Garnets were one of the first crystals that whetted my interest in minerals, rocks and eventually Geology. As I look back it was initially the color (almandine) and later the uniqueness of the dodecahedrial crystal morphology. It is almost "magical" that nature can produces these crystal shapes, although in true reductive reasoning, its all chemistry and physics in the long-run.
I grew up in the SE US (Atlanta, Georgia area) near the foothills of the Appalachian Mountains. In that heavily weathered region of metamorphic rocks we occasionally came across anthills with tiny garnets incorporated in the sand-sized gains of the hills. These little ruby red pieces of garnet were instantly interesting to us kids.
Garnets common in some metamorphic rocks of the Scottish Highlands (best I've seen are on a mountain called Beinn Oss). Legend is that they're formed from blood dripping off the 'sluaigh' or fairy host carrying away battlefield victims through the sky. There's also an alternative legend to do with 'Barrow zones'...
I also did not know garnets formed from vapors. Very cool. Er, hot! ...but that would explain the perfection of the xls, too. But the cooling must be somewhat slow, since volcanic vents into the air precipitate crusts and ' hoarfrost' type stuff, not thick xls. Right?
I should have provided a reference for this idea. The first one below suggests, but does not really address, that indeed the crystallization within the cavities was slower that the rock in general, and took place as the hot fluids (vapors and also possibly some liquids) passed through, remaining hot. Your suggestion seems logical to me, but I have difficulty grasping the nature of such crystallization.
Moscati and Johnson, 2014, Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA: Mineralogical Magazine 78:4, 1029-1041, which you can find online, and this one (which I have not read): Hollabaugh and others, 1989, The petrology and vapor phase mineralogy of rhyolite and tuffs from Garnet Hill, White Pine County, Nevada: Northwest Science, 63:5.
Garnets were one of the first crystals that whetted my interest in minerals, rocks and eventually Geology. As I look back it was initially the color (almandine) and later the uniqueness of the dodecahedrial crystal morphology. It is almost "magical" that nature can produces these crystal shapes, although in true reductive reasoning, its all chemistry and physics in the long-run.
I grew up in the SE US (Atlanta, Georgia area) near the foothills of the Appalachian Mountains. In that heavily weathered region of metamorphic rocks we occasionally came across anthills with tiny garnets incorporated in the sand-sized gains of the hills. These little ruby red pieces of garnet were instantly interesting to us kids.
Garnets common in some metamorphic rocks of the Scottish Highlands (best I've seen are on a mountain called Beinn Oss). Legend is that they're formed from blood dripping off the 'sluaigh' or fairy host carrying away battlefield victims through the sky. There's also an alternative legend to do with 'Barrow zones'...
I also did not know garnets formed from vapors. Very cool. Er, hot! ...but that would explain the perfection of the xls, too. But the cooling must be somewhat slow, since volcanic vents into the air precipitate crusts and ' hoarfrost' type stuff, not thick xls. Right?
I should have provided a reference for this idea. The first one below suggests, but does not really address, that indeed the crystallization within the cavities was slower that the rock in general, and took place as the hot fluids (vapors and also possibly some liquids) passed through, remaining hot. Your suggestion seems logical to me, but I have difficulty grasping the nature of such crystallization.
Moscati and Johnson, 2014, Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA: Mineralogical Magazine 78:4, 1029-1041, which you can find online, and this one (which I have not read): Hollabaugh and others, 1989, The petrology and vapor phase mineralogy of rhyolite and tuffs from Garnet Hill, White Pine County, Nevada: Northwest Science, 63:5.
Cool. thanks