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I gather that chemical sedimentary rocks are formed when minerals in solution are supersaturated and therefore they precipitate out.

I also understand that desert sandstone is formed as Iron Oxide rich water evaporates leaving a hematite cement.

Does this mean Desert Sandstone is a chemical sedimentary rocks?

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No, it's not.

The overwhelming majority of the material in the rock is clastic, not chemical. Chemical sedimentary rocks are rocks where (almost) all of the material was precipitated, such as travertine, evaporites, etc.

Cementation is a chemical process. As a clastic sedimentary rock has to be cemented somehow, otherwise it would be a sediment rather than a rock, all clastic sedimentary rocks have undergone some chemical process. This does not mean that they are chemical though.

As diagenesis progresses and the rock undergoes more chemical changes it may become a diagenetic clastic sedimentary rock of even a metamorphic one, but never a chemical sedimentary rock. This implies that most, if not all, of its initial mass precipitated out of solution.

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    $\begingroup$ Michael is correct. There are many cementation process even in desert areas, and they frequently do not involve iron oxide precipitation. The cementing mineral can be silica, carbonate, sulphate or oxy-hydroxide as well as pure iron oxide. None of this alters the underlying classification of the rock being classic sedimentary. $\endgroup$
    – Gordon Stanger
    Commented Apr 16, 2016 at 6:57

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