What is "tungsten trioxide"

Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.

2 WO + 3 C → 2 W + 3 CO (high temperature) WO + 3 H → W + 3 HO (550 - 850 °C)

Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO·HO, meymacite WO·2HO and hydrotungstite (of the same composition as meymacite, however sometimes written as HWO). These minerals are rare to very rare secondary tungsten minerals.