3D framework of linked tetrahedra
|Crystal system||Hexagonal (rhombohedral)|
White (milky quartz)
Pink (rose quartz))
Brown or black (smoky quartz)
|Transparency||Translucent to transparent|
|XPL||1st order grey
Will appear black through full
rotation if looking down the C axis
|after Perkins, 303|
Quartz in Hand Sample
Cryptocrystalline Quartz Minerals
Beekite, a type of chalcedony which forms when silica replaces carbonate minerals in fossils, present as concentric rings in a Devonian-aged fossiliferous sandstone
Agate (dyed, but you’ll get the idea)
Agate in volcaniclastic breccia (!) from Mount St. Helens
Quartz in Thin Section
Thin Section GigaPans
Looking for ghost boundaries/ghost structures? Head on over to the quartzite page.
Quartz in quartzite, crossed polars (with bonus hair in the field of view; oops)
The Vagaries of Quartz Extinction
All the textbooks say that quartz is characterized by undulatory (or undulose) extinction.
It’s not that simple.
Sometimes it’s nice and smooth. Sometimes it’s genuinely undulatory. Sometimes the grain has been through an orogeny or something and is just a mess.
If it’s more than 30µ thick, quartz will display 1st order yellows instead of 1st order greys. This is pretty common; sometimes it’s better to let the quartz be a little thick than to risk grinding too much material off your thin section. Yellowish quartz is an acceptable sacrifice, because you can still infer that it’s quartz by its extinction patterns. The quartz in this biotite gneiss is a great example. Had the person who made the thin section ground the sample down any farther, much of the feldspars would have been lost.
This slide obligingly provided a 30µ quartz grain beside a thicker one, so you can see the comparison.
Sometimes you’ll be staring at a grain that you just know is quartz, but it’s black, and it stays black through a whole rotation of the stage. It’s okay. It’s quartz. It’s just angled so that you’re looking straight down the C-axis.