Mineral phase
IMA name
Volcanic glass
Secondary names
Volcanic glass, Obsidian
Type
Class
silicate mineral
Compound type
tektosilicate
Comments
Volcanic glass is classified as a mineraloid
Chemical composition
Formula
$(Ca,Na,K,Mg,Mn,Fe^{2+},Fe^{3+},Ti,Al,...)SiO_2$
Chemical formula
(Ca,Na,K,Mg,Mn,Fe2+,Fe3+,Ti,Al,...)SiO2
Elemental formula
Si O2 Al0 Ti0 (Fe2+)0 (Fe3+)0 Mn0 Mg0 Ca0 Na0 K0
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Si$ Silicon element 1 constituent element main
$O$ Oxygen element 2 constituent element main
$Al$ Aluminium element 0 constituent element main
$Ti$ Titanium element 0 constituent element main
$Fe^{2+}$ Iron(II) cation element 0 constituent element main
$Fe^{3+}$ Iron(III) cation element 0 constituent element main
$Mn$ Manganese element 0 constituent element main
$Mg$ Magnesium element 0 constituent element main
$Ca$ Calcium element 0 constituent element main
$Na$ Sodium element 0 constituent element main
$K$ Potassium element 0 constituent element main
Composition comments
contain a number of other minor elements
Oxides
Formula $SiO_{2}$ $Al_{2}O_{3}$ $TiO_{2}$ $Fe_{2}O_{3}$ $MnO$ $MgO$ $CaO$ $Na_{2}O$ $K_{2}O$
Mass fraction (%)
Oxides comments
SiO2: 64-80%, Al2O3: 10-15%, TiO2: 0.03-0.65%, Fe2O3 ...

Strunz

Class
silicates (09)
Division
unclassified silicates (09.H)
Family
Unclassified Silicate (09.HX)

Dana

Major class
silicates (VIII)
Class
unclassified silicates (78)
Type
Without Dana Classification Numbers (78.00)
Group
Unclassified Silicate (78.00.00)
Members
  • Glass-bearing (0-20% glass)
  • Glass-rich (20-50% glass)
  • Glassy (50-80% glass)
  • Obsidian (80-100% glass)
Phase type
glassy
Crystal system
amorphous
Density
2.35 - 2.60 $g/cm^3$
State NTP
solid
Refringence type
isotropic
Birefringence
none
Refraction index $n_{\alpha}$
1.45 - 1.55
Pure color
various
Color
various
Diaphaneity
transparent to opaque
Luster
vitreous
Links