Atlas of Igneous Rocks
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Part 1: Crystallinity
•Igneous rock range in crystallinity from entirely glass. Adjectives used to describe these states
are shown on the following scale:
•
100 % crystals
holocrystalline
hypocrystalline1
or hypohyaline
100 % glass
holohyaline
The adjectives glassy, vitreous and hyaline all indicate that a rock is more or less completely
glass.
1 Hypocrystalline rocks can be described more precisely by stating the relative proportions of
crystals to glass.

Holocrystalline anorthositic gabbro
•


Holocrystalline granite
•


Hypocrystalline pichstone with perlitic cracks
•


Hypocrystalline basalt



Glassy rock
•


Glassy basalt  threads -  Pole’s hair
•


Glassy particles of mare basalt in lunar soil
•


Liquid Immiscibility



Glassy unwelded rhyolite tuff
•Glass, or devitrified glass, is often an important constituent of the pyroclastic rocks known as
ash-flow tuffs ( or ignimbrites). Such rocks typically have fragmental textures. They comprise
mixture of fragments of rocks of rocks, crystals and glass, predominantly less than a millimetre in
size.

Tuff



Glassy welded crystal tuff
•In an Ash-flow  deposit the glass fragments may initially be plastic enough to be partly or wholly
welded together as the weight of overlying material  causes compaction of the constituent
fragments; such a rock is known as a welded tuff. If sufficient heat is available, glassy fragments
devitrify.