Anorthosite and asphalt

I spent two days this week on a field trip in western Norway, where our guide assures us that the world’s best (ultra) high pressure rocks are located. It is my plan to do a proper field trip post, including photos, but in the mean time I’ll share with you a snippet of information I learned about one of the rock types we drove past, but did not stop at.

Near the town of Florø, Norway, there are a variety of white road cuts which are made of anorthosite. The primary (only?) mineral in anorthosite is plagioclase feldspar. In this area anorthosite is often quarried and used for making asphalt. Why? Several reasons:

* White colour of anorthosite makes whiter asphalt = better visibility for driving at night or under cloudy conditions.

* Plagioclase hardness (~6 on Mohs scale) makes for more durable, and therefore longer lasting asphalt.

* The plagioclase cleavage planes means that as cars drive on it and break it down the crystals maintain their edges, which provides better traction, even when wet.

Alas, I didn’t manage to get a photo of any of the anorthosite outcrops we passed—my camera has too long of a delay between pushing the button and taking a photo, which means that by the time I saw the outcrop it was too late to photograph it at the speeds our bus was traveling. But I was able to find a photo on line of an outcrop down near Bergen, which appears on this web page.