According to the RAW every non-living solid is partly Terram.
Anyway, if we actually read the RAW, it turns out we are both right. You more than me.
In the section on The Elemental Forms (after the definition of the Forms), in ArM5 it says:
Aquam, Auram, Ignem, and Terram are the elemental forms, dealing with the non-living things in the world. Often, a thing falls under more than one of these Arts... Similarly, ice is basically Aquam, being a form of water, but its solidity is covered by Terram and its coldness by Ignem....Thus, to create ice you need only Creo Aquam...A Creo Aquam spell with an Ignem requisite could create warm ice — still solid, but warm.
So, the solid nature of ice is governed by Terram, but you don't need a Terram requisite to create it. Ignem is likewise irrelevant to the creation of ice unless you happen to want warm ice.