A common idea in science fiction is that life elsewhere may differ substantially from our own. In many cases a key difference in biochemistry has been hypothesised: the idea that life may be based on compounds of silicon, rather than the carbon that forms an integral part of our own cell structures. Here I look at mineral life in science fiction

[Note: I will not look at cybernetic artificial intelligences or similar electronics-based models for life this time - although these are usually based on silicon semi-conductors, the life as such is deemed to reside in the software rather than the hardware.]

Solar System Silicates

Silicon life has formed part of the picture of our solar system since some of the earliest science fiction to consider realistic planetary ecologies. A Martian Odyssey (novella, 1934) by Stanley Weinbaum described a trek across the Martian landscape by one of the first human explorers. As well as an intelligent, bird-like native he names Tweel, the explorer describes a bizarre phenomenon:

“There was a line of little pyramids—tiny ones, not more than six inches high, stretching across Xanthus as far as I could see! Little buildings made of pygmy bricks, they were, hollow inside and truncated, or at least broken at the top and empty. I pointed at them and said 'What?' to Tweel, but he gave some negative twitters to indicate, I suppose, that he didn't know. So off we went, following the row of pyramids because they ran north, and I was going north.” [...] “I examined a brick or two as well; they were silica, and old as creation itself!”

Following this line of pyramids, each of increasing size, they find the last one moving:

“The top tiers of bricks were heaving, shaking, and suddenly slid down the sides with a thin crash. And then—something—something was coming out! A long, silvery-grey arm appeared, dragging after it an armored body. Armored, I mean, with scales, silver-grey and dull-shining. The arm heaved the body out of the hole; the beast crashed to the sand. It was a nondescript creature—body like a big grey cask, arm and a sort of mouth-hole at one end; stiff, pointed tail at the other—and that's all. No other limbs, no eyes, ears, nose—nothing! The thing dragged itself a few yards, inserted its pointed tail in the sand, pushed itself upright, and just sat.

As becomes clear, this non-intelligent but silicate-based creature excretes the bricks and builds a pyramid around itself over time before eventually breaking free of this shell and moving on a short distance to build a larger one. In this case, the explorers conclude that the behaviour of the creature is essentially acting out a response to a chemical stimulus, not subject to biological delay, and only dependent on having the correct minerals in its environment: 

“Blind, deaf, nerveless, brainless—just a mechanism, and yet—immortal! Bound to go on making bricks, building pyramids, as long as silicon and oxygen exist, and even afterwards it'll just stop. It won't be dead. If the accidents of a million years bring it its food again, there it'll be, ready to run again, while brains and civilizations are part of the past.”

Weinbaum’s pyramid builder, then, is as much a machine or chemical reaction process as it is life as we know it. By contrast, the 1966 feature film Thunderbirds Are Go (dir. Lane; based on the 1965 television series Thunderbirds), featured a more active form of mineral based life on Mars. The Martian Rock Snakes initially appear to be conical piles of stone, that gradually unwind when disturbed to raise heads that can spit balls of plasma at any aggressor. The biology of these creatures isn’t really explored, but it’s interesting to note that these creatures survive in the toxic near-vacuum of the Martian atmosphere (against which human astronauts in the film are protected) and that their ability to project fireballs suggests a higher range of thermal tolerance in their tissues than most organic life.

However most science fiction describes Mars as primarily hosting carbon-based life (if any), and silicon life is relatively rare in Martian fiction.

By contrast, rock creatures come into their own in the low gravity, vacuum-encased worlds of the asteroid belt. An example is described by another relatively early short story: “The Beast of Space” by F E Hardart (which originally appeared in Comet, July 1941, text, audio). However this creature is rather less passive and more of a threat:

“This thing, known to man as Asteroid Moira, is, in actuality, one of the gigantic mineral creatures which inhabited a planet before it exploded, forming the asteroids. Somehow it survived the catastrophe, and, forming a hard, crustaceous shell about itself, has continued to live here in space as an asteroid. It is apparently highly intelligent and has acquired an appetite for human flesh.”

In this case, the creature captures an aged professor and his attractive young daughter, who is - perhaps inevitably for this period - rescued by Nat Starrett, a virile young asteroid miner, who promptly ensures that this ancient and intelligent survivor does not survive any longer. Interestingly, in this case, the basic composition of the creature is not based on silicon (Si, atomic number: 14) compounds but instead on zirconium (Zr, atomic number: 40).

Author Isaac Asimov is best known for his Robots and Foundation future history, in which aliens do not feature. However he did occasionally write about alien life, and one example is his short science fiction mystery “The Talking Stone” (first published in the Magazine of Fantasy and Science Fiction, Oct 1955). This establishes that the asteroid belt is occupied by a silicon based life form known as a silicony, which consumes asteroidal rock and gamma radiation, and excretes small pebbles of pure silica. Usual examples are just an inch or two across, and the mystery is driven by the discovery of a giant creature a foot across:

“It’s skin was of an oily, smooth grayness. Its motions were slow, as became a creature who borrowed in stone and was more than half stone itself. There was no writhing of muscle beneath that skin; instead it moved in slabs as thin layers of stone slid greasily over one another.
It had a general ovoid shape, rounded above, flattened below, with two sets of appendages. Below were the “legs,” set radially. They totalled six and ended in sharp flinty edges, reinforced by metal deposits. Those edges could cut though rock, breaking it into edible portions.”

This creature is clearly intelligent, and - like The Beast in Space - it’s hinted that its species may have originated on a now-destroyed Fifth Planet. However, since Asimov’s focus is on unravelling a mystery involving space smugglers, the silicony’s role is somewhat relegated to the source of a clue and not explored further.

Poul Anderson, a writer who appears to have been fond of the asteroid belt, also explored this theme in his short story “Barnacle Bull” (originally appearing in Analog/Astounding, September 1960, under the pen name Winston P. Sanders). This describes a Norwegian spaceship, the Hellik Olav, which is attempting to make the first successful passage through the asteroid belt. The ship begins to encounter problems as its surface becomes encrusted with “space barnacles”. These have curious properties that appear to straddle the biochemical boundaries:

“The basic chemistry does remain that of carbon, of proteins, albeit with an extensive use of complex silicon compounds.” […]
“When a spore does chance on a meteorite or an asteroid it can use, it develops rapidly. It requires silicon and carbon, plus traces of other elements; hence it must normally flourish only on stony meteorites, which are, however, the most abundant sort. Since the barnacle’s powerful, pseudo-enzymatic digestive processes - deriving their ultimate energy from sunlight - also extract metals where these exist, it must eliminate same, which it does by laying down a plating, molecule by molecule under its shell. Research into the details of this process should interest both biologists and metallurgists.”

Organosilicates - creatures with carbon-based tissues which utilise silicon for protective shells and other features - are rare but do exist on Earth. The best known are diatoms - microscopic aqueous creatures which build their complex shells from silica rather than the calcium compounds used by most Earth life. Interestingly, Anderson also makes an attempt to explain the rather unlikely evolutionary pathway of his creatures:

“Winge believes the barnacles originated as a possibly mutant life form on the ancient planet before it was destroyed. The slower breakup of the

resulting superasteroidal masses gave this life time to adapt to spatial conditions.”

As I’ve discussed before, we’re now rather confident that such a fifth planet never existed - its possible formation site was disrupted by gravitational disturbance from Jupiter. While we are less certain that silicon life cannot exist elsewhere in the solar system, we’ve certainly found no evidence of it. 

Moving further afield, the novel Sentenced to Prism by Alan Dean Foster, published in 1985, presents an example of silicate life sited well outside our Solar System. In this distant future, giant corporations explore and exploit new worlds. Sent as a troubleshooter to investigate the silence of a research outpost, Evan Orgell has to fight for survival on a world in which organic carbon-based life, silicon-based life and organosilicates exist in mutual competition, including in highly complex - and even intelligent - forms.

This novel takes a less hard science fictional approach than some of the other examples mentioned here, and does not explore the chemistry and physics in detail, although it does describe the photovore (i.e. directly light-consuming) nature of the silicaceous beings, and their need to extract trace elements from the soil or other creatures (going someway towards answering the mystery of why a silicate lifeform would want to eat organic flesh).

Life, but not as we know it

In the cases above, those encountering mineral life have little difficulty in recognising it for what it is. However science fiction has also explored the difficulty in determining what is and is not life as we would understand it.

Perhaps the most famous mineral lifeform in science fiction was encountered by the USS Enterprise in the Star Trek (TV, 1966-1969) episode “The Devil in the Dark” (1967). In this story, a mining colony on the planet Janus VI experiences an unexplained series of murders and sabotage incidents. In investigating, Captain Kirk and his crew find mysterious silicon nodules, which are being broken up and discarded by the miners . It soon becomes clear that the damage and deaths are being caused by a creature known as the Horta, which is capable of exuding strong corrosives, and is simply trying to protect the silicon nodules - the eggs of its species.

A large part of the drama in the episode is derived from the gradual realisation that what they had assumed was either an unintelligent animal or simply not life at all is in fact a highly intelligent and self-aware life form, which both needs and deserves their help. Captain Kirk, science officer Mr Spock and Dr McCoy actually discuss the possibility of silicon life, with Spock advocating for its probability, Kirk acknowledging that he’d heard of it, and McCoy expressing scepticism: 

Spock: Life as we know it is universally based on some combination of carbon compounds, but what if life exists based on another element? For instance, silicon.

McCoy: You're creating fantasies, Mister Spock.

Kirk: Not necessarily, Bones. I've heard of the theoretical possibility of life based on silicon. A silicon-based life would be of an entirely different order. It's possible that our phasers might not affect it.

Spock: Certainly not phaser one, which is far less powerful than phaser two.

Kirk: All right, how about this? A creature that lives deep in the planet below us, at home in solid rock. It seems to me that in order to survive, it would have to have some form of natural armour plating.

Spock: It could explain much, especially since the colonists are armed only with phaser one.

Kirk: But our people have phaser number two.

Spock: Which I could adjust to be more effective against silicon.

McCoy Silicon-based life is physiologically impossible, especially in an oxygen atmosphere.

Spock: It may be, Doctor, that the creature can exist for brief periods in such an atmosphere before returning to its own environment.

McCoy: I still think you're imagining things.

However, the willingness of the Enterprise crew to ultimately accept this unknown form of life and provide help is exemplified both by the difficulty Spock endures in trying to link telepathically to such an alien creature and by Doctor McCoy’s intent efforts to repair an injury - efforts which are accompanied by the immortal line: “I’m a doctor, not a bricklayer!”

Star Trek: The Next Generation (TV, 1987-1994) returned to consider similar questions regarding how we recognise life in the episode “Home Soil” (1988). Here the soil of Velera III was found to be permeated with silicate crystal microbrains, living in moist saline-rich soil. Initially overlooked by a terraforming team, these crystal beings in turn described humans as “ugly bags of mostly water”. The lack of recognition of their status of life rises to the level of prospective, unintentional genocide on the part of the Federation, which the Enterprise crew must avert. However in this case, while each crystal meets the criteria for life, their intelligence appears to be an emergent property of an array of such beings - i.e. more akin to a network of silicon chips in a computer than neurons in a brain.

Mineral Intelligences

Since “The Devil in the Dark” though, many science fiction serials have treated mineral-based intelligences as just one more aspect of the variety of life in the Universe. They are less common in fiction than, for example, animal or plant-evolved life, but they still represent a fairly common feature of any interstellar community. In the Star Trek universe, the Horta and the crystals of "Home Soil" are complemented by the Tholians. These mysterious crystal-shaped and based aliens were introduced in “The Tholian Web” (1968), and their role and biology expanded upon in the Star Trek: Enterprise (TV, 2001-2005) episodes “Future Tense” (2003) and “In a Mirror, Darkly” (2005). The Enterprise episodes establish that Tholians have a hard crystal carapace, glowing eyes, and require a typical environmental temperature of around 480 K (200 C), although little else about their physiology is made clear.

Similarly mysterious are the enormous crystalline entities which were encountered in space by the USS Enterprise in Star Trek: The Next Generation (episides “Datalore” (1988), “Silicon Avatar” (1991)). These appear to convert solar energy to matter, strip both vegetation and minerals from planets, and leave behind a fictional element, bitrium, in their wake. They also have a sensitivity to graviton pulses, suggesting they are attracted by gravitational mass. Several ships in the Star Trek universe encountered such entities, including the USS Cerritos in the animated series Star Trek: Lower Decks (TV, 2020-date; episode “I, Excretus” (2021)).

Another long running science fiction universe, that of television series Doctor Who, also treats mineral-based life as a relatively rare but largely normal variant on the spectrum of life. To survey some of the most prominent of the mineral-based species described in the Whoniverse:

• The Krotons (from “The Krotons”, 1968) were crystalline beings comprised primarily of tellurium. Their crystal forms were given mobility by suits of robotic armour, in which their crystals were carried. They were able to drain the mental energy of humanoids to act as a power source, and this could also reconstitute broken down Kroton crystals. They were vulnerable to sulphuric acid.
• The Kastrians (from “Hand of Fear”, 1976) were silicon-based humanoids, who - thanks to the work of their scientist Eldrad - were able to reconstitute themselves from small fragments, once exposed to irradiation. Dwellers of underground thermal cave networks after their planetary environment was destroyed, they migrated back to the surface after protection was developed against a strong solar wind, and then suffered near-extinction when that protection was sabotaged. We're told that silicon-based life is sufficiently rare that finding two such species in the same galaxy would be surprising.
• The Ogri (from “Stones of Blood”, 1978) were classic silicon life forms in the form of large rocks. A group which crashed on Earth had been mistaken for a prehistoric megalithic circle, the stones of which derived nutrient from the proteins in mammalian blood.
• The Slitheen from Raxacoricofallapatorius (first seen in “Aliens of London”, 2005) had a calcium phosphate-based metabolism. While distinct from silicon based life, this is a similar mineral substance to chalk, and very different from the hydro-carbon based metabolisms of life elsewhere - although this didn’t seem to stop them enjoying human food. They were vulnerable to acetic acid.
• Weeping Angels (first seen in “Blink”, 2007) are a humanoid species who are, arguably, not mineral based at all, but who are only ever observed when transformed into stone statues since they are “quantum-locked” in this form when overseen. They feed on the life energy released by time-displaced individuals.
•  The Pyroviles (“Fires of Pompeii”, 2008) were fire-breathing silicon-based humanoids which appear to comprise stone fragments floating on a form of molten lava (a little like Star Trek’s Tholians in their later incarnation). Humans inhaling the powdered matter of pyroviles were themselves gradually transformed into rock, while also manifesting precognitive abilities.

Of course, Star Trek and Doctor Who aren’t alone in exploring this parameter space. The Stargate universe (although overwhelmingly human- or near-human based) introduced the silicon based Sekkari in the Stargate Atlantis episode “Remnants” (TV, 2008), while the Star Wars universe had several such creatures, including the Mynocks (first seen in The Empire Strikes Back, film, 1980) - vacuum-tolerant winged creatures which are capable of consuming starship hulls and wiring and also canonically have a silicon biology.

Silicon life also continues to appear in literary science fiction, although here models for emergent intelligence of the distributed software variety have become more common as information technology has developed. An interesting recent example of a science fiction short story on the theme is “Lumenfabulator” by Liu Yang (trans. Ladon Gao) which appeared in Life Beyond Us (anthology, 2023; eds. Novakova, Law & Forrest), an anthology which explicitly set out to explore astrobiology. The titular aliens are shown as clearly sentient individual crystals, which derive energy from tidal distortion stresses and add bulk to their forms through ingesting magma with potassium, aluminium and nickel contaminants. This crystalline form resonates with electromagnetic signals (including those transmitted by a far-distant humanity).

Animal, Vegetable or Mineral?

The fact that all animal and vegetable life that we know of, or would recognise as such, is based on a combination of hydrocarbon and water molecules has shaped our definition of a habitable zone (i.e. a region where liquid water could exist). Carbon is unique in the periodic table for the complexity and stability of molecules, molecular chains and carbon rings that it is capable of forming in the conditions of the Earth’s surface. But the hypothesis of mineral life raises the question of what conditions such life might require - what does habitability look like for these creatures? 

An interesting question - and the one most often applied to water for carbon life - is the range of permitted temperatures.

Several writers have suggested that silicon life would be favoured by high temperatures, with examples including the Tholians of Star Trek and the Pyroviles of Doctor Who. The argument here is that it is only at high temperatures that liquid magma or similar substances can interact chemically with mineral compounds that are inert at room temperature and so act as a lubrication permitting movement or a life-giving fluid analogous to blood. The higher melting/boiling point of silicon compounds might also permit mineral life to exist at higher temperatures, even if these are not strictly required.

By contrast, many writers have suggested that mineral-based life might thrive at very low temperatures and particularly the temperatures of deep space. This is true, for example of the asteroid-belt based silicon life popular in the mid twentieth century. Most of the reason for this is that the physical structure of minerals are deemed more robust against vacuum and low temperatures than organic compounds - although this takes little account of any necessary non-organic biochemistry (which, by definition, requires less stability). A biochemical argument is that a solvent which might act for silicon as water does for carbon is ammonia. This is liquid only at -75 to -34C. Other possible solvents such as liquid nitrogen or methane require similar cryogenic temperatures, and this sort of environment might allow the formation of more complex silicon-based molecules than Earth-like conditions. Indeed within our solar system, the gas giant moons Titan and Triton have been proposed as possible locations to search for silicon-based life due to their cold, reducing (oxygen-poor) environments and the abundance of possible solvents.

Another, alternate, argument in favour of low temperatures for silicon life actually emerges from the Discworld fantasy novels of Terry Pratchett, in which he hypothesises that (silicon-based) trolls become more intelligent when their brains are cooled to allow improved conductivity of electricity-based neural impulses. Arguably the same applies (although is not discussed, and borders on the silicon-life-as-software topic I wasn’t going to discuss) in the same author’s The Dark Side of the Sun (novel, 1976), in which a silicon planetary crust becomes self-aware through similar superconducting currents.

In reality though, the permitted temperature range for mineral-based biochemistries remains largely unconstrained. This is largely because the functioning of such a theorised biochemistry itself is not well constrained. As we’ve seen the majority of examples use silicon as the key element in a direct substitution for carbon. The reasoning behind this is relatively straightforward: silicon belongs to the same group in the periodic table as carbon and so has the same outer electron structure and can form the same sorts of compounds. However a silicon atom is larger than carbon and so those bonds are likely to be weaker and the production of such compounds less energetically favourable. In oxygen-rich, liquid water conditions, silicon preferentially forms very simple molecules - notably silicon dioxide (also known as sand). A common assumption, including one made by Dr McCoy in “Devil in the Dark”, is that free-oxygen atmospheres are inimical to silicon life entirely. In the case of the Horta, Spock suggests that brief exposure to human-habitable atmospheres might be tolerable, even if the life itself usually resides in an oxygen-free environment.

The instability of large silicon molecules is another reason to favour low temperatures - at high temperatures hydro-silicon compounds may be more likely to decompose than remain robust, and at low temperatures more complex silicon molecules are stable. By contrast, oxygen-silicon compounds, an alternative basis for long-chain molecules are, if anything, too robust. The two atoms bond tightly, preventing the constant reconfiguring of molecules that is necessary for life processes. Whatever its composition and environment though, silicon life would likely be slow: the chemical reactions involving this element, whether with oxygen, hydrogen or other elements, are all slower than those of carbon.

However other elements explored in SF may also have their advantages (whether or not these were thought through by the original authors). Examples mentioned above include germanium, tellurium and calcium.

Germanium is the next element in the same chemical group as silicon and carbon. Many of the same arguments that apply to silicon also apply in this case, with the element showing still less capacity for forming stable bonds than either of its lower mass brethren. An additional complication is that germanium is produced in very small quantities through stellar nucleosynthesis (unlike carbon and silicon, it is only generated in the s-process of slow-evolving, low-mass stars) and so is an extremely rare element.

Tellurium is rarer still (particularly on Earth) and has no obvious advantages as a basis for life, sitting in a very different chemical group (the metalloids). However, of its six natural isotopes, two are radioactive with a very long half-life, and so it is remotely plausible that life with some other basis might adapt to use this slow energy release in place of photosynthesis or similar biological energy fixing compounds.

By contrast with carbon, germanium and silicon, each of which can bond to four other atoms (tetravalent), calcium is only bivalent (bonding with two others) which prevents formation of complex structures such as DNA or biological proteins. Thus calcium life would be surprising and likely fragile - with less scope for variation than carbon-based life.

Whichever mineral is considered as the basis for life, an interesting question is whether or not it also requires hydrocarbon compounds. The space barnacles of Anderson’s Barnacle Bull, for example, are clearly stated as being based on both carbon and silicon compounds, as are the organosilicates of Foster’s Prism, and in several other cases, there is potential for ambiguity.

One frequent feature of science fiction implying a mixed basis is the presence of boundary crossing between different biospheres. The Ogri of Doctor Who, for example, consumed proteins from blood - which would make no sense if their biochemistry was not able to make use of such compounds in some manner. It is also common to see humans exposed to such mineral-based life also affected by petrification - as is seen for example in Doctor Who in “The Hand of Fear”, “Fires of Pompeii” and even “Ghost Light” (1989), in the Voyage to the Bottom of the Sea episode “The Fossil Men” (TV, 1967) in which humans exposed to a magma-like fluid are transformed to rock) and in films such as The Monolith Monsters (film, 1957; dir. Sherwood; in which an asteroid delivers crystals to Earth that not only grow into towering collossi, but also petrify anyone who touches them). Again this would require some biochemical coupling between carbon and other compounds - replacing one biochemistry with another (if only superficially).

This may not be entirely implausible. No elemental chemistry exists entirely in isolation from others. Certain human medical conditions, for example, result in the deposit of excess calcium, numerous animals create mineral shells around themselves, while chemical processes in any carbon-based life also requires trace amounts of other elements (notably iron, iodine, phosphorus, calcium and fluorine). However in each case that we know of, the active biochemistry (i.e. the source of energy, nutrition and tissue growth) remains based on long-chain carbon molecules.

Speculative Life

Many examples of mineral life in science fiction - as was the case for Barnacle Bull - explore the physics and chemistry of the elements on which they are based in a fair level of detail. They represent an opportunity for science-enthused writers to engage with our ever-changing understanding of the physical world - and for science-interested readers to do the same. They appeal to the basic remit of science fiction: to extrapolate from the known cutting edge of scientific understanding to explore possibilities that remain consistent with known science. Through the mid-twentieth century, stories of mineral life also provided an opportunity to circumvent our growing understanding of just how hostile most solar system environments are to Earth-like carbon-based life. The concept of silicon-based life in science fiction was originally inspired by our scientific knowledge of atomic structure. However it is also undoubtedly true that science fiction of silicon life has inspired further scientific exploration of the possibilities - leading to ongoing examples of peer-reviewed articles on the topic in academic journals dedicated to astrobiology and other areas of science. Unfortunately the majority of recent studies place fairly stringent limitations on the low probability of mineral-based life, although without ruling it out entirely.

Of course, in many cases, silicon-based life is just offered as a throwaway description - a way of saying “this thing is not like us.” In such cases, the details of such alien biochemistries are never explored, and likely were never even considered by the relevant writers. Hybrid cases like Sentenced to Prism by Alan Dean Foster give some consideration to the energy and resource utilisation by their organisms, but not to the point of exploring the biophysics or biochemistry of their mineral life forms, and often ignoring the known science which would rule out the life as described. In still more cases, the animation of mineral-like life can only be presumed to depend on unknown scientific principle, or rely on what seem to be energy fields to connect their components and communicate sensation rather than biochemical tissues (as is the case, for instance, in the rock creature in the film Galaxy Quest, 1999).

Sometimes the difference between mineral life and our own - whatever its mechanism or underlying principles - is used to inspire the instinctive distrust of the unknown in readers, in others it is used to demonstrate that such distrust is unjustified, and sometimes it is simply one of many features of an alien species as complex and varied in motivation and morality as humanity itself.

Perhaps we may one day locate silicon-based life, perhaps even within our own solar system. However for the moment, this remains an area for speculative thought and imaginative writing. Mineral-based life is different, is Other, and challenges us to accept intelligence and worth in individuals which look and act very differently to ourselves. And, in many cases, failure to do so would take a heart of stone.