With Island Silicates (also known as Sorosilicates), all of the corners of the silicon tetrahedra
are linked to positively charged ions: thus, there is no direct linking of one tetrahedra to another.
Olivine is a typical island silicate. Moderately hard, and has no prominent cleavage (don't laugh, it's a geological term meaning the direction in which a mineral will typically split) because the bonds are of equal strength in all directions.
For more general information, see silicon tetrahedra.