About SiO2 - it's important to mention that it is a 'macromolecule' - each atom is bonded to other atoms by strong covalent bonds. There are no weak intermolecular forces (van der Waal's and so on) needed. Methane has tetrahedral covalent bonds too, but it forms discrete CH4 molecules.
Magnesium Phosphide - that will be an ionic solid. 3 Mg 2+ ions for each 2 P 3- ions I think. Most ionic solids will probably be crystalline (most things are crystalline, so that's not saying much), will be quite soluble in water, will conduct electricity when molten or dissolved in water. The question says "explain", so you will need to more than just state a property. (Conductivity is easy - once the charged ions become mobile they can transport a current. Solubility and crystallinity are a bit trickier)