Post by 1dave on Aug 23, 2020 11:40:41 GMT -7
Garnets are created under extreme pressure over long periods of time - such as the boundary areas when continents collide.
www.esci.umn.edu/courses/1001/minerals/garnet.shtml
In Our Earth: The Geologic Importance of Garnet
Garnets occur in some igneous rocks and pegmatites, but are more typical of high-grade metamorphic rocks. Although identifying individual varieties of garnet is not important to a non-specialist, geologists use these different varieties as a way to gauge a rock’s metamorphic grade (the degree to which it has been altered), as the different garnets’ formation and composition reflects the temperature and pressure conditions their host rock endured. Hence garnets tend to record the geologic history of their host rock.
Almandine, the iron-rich garnet variety, commonly forms from regional metamorphism of clay sediments and is the most common garnet in schists and gneisses. It also occurs in silica-rich igneous rocks, such as granite, rhyolite, or associated pegmatites.
Pyrope, the magnesium-rich variety is more common in mafic igneous rocks and metamorphosed mafic igneous rocks,
while the calcium-rich garnets, Grossular and Andradite, are commonly found in metamorphosed carbonate rocks.
Spessartine is found in granite pegmatites and metamorphosed manganese-rich rocks,
while Uvarovite is the rarest garnet variety, only occurring as crusts or seams in some chromium deposits.
As garnets are chemically and physically resistant they often survive the erosion of metamorphic rocks, becoming loose garnet grains. These garnet sands can be concentrated by wave and current action into ‘heavy’ mineral sand deposits.
In most metamorphic rocks, garnets are typically associated with the mica minerals and other metamorphic minerals such as staurolite, kyanite, and sillimanite. Calcium-rich garnet varieties usually form in metamorphosed carbonate rocks where they may be found with calcite, wollastonite, or metallic ore deposits. Garnet grains in sedimentary sandstones usually occur with other heavy mineral grains or resistant minerals such as quartz.
However I ran across this image recently which helps visualize garnet group relationships by color and chemistry (no R.I. info unfortunately). But it helps one grasp how the groups are linked and the almost endless number of hues garnets can exhibit.
@rockpowell and Sugar Babe visit one of their secret slave labor Garnet Mines in the Wah Wah Mountains of Utah.
Ants labor without pay extracting garnets, piling them on top of their ant hill.
As garnets have lots of iron in them, a magnet covered with a baggie after being connected to an old ski pole easily attracts the garnets. Reverse the baggie and pull it off the magnet full of the stolen garnets, then off to the next slave mine.
Checking anthills will reveal what crystals are in the area - quartz, garnet, peridot, etc.
Those little guys explore the underground far better than we can.
EDIT: @gingerkid asked "What kind of garnets?" I blithely replied "Almandine because they are magnetic so must have a lot of iron."
@gemfeller pointed out "All garnets are magnetic!" and pointed out a fantastic website!
A couple of brief glimpses:
www.esci.umn.edu/courses/1001/minerals/garnet.shtml
Description and Identifying Characteristics
The word ‘garnet’ comes from the Latin word granatus, which means ‘seed-like,’ a reference to the common appearance of garnets as discrete small red rounded crystals that look like pomegranate seeds embedded in rock. The different garnet varieties comprise a group of minerals that all share a similar crystal structure, but whose chemical composition varies as different ions substitute for one another in that crystal structure. Most garnet crystals are actually mixtures of different garnet varieties rather than being composed solely of a single variety. As a result, there is little reason for non-specialists to worry about identifying individual garnet varieties.
Although red garnets are the most common variety, garnets occur in nearly every color except for blue (no longer true -Dave). This variability in color results from the garnet crystals’ transparent to translucent nature, which allows minor impurities to greatly affect its appearance. Despite their variable appearance, garnets are usually easy to identify by their hardness, crystal habit and occurrence in metamorphic rock. Garnets usually form at high temperature and pressure, so they typically occur in their crystal form as rounded dodecahedrons (twelve-sided) or twenty-four sided trapezohedrons.
Iron can substitute relatively easily for either magnesium or manganese in the garnet structure, but calcium is a larger ion and does not substitute as freely for iron, magnesium or manganese.
As a result, the garnet group can be subdivided into two series of minerals – an aluminum silicate series in which Fe+2 Mg+2 and Mn+2 freely substitute for one another and a calcium silicate series in which Cr+3, Al+3 and Fe+3 substitute for one another. Natural garnets are almost always a combination of these end-members, rather than being a pure sample of one composition.
Almandine Fe3Al2(SiO4)3 Grossular Ca3Al2(SiO4)3
Pyrope Mg3Al2(SiO4)3 Andradite Ca3Fe2(SiO4)3
Spessartine Mn3Al2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3
The word ‘garnet’ comes from the Latin word granatus, which means ‘seed-like,’ a reference to the common appearance of garnets as discrete small red rounded crystals that look like pomegranate seeds embedded in rock. The different garnet varieties comprise a group of minerals that all share a similar crystal structure, but whose chemical composition varies as different ions substitute for one another in that crystal structure. Most garnet crystals are actually mixtures of different garnet varieties rather than being composed solely of a single variety. As a result, there is little reason for non-specialists to worry about identifying individual garnet varieties.
Although red garnets are the most common variety, garnets occur in nearly every color except for blue (no longer true -Dave). This variability in color results from the garnet crystals’ transparent to translucent nature, which allows minor impurities to greatly affect its appearance. Despite their variable appearance, garnets are usually easy to identify by their hardness, crystal habit and occurrence in metamorphic rock. Garnets usually form at high temperature and pressure, so they typically occur in their crystal form as rounded dodecahedrons (twelve-sided) or twenty-four sided trapezohedrons.
Iron can substitute relatively easily for either magnesium or manganese in the garnet structure, but calcium is a larger ion and does not substitute as freely for iron, magnesium or manganese.
As a result, the garnet group can be subdivided into two series of minerals – an aluminum silicate series in which Fe+2 Mg+2 and Mn+2 freely substitute for one another and a calcium silicate series in which Cr+3, Al+3 and Fe+3 substitute for one another. Natural garnets are almost always a combination of these end-members, rather than being a pure sample of one composition.
Almandine Fe3Al2(SiO4)3 Grossular Ca3Al2(SiO4)3
Pyrope Mg3Al2(SiO4)3 Andradite Ca3Fe2(SiO4)3
Spessartine Mn3Al2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3
In Our Earth: The Geologic Importance of Garnet
Garnets occur in some igneous rocks and pegmatites, but are more typical of high-grade metamorphic rocks. Although identifying individual varieties of garnet is not important to a non-specialist, geologists use these different varieties as a way to gauge a rock’s metamorphic grade (the degree to which it has been altered), as the different garnets’ formation and composition reflects the temperature and pressure conditions their host rock endured. Hence garnets tend to record the geologic history of their host rock.
Almandine, the iron-rich garnet variety, commonly forms from regional metamorphism of clay sediments and is the most common garnet in schists and gneisses. It also occurs in silica-rich igneous rocks, such as granite, rhyolite, or associated pegmatites.
Pyrope, the magnesium-rich variety is more common in mafic igneous rocks and metamorphosed mafic igneous rocks,
while the calcium-rich garnets, Grossular and Andradite, are commonly found in metamorphosed carbonate rocks.
Spessartine is found in granite pegmatites and metamorphosed manganese-rich rocks,
while Uvarovite is the rarest garnet variety, only occurring as crusts or seams in some chromium deposits.
As garnets are chemically and physically resistant they often survive the erosion of metamorphic rocks, becoming loose garnet grains. These garnet sands can be concentrated by wave and current action into ‘heavy’ mineral sand deposits.
In most metamorphic rocks, garnets are typically associated with the mica minerals and other metamorphic minerals such as staurolite, kyanite, and sillimanite. Calcium-rich garnet varieties usually form in metamorphosed carbonate rocks where they may be found with calcite, wollastonite, or metallic ore deposits. Garnet grains in sedimentary sandstones usually occur with other heavy mineral grains or resistant minerals such as quartz.
May 7, 2019 10:02:03 GMT -7 @gemfeller said:
Don't even get me started on he subject of garnet trade-names. As R2D says, they make and already-difficult subject infinitely more confusing. Dr. W. William Hanneman Ph.D wrote a very interesting book on the subject: "Naming Gem Garnets." Early editions contained a visual aid for showing the garnet groups and the miscibility between them, illustrating intermediate hues by R.I. and color. Unfortunately, that edition is mostly extinct and I think the newer edition (if you can find it!) doesn't have the little pyramidal "helper."However I ran across this image recently which helps visualize garnet group relationships by color and chemistry (no R.I. info unfortunately). But it helps one grasp how the groups are linked and the almost endless number of hues garnets can exhibit.
@rockpowell and Sugar Babe visit one of their secret slave labor Garnet Mines in the Wah Wah Mountains of Utah.
Ants labor without pay extracting garnets, piling them on top of their ant hill.
As garnets have lots of iron in them, a magnet covered with a baggie after being connected to an old ski pole easily attracts the garnets. Reverse the baggie and pull it off the magnet full of the stolen garnets, then off to the next slave mine.
Checking anthills will reveal what crystals are in the area - quartz, garnet, peridot, etc.
Those little guys explore the underground far better than we can.
EDIT: @gingerkid asked "What kind of garnets?" I blithely replied "Almandine because they are magnetic so must have a lot of iron."
@gemfeller pointed out "All garnets are magnetic!" and pointed out a fantastic website!
A couple of brief glimpses: