Any chance of a quick weekend settlement to end the nation's 97-day-old coal strike evaporated yesterday as negotiations continued in an atmosphere of muted optimism, without any visible signs of imminent agreement.

"There's a lot of work to be done," said chief industry negotiator Nichols T. Camicia as the main bargaining groups broke down into smaller teams to deal with what Camicia called the "Hard issued."

Sources on both sides cautioned against taking a pessimistic view of the recess in full-scale bargaining, but conceded it wiped out chances of reaching a tentative agreement by today, when the 160,000 United Mine Workers strikers are theoretically to report for work under the government's Taft-Hartley back-to-work order.

a tentative agreement would not have immediately ended the strike because UMW bargaining council and rank-and-file ratification is required, but it might have encouraged compliance.

Amid predictions of widespread defiance and some fears of violence when the mines reopen, federal marshals continued serving the legal orders on 1,450 company and union officials covered by the strike injunction. The mines open at 12:01 a.m., but the deadline for serving the orders is not until 4:30 p.m.

Hundreds of union meetings were being held throughout the coalfields to inform miners of the order with mixed results.

"I will comply with the law," James Taranto, president of 2,200-member Local 1269 in Bakerton, Pa., was quoted by the Associated Press as saying, "It'll be my job to read it and their [the strikers'] job to ignore it."

Quinton Steel, a Jaspar, Ala., union official, was wearing his American Legion cap when a marshall knocked on his door. "I'm a soldier from World War II and the president is my commander-in-chief; I'll obey his order," Steel told United Press International.

Said Bob Hoffman, president of a UMW local at Pwwhatan Point, Ohio: "I told them to go back to work . . . They just looked at me."

Individual miners can refuse to work against their will, risking only dismissal, but union officials can be fined or sent to jail for contempt of court for refusing to obey a Taft-Hartley order.

The bargaining resumption has the effect of forestalling a breakup in industrywide negotiations, a prospect so scary to both sides that it brought them back together Friday in what appeared to be a last-ditch effort to write a national coal contract.

So-called "Balkanization" of bargaining, which the Carter administration was quietly pushing in tandem with the Taft-Hartley Act in an effort to get some coal moving again, would break up the industry's united front and could have devastating effects on the already splintered UMW.

A crucial question may now be how long the new round of industrywide bargaining can continue without results before pressures mount again for regional or company-by-company settlements.

The administration, shifting tactics again as it has done many times before in the protracted coal dispute, signaled yesterday that it was pinning its hopes again on an industrywide contract.

"I think the process of negotiations is going forward in a much better climate than before," presidential trade representative and all-purpose troubleshooter Robert S. Strauss said on "Face the Nation" (CBS, WTOP). "I think they'll pull it off and I'm very optimistic."

As the negotiators gathered shortly afterward in their ninth-floor suite at the Capital Hilton Hotel, UMW officials agreed the climate was better - attributing it largely to the participation of Camicia and Stonie Barker, president of Island Creek Coal Co., both of whom are former miners.

But 'Camicia questioned whether Strauss had firsthand knowledge of the talks and implied that he might have been overly optimistic. "We're progressing to the point of getting down to brass tacks," said Camicia, who described himself as "encouraged."

Said UMW President Arnold Miller: "I think we're making some progress . . . I think we want to get things together."

Because of overwhelming rank-and-file rejection of an earlier contract offer a week ago, which triggered Carter's invoking of the Taft-Hartley Act, it is believed that any major concessions must come from the industry side.

The Bituminous Coal Operators Association, the industry's bargaining group, reportedly agreed when the talks resumed Friday to reduce proposed health-care deductibles, one of the most controversial features of the rejected contract. But it also withdrew its earlier offer to guarantee health and pension benefits, one of the few selling points of the earlier agreement.

While this was apparently unacceptable to the union, it formed the basis for continued talks, which theoretically could go on indefinitely since a Taft-Hartley injunction requires bargaining during an 80-day "cooling-off" period.

Neither side would say yesterday when a settlement might be expected. "As long as you're moving, it's encouraging . . . but things can always break down," said Miller.

Asked if the miners would obey the back-to-work order, which the union's national officers have instructed them to do, Miller said: "I don't really know. You'll have to ask them. They don't always follow me."