Solids are heavier than liquids, as a rule—because they are denser, as a rule. Two exceptions to this rule are, incidentally, the most abundant liquid on Earth and the second most abundant mineral on Earth.
Ice floats on Water and frozen silica floats on molten silica. Water reaches its maximum density at 39 degrees Fahrenheit (4 degrees Celsius). Ice floats because at 0°C (32 °F) it is less dense (lighter) than the slightly warmer water. The hydrogen bonds are affected at this temperature, and they hold the oxygen atoms apart, thus spreading the substance out and making it less dense.
Molten silica works in the same anomalous way, becoming less dense in solid form.
Typical forms of silica are crystal, fused quartz, and silica gel. The crystals are used as jewellery, or in natural medicine.
Silica gel has been synthetically made since 1640. It has a strong affinity for water molecules and is used as a desiccant to control local humidity or in your food (without the chemicals often added to the desiccants).
Fused quartz is probably in your hand daily, as it is used in smartphone production. It is known for its purity and its use in semiconductors to make lenses and other optics for the ultraviolet spectrum. Semiconductors made from fused quartz are essential for your smartphone.
Actually, the necessity is mutual. The semiconductor business is dependent on large orders. The military developed the industry first, computers took over, and now the smartphone industry is leading.
Silica is the second most abundant mineral on Earth, and just in case you wonder what the most abundant mineral on earth is, it is feldspar (used in ceramics).
*Image of a lake in Scotland via Shutterstock