Quartz

Physical Properties
Chemical formula SiO2
Class Tectosilicate
3D framework of linked tetrahedra
Crystal system Hexagonal (rhombohedral)
Habit Prismatic
Massive
Color Colorless
White (milky quartz)
Purple (amethyst)
Pink (rose quartz))
Yellow (citrine)
Brown or black (smoky quartz)
Hardness 7
Specific gravity 2.65
Cleavage None
Fracture Conchoidal
Luster Vitreous
Transparency Translucent to transparent
Streak White
Optical Properties
PPL Colorless
Low relief
XPL 1st order grey
Undulatory extinction
Will appear black through full
rotation if looking down the C axis
δ 0.009
Twinning None
Extinction Undulatory
after Perkins, 303

Quartz in Hand Sample

Four variations on a theme

Subhedral quartz crystal

Double-terminated quartz crystals

Amethyst

Citrine

Milky quartz

Rose quartz

Smoky quartz

Blue quartz in weathered granitoid

Reddish quartz with whitish, radiating barite and purplish-blackish botryoidal hematite on quartzite

Quartz with gold-colored rutile and silvery-gray ilmenite

Scanning electron micrograph of euhedral quartz

Scanning electron micrograph of anhedral quartz

Cryptocrystalline Quartz Minerals

Chalcedony

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, plane polars

Quartz in quartzite, crossed polars (with bonus hair in the field of view; oops)

Siliceous oolites, plane polars

Siliceous oolites, crossed polars

Quartz, PPL
Quartz, XPL

Quartz ghost with biotite scarf
Alot of quartz with air bubble eyes
Quartz chestburster

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.

Quartz Oddities

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.

Quartz and thick quartz

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.

Further Reading

Quartz at webmineral.com

Quartz at mindat.org