, but is also found amongside autunite, meta-autunite and uraninite.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Mae Jemison, aboard Space Shuttle Endeavour in 1992.
