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In nature, the SiO₂ concentration in some cases can exceed 80 wt % although in the most common magma, basalt, the SiO₂ range is 45–55 wt %. By comparison, most commercial glasses have from 50 to 70 wt % SiO₂ contents (Table 1).

From the relatively small enthalpy, entropy, and volume of fusion (ΔH, ΔS, and ΔV) of crystalline SiO₂ polymorphs (see [2] for review of data), it may be inferred that silica melt and glass retain a three‐dimensional structure of interconnected SiO₄ tetrahedra that exist in its crystalline polymorphs (quartz, tridymite, and cristobalite). From vibrational, X‐ray, and NMR spectroscopic studies, one also concludes that the SiO₂ glass structure is essentially fully polymerized [3]. Vibrational spectroscopic spectra recorded at temperature above that of the glass transition of silica glass (1208 °C) do not reveal significant structural differences between glass and supercooled liquid. There is an asymmetric distribution of intertetrahedral angles, ranging from ~120 to 180° (Figure 2) with a maximum between 145 and 155° [4]. A 145–155° Si─O─Si angle is that expected in a three‐dimensionally interconnected SiO₂ glass structure consisting predominantly of six‐membered rings. The somewhat asymmetric Si─O─Si angle distribution (Figure 2) suggests that more than one exists in SiO₂ glass. Rings with three or four SiO₄ tetrahedra coexisting with six‐membered rings are those most commonly suggested.

Figure 1 Compositional environment of complex silicate melts and glasses. Peralkaline denotes compositional range where there is excess metal cations (alkali metals + alkaline earths) over that necessary for charge‐balance of tetrahedrally coordinated Al³⁺. Meta‐aluminous compositions are those where the proportion of alkali metals + alkaline earths is exactly equal to that needed for charge‐balance of tetrahedrally coordinated Al³⁺. Peraluminous compositions are those where there is excess Al³⁺ over that which can be charge‐balanced with alkali metals + alkaline earths.

Table 1 Oxide composition (wt %) of common commercial glasses and glass of common magmatic rocks with additional data from http://Earthchem.org.

Source: Modified from [1]

	Window glass	Pyrex	Glass wool	Rockwool	Rhyolite	Dacite	Andesite	Basalt	Phonolite
SiO2	72.6	81.1	65	46.6	72.18	65.13	57.51	50.29	56.56
TiO2				2.4	0.39	0.64	0.93	2.06	0.87
Al2O3	0.6	0.43	2.5	13.3	13.23	15.67	16.93	14.79	19.31
B2O3		22	4.5
FeO(T)	0.8	0.2		10.6	2.90	4.73	7.08	10.94	4.02
MnO					0.10	0.82	0.05	0.03	1.05
MgO	3.6	0.3	2.5	9.1	0.48	1.03	1.82	2.5	1.86
CaO	8.7	1.1	8	10	1.53	1.47	1.85	1.38	2.28
Na2O	14.3	1.5	16.5	5.6	4.03	0.81	0.77	0.55	1.57
K2O	0.2		0.7	1.4	3.76	0.96	0.86	0.38	1.01
NBO/T	0.79	0.00	0.62	0.99	0.08	0.18	0.36	0.72	0.22

The coexistence of distinct structural units has important consequences because it has been invoked to account for the unusual properties of SiO₂ glass such as a room‐temperature density maximum for glass quenched from temperatures near 1505 °C. Besides, a density minimum is observed near 950 °C for structurally relaxed glass. The anomalous pressure‐ and temperature‐dependence of SiO₂ glass compressibility, with maxima near 3 GPa and 100 K, respectively, can also be modeled with two coexisting three‐dimensional structures in SiO₂ glass.