Mulder:" ... I found several references to a subterranean organism."
Scully: "What are you talking about?"
Mulder: "An unknown organism, existing within the volcano. I haven't found anything yet that describes it in specific terms but ..."
Scully: "Mulder, nothing can live in a volcanic interior, not only because of the intense heat but the gases would be toxic to any organism."
In the X-Files episode "Firewalker" (season 2, episode 9) special agents Fox Mulder and Dana Scully investigate the death and the disappearance of another member of a team of scientists, busy monitoring Mount Avalon in Oregon, an active volcano ready to erupt. Mount Avalon is a fictional place, supposedly part of the Cascade Range, but in the episode real footage of the eruption of Mount St. Helens (Washington) was used. In the episode the geologists also mention Sherman Crater, found at Mt. Baker.
Soon it is discovered that a parasitic lifeform uses humans as a host, nesting into the victim´s lungs. Terrestrial in origin and the result of a parallel evolutionary process, this lifeform bases its metabolism on silicon. As it produces as waste simple silicon-dioxide or sand, the lungs will eventually fill with sand and the host suffocate.The parasite is first discovered when spores are released from rock samples collected by a robot - named "Firewalker" - from inside the crater of Mount Avalon. The idea of Firewalker is based on a real robotic probe, a joint project between NASA and Carnegie Mellon's Field Robotics Center. In 1992 "Dante", as the real eight-legged robot was named, was scheduled to explore an Antarctic volcano. Later the parasite is revealed to be a sort of sponge/mushroom-like organism, growing inside and finally bursting out of its host to release its spores.
Fig.1. The full-grown silicon-based organism as seen in the X-Files.
In first production sketches the parasite resembled more a worm than a fungus, like previously seen in the episode “Ice” (season 1, episode 8). The writing of the episode is indeed very similar to this episode, as an isolated team of scientists is threatened by an unseen killer, paranoia spreads fast (anyway both episodes are very similar to the “The Thing” from 1982 and “Alien” from 1979). In the final version the fungus appear to be inspired by specimens of Cordyceps, a genus of endoparasitic fungi of small animals.
In life as we know it only ten elements play a mayor role: carbon, oxygen, nitrogen, hydrogen, potassium, calcium, magnesium, iron, phosphorus and sulphur. Strangely carbon is among the rarest elements on earth, so why is it so important in the biosphere?
Carbon can form stable and complex macromolecules within the temperature range found on earth. Atomic bounds between the carbon-carbon, carbon-oxygen and carbon-hydrogen atoms are very stable and the formed molecules are soluble and stable in water, important property for terrestrial biochemistry. Silicon is very common in earth´s rocks but rarely used by earth´s life, even if it´s popular in aliens (according to science-fiction tropes). Microorganisms like radiolarians and diatoms use silaffins, a unique type of peptides, and silica-hydrogels Under higher organisms only siliceous sponges are known to use silicon. However all those organisms use silicon only to build their shells or skeletal structures, not directly in their metabolism.
Fig.2. Drawing by biologist Ernst Haeckel of the silicon-skeleton as seen in the group radiolaria. Charles Darwin even remarks in "Origin of Species (1866) that "Few objects are more beautiful than the minute siliceous cases of the diatomaceæ: were these created that they might be examined and admired under the higher powers of the microscope?"
Under terrestrial environmental conditions silicon-based life is highly improbable and anyway not competitive with carbon-based life. However as silicon is found in the periodic table near carbon it displays some similar properties, suitable for life in general.
Silicon forms stable (however not as stable as carbon) bounds with itself and other elements like carbon, germanium, nitrogen, phosphorus, oxygen, sulphur and many metals. Those silanes can form complex and flexible macromolecules, forming sheets, chains and tubes needed to form parts of a living cell.
However silicon shows a very strong affinity to oxygen, as shown by the very common minerals-group of the silicates, composed mostly of silicon and oxygen. Earth´s atmosphere is composed of 21 percent of oxygen and silicon lifeforms would probably have troubles as the oxygen would slowly corrode their bodies. Also water would do more harm than good to the relatively delicate silicate molecules.
A silicon-lifeform would need therefore a reducing atmosphere, scarcity of water, low temperatures to stabilize the silicon-silicon bounds and an alternative solvent for the silicon-molecules, like liquid methane (highly unlikely idea but not outside the realm of extreme possibilities, as lakes of methane are found on the moon Titan).
In “Firewalker” the science is in part plausible. The silicon lifeform is found in (presumably) anoxic conditions and it is stated in the episode that oxygen will kill the organism by destroying its molecules. However temperatures are very high inside an active volcano (strangely also in one scene they try to kill the parasite with fire... it survives a volcano but can´t stand a fire?! :/), so that may be implausible for a silicon-based organism. To be fair geneticist J. B. S. Haldane (1892-1964) proposed that silicon-lifeforms could survive inside a planet feeding on partially molten rocks.Another contradiction seems to be the parasitic nature of this hypothetical organism. It´s not clear from the episode if the organism is an obligatory parasite or the spores simply and by accident grow inside the infected researchers. Scully discovers that the spores don´t react with any carbon-based tissue and have no preferred range of temperatures for growth. However, as Scully speculates, the spores die so quickly in the superficial environment that maybe she wasn't able to collect some living specimens. The interior of a carbon-based organism, also mostly composed of water (on average, the body of an adult human being contains 60% water), would probably be anyway hostile to a silicon-based parasite.
The episode ends with a devastating eruption of Mount Avalon, pyroclastic flows destroy the research station and any evidence of the unknown organism. Only Mulder and Scully escape and the government covers up any information about this X-file:
Mulder: "[]...The data it collected from the earth's interior will never be known. And of the events that occurred at Mount Avalon between the 11th and 13th of November, 1994, mine stands as the only record."
Interested in reading more? Try:
SCHULZ-MAKUCH, D. & IRWIN, L.N. (2006): The prospect of alien life in exotic forms on other worlds. Naturwissenschaften. Vol.93: 155-172
SIMON, A. (1999): Monsters, Mutants & Missing Links: The Real Science Behind the X-Files. Ebury Digital Publisher: 256 
