Alva Smith Vanderbilt Belmont, an impulsive and imperious railroad heiress, defied convention in the 1890s by dumping her unfaithful husband and running off with a neighbor. Although her feminist streak led her to battle for women's suffrage, she forced her daughter Consuelo into a stiflingly conventional marriage to a philandering English duke who needed Vanderbilt money.Mrs. Belmont, who lived in New York, Long Island, northern France and Newport, R.I., also collected antiques on impulse. At her Versailles-inspired Newport home, called Marble House (it contains 500,000 cubic feet of marble), she filled an 1890s Gothic reception room with pinnacled oak paneling, medieval stained glass, statues of saints, rock crystal watches and Roman cameos made of amethyst and carnelian. Much of the room's contents, which she tired of and sold for $125,000 in 1927, has been reunited for an exhibition that opens on Saturday at Marble House. The show's cumbersome title, ''Gothic Art in the Gilded Age: Medieval and Renaissance Treasures in the Gavet-Ringling-Vanderbilt Collection,'' refers to Mrs. Belmont's favorite Paris dealer, Emile Gavet, and her 1920s buyer, the circus tycoon John Ringling. [ABSTRACT FROM PUBLISHER]