Zenite

From Memory Alpha, the free Star Trek reference.

Zenite is a mineral that is useful in countering plant diseases. It is found only on Ardana in soft ore material, and hand removal is the preferred method of mining.

However, the Ardanans were apparently unaware that the mineral in its raw state has an undesirable by-product: it emits a colorless and odorless gas that temporarily impairs synaptic functions, stunting the subject's intelligence and emotional control. When the subject is removed from a high concentration of the gas, brain functions return to normal.

This medical problem was discovered by Dr. Leonard McCoy, who also learned that a simple air filter is sufficient to prevent exposure. However, the Ardanans were reluctant to accept these facts, and only after concerted diplomatic efforts did they agree to take prescribed safety precautions for mining. (TOS: "The Cloud Minders")

It is possible that the planet's aristocratic population was aware of the problem and used it to subjugate the Troglytes. The Enterprise pushed the issue and forced Ardana to confront the problem officially.