Today I am going to discuss a rock that was presented to me at school; though when I first looked at it, I knew nothing about it. This specimen was part of my last lab assignment for Mineralogy, where I was given a sample and told to describe everything about it. I spent by far the most amount of time on this assignment, but also enjoyed it the most. The lab was the culmination of every diagnostic tool we had learned throughout the semester.
The rock itself was given to us in both hand sample and thin section so we could compare and combine our microscope work with the hand sample testing techniques like testing for hardness or describing luster.
Let me start by showing you the rock (please excuse the blurry photo, it looked sharper when I first took the photo):
The rock contains about 80% dark, large-grained (~3 mm), minerals that have a vitreous luster seen through the hand lens. While I was able to scratch the mineral with an iron nail some pressure had to be applied, so I called the hardness a 5 on the Mohs Scale. The same mineral leaves a gray-brown streak when rubbed against an unpolished porcelain plate. The other 20% of the rock is made up of a dark-brown/black mineral and a white/colorless mineral. The black/brown mineral has a hardness of 3 and exhibits one distinct cleavage plane, while shows a vitreous luster through the hand lens and a prismatic habit.
In hand sample I saw three minerals and this was confirmed through thin section work. The dark mineral seen in hand sample showed a yellowish-green color, with slight pleochroism, two good cleavage planes, moderate high relief and second order birefringence in thin section. Comparing these observations with the HS work I felt comfortable calling this mineral clinopyroxene.
The brown/black mineral seen in HS showed a tan-brown color, with one good cleavage plane, distinct pleochroism, high relief, and bird’s eye extinction. The mineral is biotite.
The colorless mineral in both HS and TS had no cleavage or pleochroism, had low relief, and first order gray birefringence. I called this mineral nepheline and ended up being very important in my final decision.
Since the rock’s main two constituents were clinopyroxene and nepheline the original thought was that it could be a nephelenite, but since the mineral grains were larger it leads to the assumption that this igneous rock was plutonic. Though in HS the clinopyroxene was dominant the presence of the nepheline kept me from calling it mafic as there was a significant amount and no zonation between the two minerals. For these reasons I feel good to quite-good about calling this rock an ijolite, an igneous rock containing mostly nepheline and augite (clinopyroxene).
I was planning on showing a picture of ijolite, but I am not up to date on my copyright laws so you’ll have to do your own Google-ing. Sorry.