Primary/secondary minerals

Are secondary clay sized minerals formed from the weathering of primary minerals, or are they formed to begin with under different conditions such a lower temperature or pressure?

Some secondary mineral are created as a result of partial change of crystalline structure of some other mineral (either primary or secondary). Example of that is weathering through hydration of hematite to limonite (see lecture notes from Jan 9). In addition, some other secondary mineral are created as a result of complete destruction of crystalline structure of another mineral that releases various ions into soil solution. New (secondary) minerals could be created in soil from those ions