Approximately how many atoms of silicon and oxygen would be in a cube of silicon dioxide 0.5 mm on a side? (a grain of sand)

This isn't so much an internet research question as a mathematical Chemistry question. Sand is essentially silica (SiO2), which (in my one internet search) has a density of 2.65 g/cc. 0.5 mm is 0.05 cm. A cube with a side of 0.5 mm would therefore have a volume of (0.05 cm)^3 = 0.000125 cc. The mass of this grain would be m=dV or m= 0.000125 × 2.65 = 0.00033125 g. The number of moles is the number of grams divided by the molar mass of SiO2, (which is 60.0835 g/mol), so mol = 0.00033125/60.0835 = 5.51316 × 10^(-6) mol. The number of formula units is Avogadro's number times the number of moles, or (6.022 × 10^23) × (5.51316 × 10^(-6)) = 3.320 × 10^18 formula units. There is one silicon atom per formula unit, and two oxygen atoms per formula unit in SiO2, so:

Number of silicon atoms = 3.320 × 10^18
Number of oxygen atoms = 6.640 × 10^18

https://www.quora.com/What-is-the-density-of-silica-sand