The Sisters of Mercy of the Union are selling their 346-acre national headquarters in Potomac, valued at more than $10 million, because the nuns can no longer afford to keep it up, a spokesman for the Catholic religious road said yesterday.

The tract is one of the last remaining large parcels of undeveloped land in the exclusive western part of Montgomery County.

Also up for sale is the building that has been the religious order's headquarters for the last 21 years and the St. Maurice School for learning-disabled and emotionally disturbed students, according to Jack Shank, the order's real estate adviser.

The school will close at the end of the school term next year, according to Sister Mary Jeanne Ward, the principal.

A spokesman for the headquarters said it had become a financial burden for the order to keep up the facility. Additionally, much of the space in the headquarters was unused. Like most religious orders, the 4,700-member Sisters of Mercy have been experiencing a decline in the number of women entering the sisterhood.

Only about 15 nuns still live in the motherhouse in Potomac, the spokesman said. They will move to other religious communities run by the Sisters of Mercy throughout the country, according to Sister Mary Jeanne.

Another reason for the closing, however, is that the sisters, following a mandate from Pope John Paul II, want to reaffirm their commitment to helphing the poor, the spokesman said. And they felt that affluent Potomac was no longer a suitable location for headquarters, he added.

"All our neighbors are wealthy. And while they are all nice people, we are not comfortable in a place that con-notes wealth and privilege," the spokesman said.

Potomac, said one developer, "is becoming a suburban neighborhood . . . a very, very exclusive, expensive neighborhood, but no longer this rustic, totally away-from-the-world setting."

The Sisters of Mercy property, which is at 10000 Kentsdale Dr. is zoned residential for either two-acre or half-acre lots, according to Shank.

The only remaining large undeveloped parcels in Potomac are the 894-acre avenel Farm, and the 192-acre Marwood estate, which belongs to the family of former Maryland gubernatorial candidate Louise Gore. About 450 acres of Avenel are slated to become the site of acounty sewage treatment plant, and the rest is up for sale.

James Gore, part owner of Marwood, said yesterday that although the estate "would make an ideal location for a housing development," Marwood has not been sold.

The Sisters of Mercy property was unofficially up for sale for the past few years, according to attorney Robert Linowes, who said he has at times advised the sisters in their real estate dealings. However, the decision to put it on the open market did not come until a meeting ot its 68-member governing body on June 27.

The decision to close followed an extensive feasibility study conducted by the sisters and private consultants.

According to Sister Mary Jeanne, the decision to close St. Maurice School arose from the fact that county public schools now offer a considerable number of programs for special education students similar to those provided at St. Maurice.

"We don't see a need to be here now," she said.

Sister Mary Jeanne said St. Maurice formerly received funding from the county for taking students for whom there were no suitable programs in the county school system. But the school - which will have enrollment of 96 students next year - lost out on much of that funding as county special education programs grew in number.

She said that the county, expecting to develop more programs for children with emotional and learning problem, had asked that St. Maurice School begin to enroll severely emotionally disturbed students.

But in order to get state licensing to teach those kinds of students, it would be necessary for St. Maurice to accept students ranging in age from 8 to 21.

She said St. Maurice would have to hire a different professional staff in order to serve the older students and that the sisters decided against this.

Paying the salaries of the school staff had become an increasing financial burden, Sister Mary Jeanne said, because the number of sisters, who are unsalaries, had decresed from 11 in 1972 to only two last fall.