It seems that silicon life forms in old science fiction are almost invariably described as rocky or crystalline. With the help of Science-Fiction: The Gernsback Years by Everett F. Bleiler I found four examples antedating my previous answer, "Out Around Rigel".
1931: "The Planetoid of Peril", a novelette by Paul Ernst, first published in Astounding Stories, November 1931, available at the Internet Archive.
There was that matter of the stone splinter, however, which certainly argued that the wan, prematurely white-haired fellow was a little unbalanced, and hence not to be believed too implicitly. He'd handed it to Harley, and gravely declared it to be a bit of the monster's flesh.
"Why, it's only a piece of rock!" Harley had exclaimed before he could check himself.
"Did you ever see rock like it before?"
Turning it over in his hands, Harley had been forced to admit that he never had. It was of the texture and roughness of granite, but more heavily shot with quartz, or tridymite than any other granite he'd ever seen. It had a dull opalescent sheen, too. But it was rock, all right.
"It's a piece of the thing's hide," the man had told him. "It flaked off when it tried to pry open the manhole cover of my Dart. A moment after that I got Radivision arc directions from Landon Field, aimed my sights, and shot for Earth. It was a miracle I escsped."
1931: "The Arrhenius Horror", a short story by P. Schuyler Miller, first published in Amazing Stories, September 1931, available at the Internet Archive.
"'God!' cried Tom in a stifled voice. 'The energy of it! Bill, man, have you ever seen the like? Bill, it can't be earthly — it's too huge, too colossal for earth. It would drain our little planet of energy and life, and leave it an empty husk in Space! It must be from beyond, from out there among the stars and galaxies. There are worlds out there that no man may imagine, or even dream of.'
"'Tom!' I shouted. "Tom, can't you see? It's moving! It’s growing! Tom, that Thing is alive!'
"All the great tumbled heap of crystal was stirring slowly, uneasily, with life. Huge hexagonal prisms were swelling visibly, rising toward the mist above. Now a
broad facet burst with a shattering clang, and a slender triangular shaft shot from its face, out and diagonally up, blue glory streaming from its tip. Now a thin rectangular column was thrusting up and up, with uncanny speed, from the very summit of the pile. And now a second shaft was darting toward it from one side, catching it square, bringing it down with a clash of crystal on crystal, even as new and different forms sprang from its shattered base. And as it grew upward, the great mass spread outward, toward us, with that steady creeping that gives the impression of awful relentless speed and purpose. Often, in years gone, I had dropped crystals in a beaker of water-glass, sodium silicate solution,
and watched them swell and send up pseudopods in a weird semblance of life, forming a strange submarine garden of sponge and sea-fern and tentacled polyp.
But they were but phantoms of solidity — fragile shells filled with dense solutions, while these things, growing with the same strange speed, were solid throughout. In my mock-garden, the water-glass had fed them the silica that made them grow, but here — I did not know. It must suffice that they were growing and spreading from some hidden central source, with the twang and clash of striking crystal, and the high hum that told us that the whole great body of the thing was vibrant with energy, and with something akin to life.
1930: White Lily, a novel by John Taine (pseudonym of the mathematician Eric Temple Bell), first published in Amazing Stories Quarterly, Winter 1930, available at the Internet Archive. A substantially rewritten version was published in 1952 as The Crystal Horde.
First one bubble survived, then another, until the whole desolation became a city of bleak, colossal domes. The domes began to glitter icily in the moonlight, and almost instantly the imprisoned life that was in them burst forth and multiplied. Jostling one another to ruin, the furiously increasing masses of crystal flashed out gleaming arms that branched and begot new colonies of glittering crystal; these fed for a moment on their generative substance, then instantly burst out in explosive growth in all directions, repeating the conquest of space. As yet they had no mode of locomotion; their sole power over distance was growth. In their urgency to survive, the opposing masses shot their gleaming progeny at one another, devouring the diminishing distances between them.
Their one instinct, if they had any, was to exterminate their competitors by seizing and absorbing all of the food by which they might increase.
Before the last alley closed and became a dense mass of furiously growing crystals like the rest, the mountain range that had flowed over the plain was a vast concourse of gigantic crystal shapes, towering and flashing in the moonlight, that shot upward with an accelerated growth that menaced the sky. Each glittering pinnacle budded at a thousand spax-kling points into living spears of crystal; these shot into the night, themselves to become the sources of explosive life. Neither upward nor lateral growth apparently had any natural limit, and the huger the vast bulks became the faster they grew. The whole mass pulsed and flickered with striæ of green and purple light deep within the hollow crystal masses, and from the brittle, spearlike points streamed steadily upward innumerable brushes of clear, cold blue light. The loftiest pinnacles, leaping skyward with an ever greater speed, bristled with electricity whose sharp, dry hiss all but drowned in volume the creeping rustle of an incessant growth.
1929: "The Gas-Weed", a short story by Stanton A. Coblentz, first published in Amazing Stories, May 1929, available at the Internet Archive.
It was the massed attack of several thousand men that brought the greatest surprise of the day. The plant-like growth, though apparently soft and flexible, proved to be actually hard and impermeable as granite! Again and again the attacking bayonets struck with a clatter as of iron against rock; again and again the blades were warped or broken. And the hand grenades and the aerial bombs exploded without causing any visible damage; the shells of the field artillery made only narrow gaps, which closed almost instantly, leaving all as before! The supposed plants were really stronger than steel!
[. . . .]
Within a few days, a number of other extraordinary facts about the gas-weed had been made public by Krass. For one thing, he had conducted a chemical analysis of the young shoots, when they were still relatively soft and tender and had not attained anything
of that steel-like rigidity characteristic of the developed plants. He had discovered that, in common with the seeds, they were composed of a silicon compound, so highly complex that its chemical formula defied analysis, and differed fundamentally from any other substance ever known on earth. Krass's theory — based, it is true, upon incomplete researches, but later thoroughly substantiated — was that the gas-weed had a totally different chemistry from any terrestrial organism: instead of the chlorophyl common to all green plants, it had a reddish pigment which enabled it to utilize the sunlight as a source of growth; while, in place of the well-known protoplasm, with carbon and
nitrogen as its basis, it had a molecular construction equally elaborate, but with silicon as the essential ingredient. This, in Krass's belief, explained not only why the weed throve so well in the sandy soil of the beach, where silicon dioxide existed in inexhaustible quantities, but why it was able to build up walls of the flinty construction for which various silicon compounds are noted. Such are the felspars, trap-rock and others.