QUESTION: 
In your APXS results for the Martian soil composition (oxides), iron oxide
is listed as FeO. Yet elsewhere it is stated that the red color of the soil
is due to the abundance of weathered ferric (Fe2O3) minerals. (FeO is
black.) Why isn't Fe2O3 listed in the oxides composition table? This will
clearly affect the total amount of oxygen in the soil as well.

ANSWER from Mike Mellon on August 24, 1997:
Historically, geologists list the elemental composition of rocks and soils
by specific types of oxides. The planetologists continue to use this
convention to make comparisons with known Earth analogs easier to understand
from a scientific point of view. The APXS instrument is only capable of
measuring elemental composition (for example, Fe). It can't measure mineral
composition and so can't determine if the iron is an oxide, sulfide, or
what. A mineralogical type of measurement will have to wait for a future
mission.

Mike Mellon