A mineral mined on Ardana and needed to halt a botanical plague on Merak II, which in its raw state gives off an invisible gas that dulls the mind.
Zenite appears in the episode The Cloud Minders. The Enterprise travels to Ardana to collect a consignment that is the only known means of stopping a plague destroying the plant life of Merak II.
The mineral is dug by the Troglytes, who live and labour underground while the rulers of the cloud city Stratos enjoy art and leisure. A group of miners called the Disrupters has hidden the shipment to press for better treatment.
McCoy finds that unrefined zenite releases an odourless, invisible gas that slows thought and heightens violent emotion, which explains the supposed inferiority of the miners. Kirk proves it by trapping himself, the High Advisor Plasus and the Troglyte leader Vanna in a mine until the gas takes hold, and filter masks are then accepted.