Metamorphic Garnet Schist ID'd (Alaska)
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This video identifies a metamorphic
This video identifies a metamorphic quartz-biotite schist, that contains large garnets (using 360 degree photography). The red color, almandine garnets in this sample are 25 millimeters in diameter (1.0 inch). Orange limonite weathering also occurs within areas of the rock.
Garnet crystals have a distinctive 12-sided crystal form--which other common minerals do not have). Although garnet doesn't have as many sides as a soccer ball, garnet crystals have kind of a soccer ball shape. With its general reddish-brown color (for almandine garnet), hardness more than glass, and distinctive crystal shape--garnet is fairly easy to identify. Note that the different types of garnet are: pyrope, almandine, spessartine, grossular (hessonite and tsavorite), uvarovite, and andradite.
Gem-grade garnets are used in jewelry, and common low-grade garnets are used as an abrasive (such as in sand blasting sand, and sand paper). One factor that confuses garnet identification is how it breaks. Despite the 12-sided crystals, garnet breaks into fractures--and often "bladed" fractures. Again, a bladed fracture means garnet often breaks somewhat randomly into thick, somewhat flat-shaped splinters. But these bladed forms are not cleavages--and obviously are not forming cleavages that are parallel to garnet's 12-sided crystal form. The fracture in garnet is different than the conchoidal fractures related to the mineral quartz.
This rock sample is from the Wrangle area of Alaska. Garnets from this source may be as large as golf ball size (diameter of 44 millimeters, or 1.7 inches). Here is an old, detailed report on the garnet deposit [Reference link: https://pubs.usgs.gov/bul/0963c/repor...]. Many garnets from this area are commercially sold. Reportedly, a significant part of the deposit was deeded to the Boy Scouts of Alaska (in the 1960s), and in the early 2000s, was transferred to a local Presbyterian Church. [Reference link: https://voyij.com/wrangell-garnets-44...]
Hello Rock People! This is Dave the Geology Dude. Identification notes are below in this text. Note that I have other videos that provide more rock identification information on my YouTube channel. It is best to try to see many examples of the same rock type--because only then you understand the range of variables that can be associated with each rock type. Also, be aware that the only way to really learn rock identification, is to have hands-on experience. This is because visual experiences alone do not show the physical properties like hardness, acid reactivity, and the range of variables that are common to each rock type. Don't forget that: Geology isn't rocket science--it's Rock Science! Please "like" and subscribe to this YouTube channel!
Identification information for garnet minerals (not this rock):
1) environment: garnet forms in metamorphic environments
2) shape/cleavages: garnet has no cleavages, and grows in a 12-sided crystal form--called a dodecahedron. It sort of looks like a soccer ball (although does not have as many sides). Garnet breaks in a "bladed" fashion. Which means no cleavages but tends to break into strips and flats--but do not break along a cleavage plane. This sample is a dodecahedron that was broken. So about ⅓ of the crystal is broken through in a semi-flat plane.
3) color--usually a reddish-brown color (almandine), although there are other colors too (see above for varieties of garnet). But for most geology classes, the reddish-brown almandine garnets are most often used.
4) density--it is about the same as most rocks
5) hardness--harder than glass
6) acid reactivity: will not react to acid contact
7) luster--It is non-metallic glassy--opaque.
8) other characteristics--12-sided crystal is distinctive.
9) often confused with hematite, because of the red color, but hematite is similar to glass and more earthy in luster when red colored. It can be confused with quartz, because of its relatively high hardness, although most quartz minerals are not red in color (although it can happen--such as with some jaspers)
10) summary: key factors is the generally reddish-brown color, 12-sided crystal form, and harder than glass
Music (YouTube Audio Library): Fractal of Light (by Chris Hague)
Copyright by David Knoblach, 2023. All rights reserved
Garnet crystals have a distinctive 12-sided crystal form--which other common minerals do not have). Although garnet doesn't have as many sides as a soccer ball, garnet crystals have kind of a soccer ball shape. With its general reddish-brown color (for almandine garnet), hardness more than glass, and distinctive crystal shape--garnet is fairly easy to identify. Note that the different types of garnet are: pyrope, almandine, spessartine, grossular (hessonite and tsavorite), uvarovite, and andradite.
Gem-grade garnets are used in jewelry, and common low-grade garnets are used as an abrasive (such as in sand blasting sand, and sand paper). One factor that confuses garnet identification is how it breaks. Despite the 12-sided crystals, garnet breaks into fractures--and often "bladed" fractures. Again, a bladed fracture means garnet often breaks somewhat randomly into thick, somewhat flat-shaped splinters. But these bladed forms are not cleavages--and obviously are not forming cleavages that are parallel to garnet's 12-sided crystal form. The fracture in garnet is different than the conchoidal fractures related to the mineral quartz.
This rock sample is from the Wrangle area of Alaska. Garnets from this source may be as large as golf ball size (diameter of 44 millimeters, or 1.7 inches). Here is an old, detailed report on the garnet deposit [Reference link: https://pubs.usgs.gov/bul/0963c/repor...]. Many garnets from this area are commercially sold. Reportedly, a significant part of the deposit was deeded to the Boy Scouts of Alaska (in the 1960s), and in the early 2000s, was transferred to a local Presbyterian Church. [Reference link: https://voyij.com/wrangell-garnets-44...]
Hello Rock People! This is Dave the Geology Dude. Identification notes are below in this text. Note that I have other videos that provide more rock identification information on my YouTube channel. It is best to try to see many examples of the same rock type--because only then you understand the range of variables that can be associated with each rock type. Also, be aware that the only way to really learn rock identification, is to have hands-on experience. This is because visual experiences alone do not show the physical properties like hardness, acid reactivity, and the range of variables that are common to each rock type. Don't forget that: Geology isn't rocket science--it's Rock Science! Please "like" and subscribe to this YouTube channel!
Identification information for garnet minerals (not this rock):
1) environment: garnet forms in metamorphic environments
2) shape/cleavages: garnet has no cleavages, and grows in a 12-sided crystal form--called a dodecahedron. It sort of looks like a soccer ball (although does not have as many sides). Garnet breaks in a "bladed" fashion. Which means no cleavages but tends to break into strips and flats--but do not break along a cleavage plane. This sample is a dodecahedron that was broken. So about ⅓ of the crystal is broken through in a semi-flat plane.
3) color--usually a reddish-brown color (almandine), although there are other colors too (see above for varieties of garnet). But for most geology classes, the reddish-brown almandine garnets are most often used.
4) density--it is about the same as most rocks
5) hardness--harder than glass
6) acid reactivity: will not react to acid contact
7) luster--It is non-metallic glassy--opaque.
8) other characteristics--12-sided crystal is distinctive.
9) often confused with hematite, because of the red color, but hematite is similar to glass and more earthy in luster when red colored. It can be confused with quartz, because of its relatively high hardness, although most quartz minerals are not red in color (although it can happen--such as with some jaspers)
10) summary: key factors is the generally reddish-brown color, 12-sided crystal form, and harder than glass
Music (YouTube Audio Library): Fractal of Light (by Chris Hague)
Copyright by David Knoblach, 2023. All rights reserved
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