An unexplained mercury spill closed a Guerneville street corner Thursday as officials awaited a special cleanup crew and wondered about the origin of the silvery toxin.

What was initially estimated to be around 4 ounces of the liquid metal was found spattered on the sidewalk and street at Armstrong Woods Road and Palo Alto Drive shortly after 9 a.m.

The discovery prompted firefighters to close Palo Alto Drive to foot and motor vehicle traffic to prevent further contamination and human exposure.

“Of course, mercury is toxic, and the vapors can be harmful, certainly,” said Andrew Parson, Sonoma County hazardous materials manager. “So that’s one of the reasons we’ve blocked off access to the area. We don‘t want people walking through it and tracking it to their cars or homes or somewhere else.”

Droplets of the mercury were dispersed along about 25 feet at the corner where Palo Alto Drive meets Armstrong Woods, just off Main Street in central Guerneville, Russian River Fire Capt. Rob Cassady said.

Some mercury also may have entered a storm drain below the curb near the intersection, though the ground below was dry, Parsons said. Water from the storm drain eventually feeds into the Russian River.

The owner of a building at the corner, located across Armstrong Woods Road from the fire station, reported the spill Thursday morning Cassady said.

A specialized hazmat crew was dispatched from the Bay Area and was awaiting the arrival late Thursday afternoon of a technician from the state Department of Toxic Substances Control who was to monitor the cleanup, test the storm drain and try to determine the extent of the contamination, Parsons said.

“Nobody saw it get spilled,” he said, “so we’re not sure what was the reason — whether it was an accident or someone carrying some mercury around for some reason, or whether it was something else going on.”