Post by Admin on Apr 29, 2022 13:50:22 GMT -7
The most common (and UNCOMMON) CRYSTALS ON EARTH! Made of silicon, aluminum, sodium, calcium, potassium or barium, they make up about 60% of the Earth's crust. They have Mohs Hardness: 6.0 to 6.5 so make good gemstones. Feldspars, especially plagioclase feldspars, are not very stable at the earth's surface due to their high formation temperature.[32] This lack of stability is why feldspars are easily weathered to clays.
One of the most diagnostic properties of feldspar is its two directions of cleavage that usually intersect at or close to ninety degrees. They all have a vitreous luster that is often pearly on cleavage faces.
Cats eye
Moonstone is a sodium potassium aluminium silicate of the feldspar group that displays a pearly and opalescent schiller
Adularescence
Moonstone’s unearthly glow is caused by light scattering between microscopic layers of feldspar. Other feldspar minerals can also show adularescence including labradorite and sanidine.
Sunstone is found in Pleistocene basalt flows with small plageoclaze at Sunstone Knoll in Millard County, Utah.
dustdevilmining.com/about-oregon-sunstone/
Star Stones
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".
The optical effect is due to reflections from inclusions of red copper, hematite, or goethite, in the form of minute scales, which are hexagonal, rhombic, or irregular in shape, and are disposed parallel to the principal cleavage-plane. These inclusions give the stone an appearance something like that of aventurine, hence sunstone is known also as "aventurine-feldspar". The optical effect called schiller and the color in Oregon Sunstone is due to copper. The middle part of this crystal sparkles, and usually the color is darkest in the middle and becomes lighter toward the outer edges.
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".
"Oregon sunstone is a type of labradorite, a sub-group of plagioclase feldspar. The rock in which the sunstone is found outside of Plush is thought to be part of the Steens Mountain Formation, an eruption that occurred about 16 million years ago that pushed the feldspar crystals onto the surface via the molten lava, according to Jason McClaughry, a field geologist for the Oregon Department of Geology and Mineral Industries.
Scientists are still not sure if the sunstone-bearing flows in Harney and Lake counties are related. In fact, there isn’t much scientific research conducted on Oregon sunstone as there are only theories on how the gems were formed and what creates the “schiller” inside the stone, traditionally thought to be copper inclusions."
www.gia.edu/doc/Spring-2011-Gems-Gemology-Chinese-Red-Feldspar-Controversy.pdf
The feldspar group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
potassium feldspar (K-spar) endmember KAlSi3O8[11]
albite endmember NaAlSi3O8[11]
anorthite endmember CaAl2Si2O8[11]
Solid solutions between K-feldspar and albite are called alkali feldspar.[11] Solid solutions between albite and anorthite are called plagioclase,[11] or, more properly, plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both a plagioclase and alkali feldspar.
The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of quartz, is the basis for the QAPF classification of igneous rock.[12][13][14] Calcium-rich plagioclase is the first feldspar to crystallize from a cooling magma, but the plagioclase becomes increasingly sodium-rich as crystallization continues. This defines the continuous Bowen's reaction series. K-feldspar is the final feldspar to crystallize from the magma.[15][16]
Alkali feldspars
Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these include:
orthoclase (monoclinic)[17] KAlSi3O8
sanidine (monoclinic)[18] (K,Na)AlSi3O8
microcline (triclinic)[19] KAlSi3O8
anorthoclase (triclinic) (Na,K)AlSi3O8
Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.[20] Sanidine is stable at the highest temperatures, and microcline at the lowest.[17][18] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.[21] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
Ammonium feldspar
Buddingtonite is an ammonium feldspar with the chemical formula: NH4AlSi3O8.[22] It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.
Barium feldspars
Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Barium feldspars are sometimes classified as a separate group of feldspars,[4] and sometimes they are classified as a sub-group of alkali feldspars.[23]
The barium feldspars are monoclinic and include the following:
celsian BaAl2Si2O8[24]
hyalophane (K,Ba)(Al,Si)4O8[25]
Plagioclase feldspars
The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):
albite (0 to 10) NaAlSi3O8
oligoclase (10 to 30) (Na,Ca)(Al,Si)AlSi2O8
andesine (30 to 50) NaAlSi
3O
8 – CaAl
2Si
2O
8
labradorite (50 to 70) (Ca,Na)Al(Al,Si)Si2O8
bytownite (70 to 90) (NaSi,CaAl)AlSi2O8
anorthite (90 to 100) CaAl2Si2O8
Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75).
Structure
The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Each oxygen ion, in turn, is shared by a neighbouring tetrahedron to form a three-dimensional network. The structure can be visualized as long chains of aluminosilicate tetrahedra, sometimes described as crankshaft chains because their shape is kinked. Each crankshaft chain links to neighbouring crankshaft chains to form a three-dimensional network of fused four-member rings. The structure is open enough for cations (typically sodium, potassium, or calcium) to fit into the structure and provide charge balance.[26]
Diagram showing part of a crankshaft chain of feldspar
Feldspar crystal structure viewed along the c axis
Feldspar crystal structure viewed along the a axis
Feldspar crystal structure viewed along the b axis
Etymology
The name feldspar derives from the German Feldspat, a compound of the words Feld ("field") and Spat ("flake"). Spat had long been used as the word for "a rock easily cleaved into flakes"; Feldspat was introduced in the 18th century as a more specific term, referring perhaps to its common occurrence in rocks found in fields (Urban Brückmann, 1783) or to its occurrence as "fields" within granite and other minerals (René-Just Haüy, 1804).[27] The change from Spat to -spar was influenced by the English word spar,[28] meaning a non-opaque mineral with good cleavage.[29] Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has fallen out of use. The term 'felsic', meaning light coloured minerals such as quartz and feldspars, is an acronymic word derived from feldspar and silica, unrelated to the obsolete spelling 'felspar'.
Weathering
Chemical weathering of feldspars happens by hydrolysis and produces clay minerals, including illite, smectite, and kaolinite. Hydrolysis of feldspars begins with the feldspar dissolving in water, which happens best in acidic or basic solutions and less well in neutral ones.[30] The speed at which feldspars are weathered is controlled by how quickly they are dissolved.[30] Dissolved feldspar reacts with H+ or OH− ions and precipitates clays. The reaction also produces new ions in solution, with the variety of ion controlled by the type of feldspar reacting.
The abundance of feldspars in the Earth's crust means that clays are very abundant weathering products.[31] About 40% of minerals in sedimentary rocks are clays, and clays are the dominant minerals in the most common sedimentary rocks, mudrocks.[32] They are also an important component of soils.[32] Feldspar that has been replaced by clay looks chalky compared to more crystalline and glassy unweathered feldspar grains.[33]
Feldspars, especially plagioclase feldspars, are not very stable at the earth's surface due to their high formation temperature.[32] This lack of stability is why feldspars are easily weathered to clays. Because of this tendency to weather easily, feldspars are usually not prevalent in sedimentary rocks. Sedimentary rocks that contain large amounts of feldspar indicate that the sediment did not undergo much chemical weathering before being buried. This means it was probably transported a short distance in cold and/or dry conditions that didn't promote weathering, and that it was quickly buried by other sediment.[34] Sandstones with large amounts of feldspar are called arkoses.[34]
One of the most diagnostic properties of feldspar is its two directions of cleavage that usually intersect at or close to ninety degrees. They all have a vitreous luster that is often pearly on cleavage faces.
Cats eye
Moonstone is a sodium potassium aluminium silicate of the feldspar group that displays a pearly and opalescent schiller
Adularescence
Moonstone’s unearthly glow is caused by light scattering between microscopic layers of feldspar. Other feldspar minerals can also show adularescence including labradorite and sanidine.
Sunstone is found in Pleistocene basalt flows with small plageoclaze at Sunstone Knoll in Millard County, Utah.
dustdevilmining.com/about-oregon-sunstone/
Star Stones
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".
The optical effect is due to reflections from inclusions of red copper, hematite, or goethite, in the form of minute scales, which are hexagonal, rhombic, or irregular in shape, and are disposed parallel to the principal cleavage-plane. These inclusions give the stone an appearance something like that of aventurine, hence sunstone is known also as "aventurine-feldspar". The optical effect called schiller and the color in Oregon Sunstone is due to copper. The middle part of this crystal sparkles, and usually the color is darkest in the middle and becomes lighter toward the outer edges.
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone".
"Oregon sunstone is a type of labradorite, a sub-group of plagioclase feldspar. The rock in which the sunstone is found outside of Plush is thought to be part of the Steens Mountain Formation, an eruption that occurred about 16 million years ago that pushed the feldspar crystals onto the surface via the molten lava, according to Jason McClaughry, a field geologist for the Oregon Department of Geology and Mineral Industries.
Scientists are still not sure if the sunstone-bearing flows in Harney and Lake counties are related. In fact, there isn’t much scientific research conducted on Oregon sunstone as there are only theories on how the gems were formed and what creates the “schiller” inside the stone, traditionally thought to be copper inclusions."
www.gia.edu/doc/Spring-2011-Gems-Gemology-Chinese-Red-Feldspar-Controversy.pdf
The feldspar group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
potassium feldspar (K-spar) endmember KAlSi3O8[11]
albite endmember NaAlSi3O8[11]
anorthite endmember CaAl2Si2O8[11]
Solid solutions between K-feldspar and albite are called alkali feldspar.[11] Solid solutions between albite and anorthite are called plagioclase,[11] or, more properly, plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Albite is considered both a plagioclase and alkali feldspar.
The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of quartz, is the basis for the QAPF classification of igneous rock.[12][13][14] Calcium-rich plagioclase is the first feldspar to crystallize from a cooling magma, but the plagioclase becomes increasingly sodium-rich as crystallization continues. This defines the continuous Bowen's reaction series. K-feldspar is the final feldspar to crystallize from the magma.[15][16]
Alkali feldspars
Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these include:
orthoclase (monoclinic)[17] KAlSi3O8
sanidine (monoclinic)[18] (K,Na)AlSi3O8
microcline (triclinic)[19] KAlSi3O8
anorthoclase (triclinic) (Na,K)AlSi3O8
Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.[20] Sanidine is stable at the highest temperatures, and microcline at the lowest.[17][18] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.[21] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
Ammonium feldspar
Buddingtonite is an ammonium feldspar with the chemical formula: NH4AlSi3O8.[22] It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.
Barium feldspars
Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Barium feldspars are sometimes classified as a separate group of feldspars,[4] and sometimes they are classified as a sub-group of alkali feldspars.[23]
The barium feldspars are monoclinic and include the following:
celsian BaAl2Si2O8[24]
hyalophane (K,Ba)(Al,Si)4O8[25]
Plagioclase feldspars
The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):
albite (0 to 10) NaAlSi3O8
oligoclase (10 to 30) (Na,Ca)(Al,Si)AlSi2O8
andesine (30 to 50) NaAlSi
3O
8 – CaAl
2Si
2O
8
labradorite (50 to 70) (Ca,Na)Al(Al,Si)Si2O8
bytownite (70 to 90) (NaSi,CaAl)AlSi2O8
anorthite (90 to 100) CaAl2Si2O8
Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75).
Structure
The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Each oxygen ion, in turn, is shared by a neighbouring tetrahedron to form a three-dimensional network. The structure can be visualized as long chains of aluminosilicate tetrahedra, sometimes described as crankshaft chains because their shape is kinked. Each crankshaft chain links to neighbouring crankshaft chains to form a three-dimensional network of fused four-member rings. The structure is open enough for cations (typically sodium, potassium, or calcium) to fit into the structure and provide charge balance.[26]
Diagram showing part of a crankshaft chain of feldspar
Feldspar crystal structure viewed along the c axis
Feldspar crystal structure viewed along the a axis
Feldspar crystal structure viewed along the b axis
Etymology
The name feldspar derives from the German Feldspat, a compound of the words Feld ("field") and Spat ("flake"). Spat had long been used as the word for "a rock easily cleaved into flakes"; Feldspat was introduced in the 18th century as a more specific term, referring perhaps to its common occurrence in rocks found in fields (Urban Brückmann, 1783) or to its occurrence as "fields" within granite and other minerals (René-Just Haüy, 1804).[27] The change from Spat to -spar was influenced by the English word spar,[28] meaning a non-opaque mineral with good cleavage.[29] Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has fallen out of use. The term 'felsic', meaning light coloured minerals such as quartz and feldspars, is an acronymic word derived from feldspar and silica, unrelated to the obsolete spelling 'felspar'.
Weathering
Chemical weathering of feldspars happens by hydrolysis and produces clay minerals, including illite, smectite, and kaolinite. Hydrolysis of feldspars begins with the feldspar dissolving in water, which happens best in acidic or basic solutions and less well in neutral ones.[30] The speed at which feldspars are weathered is controlled by how quickly they are dissolved.[30] Dissolved feldspar reacts with H+ or OH− ions and precipitates clays. The reaction also produces new ions in solution, with the variety of ion controlled by the type of feldspar reacting.
The abundance of feldspars in the Earth's crust means that clays are very abundant weathering products.[31] About 40% of minerals in sedimentary rocks are clays, and clays are the dominant minerals in the most common sedimentary rocks, mudrocks.[32] They are also an important component of soils.[32] Feldspar that has been replaced by clay looks chalky compared to more crystalline and glassy unweathered feldspar grains.[33]
Feldspars, especially plagioclase feldspars, are not very stable at the earth's surface due to their high formation temperature.[32] This lack of stability is why feldspars are easily weathered to clays. Because of this tendency to weather easily, feldspars are usually not prevalent in sedimentary rocks. Sedimentary rocks that contain large amounts of feldspar indicate that the sediment did not undergo much chemical weathering before being buried. This means it was probably transported a short distance in cold and/or dry conditions that didn't promote weathering, and that it was quickly buried by other sediment.[34] Sandstones with large amounts of feldspar are called arkoses.[34]