Mendipite
Encyclopedia
Mendipite is a rare mineral that was named in 1939 for the locality where it is found, the Mendip Hills
in Somerset, England. It is an oxide
of lead
, with chlorine
, formula Pb3O2Cl2.
, disphenoid
al class 2 2 2, meaning that it has three mutually perpendicular axes of twofold symmetry
, with space group
P212121, meaning that each of these axes is a screw axis
. One reference, however, gives the crystal class as orthorhombic m m m with space group Pnma, which has a higher symmetry. In each case the "P" means that the mineral has a primitive unit cell.
Unit cell parameters:
and its luster
is pearly to silky on cleavages, and resinous to adamantine on fractures. The mineral is translucent, and rarely transparent. It is biaxial (+)
, with refractive indices
Nx = 2.24, Ny = 2.27, Nz = 2.31. These values are quite high, compared with ordinary glass at 1.5. This is typical of lead
minerals.
. Because of the lead
content the specific gravity
is high, at 7.24, or 7 to 7.2, just a little less than that of mimetite
, another lead mineral. Mendipite is soluble in dilute nitric acid
, HNO3. It is not radioactive
.
, Australia, oxygenated groundwater
reacted with sulfide mineral
s during the Tertiary
Period, forming supergene
sulfides, as well as the sulfate mineral
s beudantite
, anglesite
and baryte, together with some mimetite
and native silver
. Cerussite
crystallised later, and later still chloride
-rich groundwaters reacted with many of these earlier minerals to form more mimetite, as well as blixite, laurionite
, mendipite and chlorargyrite
.
At the type locality, galena
was deposited in Carboniferous
limestones throughout the Mendip Hills
during the late Permian
or Triassic
Period. In the Jurassic
Period that followed, these deposits were exposed to the action of seawater, which deposited manganate minerals
that reacted with the galena and adsorbed heavy metals both from the seawater and surroundings. A later event heated the deposits creating the conditions which led to the formation of the suite of unusual secondary minerals
, including a number of rare oxychlorides, now found at Merehead. The mendipite occurs in nodules in manganese oxide
ores, associated with hydrocerussite, cerussite
, malachite
, pyromorphite
, calcite
, chloroxiphite
, diaboleite and parkinsonite.
is Churchill
, Mendip Hills, Somerset, England, and type material is conserved at the Royal Swedish Academy of Sciences
, Stockholm, Sweden. Other localities include Australia, Germany, Greece, Sweden, the UK and the USA.
Mendip Hills
The Mendip Hills is a range of limestone hills to the south of Bristol and Bath in Somerset, England. Running east to west between Weston-super-Mare and Frome, the hills overlook the Somerset Levels to the south and the Avon Valley to the north...
in Somerset, England. It is an oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, with chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, formula Pb3O2Cl2.
Crystal structure
Most references assert that mendipite crystallises in the orthorhombic crystal systemOrthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
, disphenoid
Disphenoid
A disphenoid is a polyhedron whose four faces are identical triangles. The faces of a tetragonal disphenoid are isosceles; the faces of a rhombic disphenoid are scalene. If the faces are equilateral triangles, one obtains a regular tetrahedron, which is not normally considered a disphenoid.All the...
al class 2 2 2, meaning that it has three mutually perpendicular axes of twofold symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...
, with space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
P212121, meaning that each of these axes is a screw axis
Screw axis
The screw axis of an object is a line that is simultaneously the axis of rotation and the line along which a translation occurs...
. One reference, however, gives the crystal class as orthorhombic m m m with space group Pnma, which has a higher symmetry. In each case the "P" means that the mineral has a primitive unit cell.
Unit cell parameters:
- a = 9.52 Å, b = 11.87 Å, c = 5.87 Å, Z = 4
- a = 11.87 Å, b = 5.806 Å, c = 9.48 Å, Z = 4
Optical properties
Mendipite is colorless to white, brownish cream, grey, yellowish, pink, red, or blue. It is nearly colorless in transmitted light. It has a white streakStreak (mineralogy)
The streak of a mineral is the color of the powder produced when it is dragged across an unweathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus...
and its luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
is pearly to silky on cleavages, and resinous to adamantine on fractures. The mineral is translucent, and rarely transparent. It is biaxial (+)
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
, with refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
Nx = 2.24, Ny = 2.27, Nz = 2.31. These values are quite high, compared with ordinary glass at 1.5. This is typical of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
minerals.
Physical properties
Mendipite is found in columnar or fibrous aggregates, often radiated but more rarely straight long fibers, and in cleavable masses. The cleavage is perfect on {110} and fair on {100} and {010}. Fracture is conchoidal (shell-like) to uneven and the mineral is soft, with hardness only 2½ to 3, a bit less than that of calciteCalcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
. Because of the lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
content the specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is high, at 7.24, or 7 to 7.2, just a little less than that of mimetite
Mimetite
Mimetite, whose name derives from the Greek Μιμητής mimethes, meaning "imitator", is an arsenate mineral which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name is a reference to mimetite's resemblance to the mineral pyromorphite...
, another lead mineral. Mendipite is soluble in dilute nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
, HNO3. It is not radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
Environment
At the Eleura Mine near Cobar, New South WalesCobar, New South Wales
-Notable people:*Nik Kosef, former rugby league player for the Manly-Warringah Sea Eagles, 1996 premiership player, NSW & Australia representative*Ernie Toshack, cricketer, member of Bradman's Invincibles* Jemma Heather, state representative 4 times....
, Australia, oxygenated groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
reacted with sulfide mineral
Sulfide mineral
The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...
s during the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
Period, forming supergene
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
sulfides, as well as the sulfate mineral
Sulfate mineral
The sulfate minerals are a class of minerals which include the sulfate ion within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal veins and as secondary minerals in the oxidizing zone of sulfide mineral deposits...
s beudantite
Beudantite
Beudandite is a secondary mineral occurring in the oxidized zones of polymetallic deposits. It is a lead, iron, arsenate, sulfate with endmember formula: PbFe36SO4AsO4....
, anglesite
Anglesite
Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and celestine. It contains 74% of lead by mass and...
and baryte, together with some mimetite
Mimetite
Mimetite, whose name derives from the Greek Μιμητής mimethes, meaning "imitator", is an arsenate mineral which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name is a reference to mimetite's resemblance to the mineral pyromorphite...
and native silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
. Cerussite
Cerussite
Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...
crystallised later, and later still chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
-rich groundwaters reacted with many of these earlier minerals to form more mimetite, as well as blixite, laurionite
Laurionite
Laurionaite is a halide mineral, named after the town Laurium in Greece. Laurionite can also be produced by lead oxide in a salt solution....
, mendipite and chlorargyrite
Chlorargyrite
Chlorargyrite is the mineral form of silver chloride . Chlorargyrite occurs as a secondary mineral phase in the oxidation of silver mineral deposits. It crystallizes in the isometric - hexoctahedral crystal class. Typically massive to columnar in occurrence it also has been found as colorless to...
.
At the type locality, galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
was deposited in Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
limestones throughout the Mendip Hills
Mendip Hills
The Mendip Hills is a range of limestone hills to the south of Bristol and Bath in Somerset, England. Running east to west between Weston-super-Mare and Frome, the hills overlook the Somerset Levels to the south and the Avon Valley to the north...
during the late Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
or Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
Period. In the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
Period that followed, these deposits were exposed to the action of seawater, which deposited manganate minerals
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
that reacted with the galena and adsorbed heavy metals both from the seawater and surroundings. A later event heated the deposits creating the conditions which led to the formation of the suite of unusual secondary minerals
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
, including a number of rare oxychlorides, now found at Merehead. The mendipite occurs in nodules in manganese oxide
Manganese oxide
Manganese oxide is a generic term used to describe a variety of manganese oxides and hydroxides. It may refer to:* Manganese oxide, MnO* Manganese oxide, Mn3O4* Manganese oxide, Mn2O3* Manganese dioxide, , MnO2...
ores, associated with hydrocerussite, cerussite
Cerussite
Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...
, malachite
Malachite
Malachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
, pyromorphite
Pyromorphite
Pyromorphite is a mineral species composed of lead chlorophosphate: Pb53Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal planes, sometimes combined with narrow faces of a hexagonal...
, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, chloroxiphite
Chloroxiphite
Chloroxiphite is an olive green to black halide mineral sometimes found in the Mendip Hills, Somerset, England associated with mendipite. Like mendipite it is an oxychloride mineral and formed from the alteration of lead ore by a prehistoric sea.It is rare but can be found in most of the quarries...
, diaboleite and parkinsonite.
Type locality
The type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
is Churchill
Churchill, Somerset
Churchill is a village and civil parish in Somerset, England. It is located in the unitary authority of North Somerset, on the western edge of the Mendip Hills about east of Weston-super-Mare...
, Mendip Hills, Somerset, England, and type material is conserved at the Royal Swedish Academy of Sciences
Royal Swedish Academy of Sciences
The Royal Swedish Academy of Sciences or Kungliga Vetenskapsakademien is one of the Royal Academies of Sweden. The Academy is an independent, non-governmental scientific organization which acts to promote the sciences, primarily the natural sciences and mathematics.The Academy was founded on 2...
, Stockholm, Sweden. Other localities include Australia, Germany, Greece, Sweden, the UK and the USA.