The fracture strength of SiO2... Silica , not silicon (or silicone) will
range from nearly the same high to the same low.
For brittle crack-surface-crack sensitive materials like glass, silica
and silicon, it is perfectly meaningless to imagine a definitive value
of fracture stress.
It is sort of meaningful to imagine a value of fracture stress for a
highly specific way of making and handling of such materials.
Brittle fracture is not well covered by looking for numbers with no
physical understanding of the process and the intrinsic statistical
nature of brittle fracture as well.
If this is important to you, it is time to dig into brittle fracture
with some of the many articles and books on the subject.
I would hazard a guess that the critical resolved shear stress to fracture
would be a better classifier parameter for brittle materials. The
original poster mentioned silicon 111 crystal. Si is diamond cubic and
the  planes would be the preferred slip planes.
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