A developer has halted work at a construction site near the Fair Oaks shopping mall in western Fairfax County after discovering highly unusual pockets of apparently pure, naturally formed asbestos in the soil, county officials said yesterday.

Edgar G. Chase, director of the county health department's air pollution control division, said work at the Fair Oaks Commerce Center was suspended July 26 after the "unprecedented" discovery of "high concentrations of what would appear to be substantially pure asbestos" at the site.

"It has left everyone a little nonplused because it's a problem we've never encountered," Chase said. "The extent to which it poses a hazard is what we're trying to ascertain. We'll be concerned {about workers} during the construction phase."

Officials said it was unlikely that anyone at the site, southwest of Rte. 50 and Random Hills Road near I-66, had received a dangerous dose of asbestos. When breathed in large quantities over a long period of time, asbestos, a rock-formed mineral, can cause lung cancer.

Claude G. Cooper, director of the county's Department of Environmental Management, said that authorities know of a belt of asbestos-contaminated rock that runs through the west-central portion of the county, between the area near I-66 and Rte. 50 and the area around Bull Run Regional Park. He said authorities do not consider areas in or near the belt to be hazardous because asbestos particles are imbedded in rock and are not airborne.

"But when we learned of this, we said, 'Here is something we need to be concerned about,' " said Cooper, adding, "We don't think there's any reason to panic."

Cooper said that county officials have asked the federal Environmental Protection Agency to help determine "what type of hazard it is and what precautions need to be taken." He said a soil scientist is mapping the belt of asbestos-carrying rock, called actimolite, so county officials can monitor asbestos levels at other projects in and near the belt.

Chase said that work was suspended at the construction site after workers complained of possible safety hazards to the developer, the Property Corporation of America.

Michael Windsor, regional partner of the corporation, did not return a reporter's telephone calls yesterday.

Chase said that the particles were discovered beginning at a depth of about two feet below the surface and increased as workers dug deeper. Although nearly all of the site contains asbestos-carrying rock, the fibers were generally found in small pockets or veins.

Jack Reynolds, an official with the EPA regional headquarters in Philadelphia, said that the EPA is studying whether there are regulations that would cover this situation because rules governing removal of asbestos from old buildings or mines probably would not apply.

Malcolm Ross, a research minerologist with the U.S. Geological Survey, said that although the discovery may be a first in Fairfax County, pockets of pure asbestos have been discovered before.

"There are geologic formations {of asbestos rock} from Canada to Georgia, and deposits like that used to be quarried in this general area, in Maryland, Virginia and Pennsylvania," Ross said. "They probably encountered it before, but no one would have worried about it until a few years ago."