There were no small forms for Johannes Brahms. That is certainly true of his ambitious A-major piano quartet, which brought to a triumphant close Saturday's concert of Music from Marlboro at the Library of Congress.

Pianist Peter Orth showed the delicacy of a summer drizzle in the gentle second movement, and the heroic stamina needed for the interminably joyful finale. Violinist Carmit Zori often added a touch of romantic desperation while retaining firm musical control, and Philipp Naegele and Marcy Rosen completed this symphony of emotions that Brahms called a quartet. Their sound was far from homogeneous, and never less than beautiful.

Beethoven's youthful B flat-major trio for clarinet, piano and cello opened the concert. A work whose myriad colors are all shades of pastel, it was given a very energetic reading, from the first unison phrase through all its unusual harmonic contrasts and Haydnesque dynamics. Clarinetist John Bruce Yeh spun a smooth, warm tone. With no scherzo, the trio barged into a final set of nine variations on an operatic theme that teased with Orth's virtuosity and filled the Coolidge Auditorium with playful hope.

The thankless transparency of Martinu's 1936 Trio was less rewarding, in spite of Rosen's dignified, starkly captivating cello phrasing.