Are there minerals which are not been found on earth but which are still possible to exist?
To answer the first part, there are many minerals on earth which do not involve silicate bonded structures. For instance, non-silicates (minerals of carbonates, sulfides, sulfates, phosphates, and oxides are common non-silicate varieties). For a detailed list of mineral species, you should check out the Dana Classification here: Minerals Arranged by the New Dana Classification.

There are indeed Fe minerals which have no occurrence on Earth, outside originating from meteorite impacts. These minerals are various lattice polymorphs of Fe wit Ni. Kamacite and Taenite are examples of minerals which are found exclusively in meteorites, often distinguished by a Widmanstätten pattern.

The question as to how many possible mineral-producing elemental combinations are possible outside the bounds of our own planet, is outside my personal knowledge in a numerical sense, but the current known naturally occurring mineral species recognized by the IMA numbers around 5,141 valid species: The official IMA-CNMNC List of Mineral Names. 

I may be outside the bounds of certain knowledge here, but restrictions on extraterrestrial mineral formation are likely akin to those on Earth: temperature, pressure, time, fluid composition (be it magma, or some aqueous solution), gravity, and atmospheric composition, just to name a few. Some minerals may be unable to exist on our planet because the formational/stability conditions of Earth do not offer much permanence. These prohibiting conditions could well be absent on certain other planets/stellar bodies.