Julgoldite
Encyclopedia
Julgoldite is a member of the pumpellyite
mineral series, a series of minerals characterized by the chemical bonding of silica tetrahedra with alkali and transition metal
cations. Julgoldites, along with more common minerals like epidote
and vesuvianite
, belong to the subclass of sorosilicates, the rock-forming minerals that contain SiO4 tetrahedra that share a common oxygen to form Si2O7 ions with a charge of 6- (Deer et al., 1996). Julgoldite has been recognized for its importance in low grade metamorphism
, forming under shear stress
accompanied by relatively low temperatures (Coombs, 1953). Julgoldite was named in honor of Professor Julian Royce Goldsmith
of the University of Chicago.
and barite
, comprising a fissure inside granular hematite
-magnetite
ore in Långban
, Sweden (Moore, 1971). Julgoldite has since been discovered in other parts of the world: Edinburgh, Scotland (in quartz dolerite) (Livingstone, 1976) and Norilsk
, Taymyr Peninsula
, Russia, one of the largest nickel
deposits in the world, in metamorphosed basalt
s and diabase
s associated with prehnite
and laumontite
(Zolotukin et al., 1965). Julgoldite has also been found exposed in basalt cavities in the Khondivili Quarry near Bombay, India along with other silicate
s, including pumpellyite-Fe2+, ilvaite
, babingtonite
, hydroandradite, prehnite
, and chlorite
(Wise and Moller, 1990). These minerals crystallized in the same basaltic cavities, which were primarily formed from gas bubble
s in the compound lava
flows; all of these Ca-Fe silicates formed in different phases of a low temperature environment (Wise and Moller, 1990).
, a thin section of the mineral will display brilliant interference figures in greens or blues (Moore, 1971). The mineral is classified under the space group A2/m (Moore, 1971). A monoclinic mineral, julgoldite is isostructural to pumpellyite and epidote (Allman and Donnay, 1973).
Pumpellyite
Pumpellyite is a group of closely related sorosilicate minerals:*pumpellyite-: Ca2MgAl2[2|SiO4|Si2O7]·*pumpellyite-: Ca2Fe2+Al2[2|SiO4|Si2O7]·*pumpellyite-: Ca22[2|SiO4|Si2O7]·H2O...
mineral series, a series of minerals characterized by the chemical bonding of silica tetrahedra with alkali and transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
cations. Julgoldites, along with more common minerals like epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
and vesuvianite
Vesuvianite
Vesuvianite, also known as idocrase, is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism...
, belong to the subclass of sorosilicates, the rock-forming minerals that contain SiO4 tetrahedra that share a common oxygen to form Si2O7 ions with a charge of 6- (Deer et al., 1996). Julgoldite has been recognized for its importance in low grade metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
, forming under shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
accompanied by relatively low temperatures (Coombs, 1953). Julgoldite was named in honor of Professor Julian Royce Goldsmith
Julian Goldsmith
Julian Royce Goldsmith was a mineralogist and geochemist at the University of Chicago . Goldsmith, along with colleague Fritz Laves, first defined the crystallographic polymorphism of alkali feldspar . Goldsmith also experimented on the temperature dependence of the solid solution between calcite...
of the University of Chicago.
Composition
The chemical formula of julgoldite is (Ca,Mn)2(Fe2+,Fe3+,Mg)(Fe3+,Al)2(SiO4)(Si2O7)(OH)2(H2O) (Moore, 1971). Pumpellyites are classified according to the prevailing metals in the X and Y sites (Moore, 1971). When Mg in the X position and Al in the Y position and both occupy greater than 50% molarity of their positions, then the mineral is identified as a pumpellyite (Moore, 1971). Julgoldites are identified when Fe2+ in the X and Fe3+ in the Y each occupy greater than 50% molarity of their positions (Moore, 1971).Geologic occurrence
Julgoldites were first collected as samples entrenched in large plates of apophylliteApophyllite
The name apophyllite refers to a specific group of phyllosilicates, a class of minerals that also includes the micas. Originally, the group name referred to a specific mineral, but was redefined in 1978 to stand for a class of minerals of similar chemical makeup that comprise a solid solution...
and barite
Barite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
, comprising a fissure inside granular hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
-magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
ore in Långban
Långban
Långban is a mining area in Värmland in Sweden. It belongs to Filipstad Municipality, with the nearest city being Filipstad, 21 km south. It was systematically mined through 1711-1972, but has traces from the 15th century. It is the birthplace of Swedish-American inventor John EricssonIt has...
, Sweden (Moore, 1971). Julgoldite has since been discovered in other parts of the world: Edinburgh, Scotland (in quartz dolerite) (Livingstone, 1976) and Norilsk
Norilsk
Norilsk is an industrial city in Krasnoyarsk Krai, Russia, located between the Yenisei River and the Taymyr Peninsula. Population: It was granted city status in 1953. It is the northernmost city in Siberia and the world's second largest city north of the Arctic Circle...
, Taymyr Peninsula
Taymyr Peninsula
The Taymyr Peninsula is a peninsula in the Far North of Russia, in the Siberian Federal District, that forms the northernmost part of mainland Eurasia and Asia...
, Russia, one of the largest nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
deposits in the world, in metamorphosed basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s and diabase
Diabase
Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...
s associated with prehnite
Prehnite
Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small...
and laumontite
Laumontite
Laumontite is a mineral, one of the zeolite group. Its molecular formula is 2·4H2O, a hydrated calcium-aluminium silicate. Potassium or sodium may substitute for the calcium but only in very small amounts....
(Zolotukin et al., 1965). Julgoldite has also been found exposed in basalt cavities in the Khondivili Quarry near Bombay, India along with other silicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
s, including pumpellyite-Fe2+, ilvaite
Ilvaite
Ilvaite is a sorosilicate of iron and calcium with formula: CaFe2+2Fe3+Si2O7O. Both manganese and magnesium substitute in the structure. Ilvaite crystallizes in the orthorhombic system in black prismatic crystals and columnar masses. It is black to brownish black to gray and opaque. It has a Mohs...
, babingtonite
Babingtonite
Babingtonite is a calcium iron manganese inosilicate mineral with the formula Ca2FeSi5O14. It is unusual in that iron completely replaces the aluminium so typical of silicate minerals. It is a very dark green to black translucent mineral crystallizing in the triclinic system with typically radial...
, hydroandradite, prehnite
Prehnite
Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small...
, and chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
(Wise and Moller, 1990). These minerals crystallized in the same basaltic cavities, which were primarily formed from gas bubble
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
s in the compound lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows; all of these Ca-Fe silicates formed in different phases of a low temperature environment (Wise and Moller, 1990).
Atomic structure
The atomic structures of pumpellyites and julgoldites consist of chains of edge-sharing octahedra linked by SiO4 , Si2O7, and CaO7 polyhedra: in the julgoldite atomic structure, the Ca site, the W site, is a seven-coordinated site with oxygen; the X and Y are two crystallographically independent octahedral sites; and the SiO4 site is tetrahedral (Passaglia and Gottardi, 1973). Like epidote, for which the chemical formula is Ca2(Fe3+,Al)Al2(SiO4)(Si2O7)(OH), julgoldite contains additional SiO4 tetrahedra that are independent of the Si2O7 structural units (Deer et al., 1996). The octahedral sites form chains along the b axis by sharing opposite edges (Allman and Donnay, 1973). The octahedral chains are joined in the ac plane by SiO4 and Si2O6 (OH) groups, forming five-member rings of two octahedra and three tetrahedral (Allman and Donnay, 1973). Half of the rings are open ended and have a Ca2+ ion in their center; the other rings are closed, and they surround a Ca2+ ion (Allman and Donnay, 1973). The X and Y chains are parallel to the crystallographic direction [010]; therefore, the two edge sharing polyhedra cause variations in the b cell parameter (Artioli et al., 2003). Two layers of X chains and one layer of Y chains occur along the [100] direction, whereas two layers of both X and Y chains occur along the [001] direction (Artioli et al., 2003).Physical properties
Moore (1971) sampled flat prismatic or bladed crystals, with the greatest dimension of each mineral to be no more than 2mm. The julgoldites found in the basalt cavities in India were almost 10 mm in length (Wise and Moller, 1990). Julgoldite is elongated parallel b [010] and flattened parallel a {100} (Moore, 1971). Allman and Donnay (1973) calculated the cell dimensions to be a 8.922(4), b 6.081(3), c 19.432 (9) Å. The color of julgoldite is usually a deep lustrous black, and it has a hardness of 4.5 and cleavage on the a-axis {100} (Moore, 1971). It has a greenish-olive powdery streak with a blue tinge (Moore, 1971). Under the petrographic microscopePetrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...
, a thin section of the mineral will display brilliant interference figures in greens or blues (Moore, 1971). The mineral is classified under the space group A2/m (Moore, 1971). A monoclinic mineral, julgoldite is isostructural to pumpellyite and epidote (Allman and Donnay, 1973).