The Smithsonian Institution, in a move guarded in secrecy, is about to pay $7 million for one of the world's most distinguished collections of Persian art -- to form a centerpiece of its new Quadrangle project, scheduled for completion in May 1987.

The collection consists of precious works -- mostly texts and illustrations -- amassed by Henri Vever, a Paris jeweler who died at 89 during World War II. His ancient Persian rarities, secretly slipped out of France during the Nazi occupation, have been in storage ever since. For years experts lost track of the several hundred objects, which have been housed in a New York warehouse since 1945.

"They knew the collection existed, but they didn't know where it was," said New York lawyer Lawrence E. Brinn yesterday. Brinn, who represents Vever's heirs, confirmed details of the sale and said the contract for the sale is scheduled to be signed here tomorrow.

Dean W. Anderson, the Smithsonian's assistant secretary for history and art, refused to answer questions about the impending purchase, declaring, "I feel it's premature to comment."

The Vever collection will be housed in the new $75 million museum of Middle Eastern, African and Asian art, now under construction adjoining the Smithsonian Castle on the Mall.

"This acquisition will make the Smithsonian the center for study of Near Eastern art in the world," speculated Brinn.

The museum's silence reflected a long history of secrecy by the Smithsonian on certain details of its acquisitions, despite the fact that the vast enterprise is a semipublic institution that exists in part on government funds. It also reflected a desire to announce the purchase at its own convenience, presumably on the day the documents are signed.

The purchase was approved by the Smithsonian's board of regents Sept. 16, but details of the arrangement were omitted from the board's published summary of the meeting. Meetings are closed to the press and the public. Details emerged when The New York Times obtained a copy of the omitted passages and published them in Tuesday's edition.

According to The Times' story, Smithsonian Secretary Robert McC. Adams, who was said to be away yesterday and unavailable for comment, told the regents that two Smithsonian experts had examined the collection and found 39 full volumes, 291 miniatures, 98 calligraphies and illuminations, 29 bookbindings, 4 textiles and examples of almost all the known classical Persian texts, plus several important Arabic texts. Many were previously unknown, the regents were told, and are of "unquestionable provenance."

"The Vever collection fully lives up to its legendary reputation," Adams told the regents. "It is a dazzling selection of one of the greatest artistic traditions known and would be impossible to duplicate today at any price. Works in abundance of this quality have been unavailable on the art market since the first decades of this century."

The Quadrangle building will also house the Arthur M. Sackler Gallery, with 1,000 works of Asian art given to the Smithsonian in 1982 by a New York research psychiatrist, as well as the National Museum of African Art.

Brinn, who said he has represented Vever's heirs "for over 30 years," said the decision to sell "was not premeditated."

He said the Smithsonian deal originated last February when the museum "became aware" of where the collection was and expressed an interest in buying it. The transaction was triggered "by indirect conversations by people who knew the collection, and people who knew the Smithsonian. This is a very small world that is concerned with Persian art," Brinn said. "There have always been certain individuals and families who knew this field. The right people get in touch with the right people."

The original asking price, Brinn confirmed, was $11 million, but came down when the Smithsonian said it could not afford the higher figure.

Though one reason the collection was finally brought out of hiding was the Smithsonian's bid, another, said Brinn, was the current high interest in the art world. "There is a time for everything," he mused. "A few years ago, everything was Japanese prints. Now it is the Near East."

Adams was quoted in The Times' article as telling the regents: "The collection was assembled in Paris during the first half of this century . . . [It] is the only remaining collection still intact assembled during the heyday of the availability of this material. The Rothschild, Cartier and Goloubev collections, for example, have been dispersed. More importantly, Henri Vever was the greatest of all these collectors."