Integrative Natural History of Minnesota's North Shore, Part GM-A: A Rapakivi Rant | Grand Marais
(Updated September 4, 2024)
Looking more or less straight down at a portion of the check-in counter in the lobby of the Best Western Plus Superior Inn.
The marking pen is provided for scale. Its total length = 5.4 in (13.8 cm).
It's time for a little motel petrology.
One of the great dangers of being an architectural geologist is that you find something interesting almost anywhere you look in the built environment. In this case, I came upon this nicely polished surface of what is marketed as "Baltic Brown Granite," and what I call, in my own classification system, Baltic Brown Wiborgite.
Quarried from the massive Wiborg Batholith in southeastern Finland, it's the quintessential rapakivi granite. Because the entire batholith's isotopic date range is from 1.646 to 1.627 Ga, this particular rock type must be late-Paleoproterozoic in age. (My source for this is "Age and Isotopic Fingerprints of Some Plutonic Rocks in the Wiborg Rapakivi Granite Batholith with Special Reference to the Dark Wiborgite of the Ristisaari Island," Rämö et al., 2014.)
Rapakivis, including Baltic Brown and Wisconsin's Waupaca Granite, are strikingly patterned igneous intrusive rocks that have large zones of an alkali feldspar haloed in pale sodic plagioclase feldspar. And wiborgites are those rapakivis with their alkali feldspar crystals in the form of large ovoids. In other words, they're rather egg-shaped.
One of the things I like the most about this popular ornamental-stone selection is how thick and pale-greenish the plagioclase jackets are. Rapakivis characteristically have as their plagioclase component oligoclase, and the alkali feldspar interiors are orthoclase. So I assume that's the case here. Other constituents include gray, glassy quartz and the black hornblende that makes the contrasting dark matrix for the mantled ovoids.
In the next shot of this set, I'll show one other part of this countertop, and discuss how it's thought rapakivis come into being in the first place.
In the meantime, if you're intrigued by Baltic Brown Wiborgite and would also like to learn more about other related granites produced in the Wiborg Batholith, take a look at the nicely produced and clearly written "Natural Stone Production in the Wiborg Rapakivi Granite Batholith in Southeastern Finland" (Härmä & Selonen 2018).
To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.
Integrative Natural History of Minnesota's North Shore, Part GM-A: A Rapakivi Rant | Grand Marais
(Updated September 4, 2024)
Looking more or less straight down at a portion of the check-in counter in the lobby of the Best Western Plus Superior Inn.
The marking pen is provided for scale. Its total length = 5.4 in (13.8 cm).
It's time for a little motel petrology.
One of the great dangers of being an architectural geologist is that you find something interesting almost anywhere you look in the built environment. In this case, I came upon this nicely polished surface of what is marketed as "Baltic Brown Granite," and what I call, in my own classification system, Baltic Brown Wiborgite.
Quarried from the massive Wiborg Batholith in southeastern Finland, it's the quintessential rapakivi granite. Because the entire batholith's isotopic date range is from 1.646 to 1.627 Ga, this particular rock type must be late-Paleoproterozoic in age. (My source for this is "Age and Isotopic Fingerprints of Some Plutonic Rocks in the Wiborg Rapakivi Granite Batholith with Special Reference to the Dark Wiborgite of the Ristisaari Island," Rämö et al., 2014.)
Rapakivis, including Baltic Brown and Wisconsin's Waupaca Granite, are strikingly patterned igneous intrusive rocks that have large zones of an alkali feldspar haloed in pale sodic plagioclase feldspar. And wiborgites are those rapakivis with their alkali feldspar crystals in the form of large ovoids. In other words, they're rather egg-shaped.
One of the things I like the most about this popular ornamental-stone selection is how thick and pale-greenish the plagioclase jackets are. Rapakivis characteristically have as their plagioclase component oligoclase, and the alkali feldspar interiors are orthoclase. So I assume that's the case here. Other constituents include gray, glassy quartz and the black hornblende that makes the contrasting dark matrix for the mantled ovoids.
In the next shot of this set, I'll show one other part of this countertop, and discuss how it's thought rapakivis come into being in the first place.
In the meantime, if you're intrigued by Baltic Brown Wiborgite and would also like to learn more about other related granites produced in the Wiborg Batholith, take a look at the nicely produced and clearly written "Natural Stone Production in the Wiborg Rapakivi Granite Batholith in Southeastern Finland" (Härmä & Selonen 2018).
To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.