Constituent
Name
opal
IMA name
Opal
Class
silicate mineral
Compound type
tektosilicate
Constituent abundance in material
Relevance
main - major
Arrangement
single phase
Mass fraction
1.0
Chemical composition
Formula
$SiO_2 \bullet n(H_2O)$
Chemical formula
SiO2(H2O)n
Elemental formula
Si (O)2.1-2.9 (H)0.2-1.8
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
$H_2O$ Water molecule 0.1 - 0.9 hydration main
Composition comments
water content may range from 3 to 21% by weight, but is usually between 6 and 10% (n = 0.2 - 0.35
Chemical functions
  Function Number Relevance
$R-OH$ 2 main
Chemical bonds
  Bond Number Relevance
$\geq Si-O-$ 4
$-O-H$ 2 main
Oxides
Formula $SiO_{2}$ $H_{2}O$
Mass fraction (%) 92.0 8.0
Oxides comments
values for n=0.3, but mostly vary from 3 to 21% for H2O

Strunz

Class
oxides - hydroxides - arsenites (04)
Division
metal:oxygen = 1:2 and similar (04.D)
Family
with small cations: Silica family (04.DA)
Code
04.DA.10

Dana

Major class
silicates (VIII)
Class
tectosilicate Si tetrahedral frameworks (75)
Type
with H2O and organics (75.02)
Group
-Opal- group (75.02.01)
Code
75.02.01.01
Phase type
amorphous
Crystal system
amorphous
Polytype
opal-CT
Molar mass
61.8 - 76.2 $g/mol$
Density
1.25 - 2.3 (av. 2.09) $g/cm^3$
Refringence type
isotropic
Refringence sign
no
Birefringence
0
Refraction index $n_{\alpha}$
1.370 - 1.470 (typ. 1.43)
Color
light orange
Diaphaneity
transparent to opaque
Luster
vitreous