# What minerals that are more durable than calcium would be suitable for bones?

Edit: I am asking in the context of alien life, should have specifyed that to begin with.

I have many ideas for fantasy/sci fi races, and one that I kept coming back to is a species with bones that are based on an element other than calcium.

Is this possible and what would a good element be?

• Bones are not made of calcium, they are made of hydroxyapatite and collagen. Hydroxyapatite and calcium are very different substances, with very different properties. Calcium is a metal; if you have ever seen a bone you have certainly noticed that it is not metallic. Calcium reacts spontaneously with water producing calcium hydroxide (a.k.a. slaked lime) and hydrogen; bones most definitely do not. Confusing hydroxyapatite with calcium is like confusing table salt with chlorine. – AlexP Apr 30 at 2:43
• What @AlexP meant by that (and he's 100% correct) is that what you really need to ask is, "is it possible to create an analogue of hydroxyapatite using a metal other than calcium that is suitable for bones?" (Hint: something in column 2 of the periodic table of elements could conceivably do the job, if the basis of life were appropriately modified.) – JBH Apr 30 at 2:57
• Edited to based on. Yea I kind of skipped most advanced biochemistry – Snydwell Apr 30 at 3:35
• @JBH after looking around for analogues for hydroxyapatite, I am sure using other elements in column II is not the best choice, since most of the other x(PO4) chemicals either dissolve in water readily or do not form a nice crystalline structure like Cax(PO4)x chains do. I have to do more research, but maybe changing up the anion for something like NOx or SOx might make pretty and stable crystals to act as bone. – Sonvar Apr 30 at 5:36
• @Sonvar, that's the basis of a good answer. I'm not at all surprised that the Potassium-Oxy bonds would need to change to something else. – JBH Apr 30 at 13:58

Bones are really fantastic, from an engineering point of view. We intensively use them for 80+ years with practically no big issues and they can even self repair, while whatever thing we engineer design is prone fatigue and wear.

This awesomeness doesn't come from the particular element which is used to make them, but from the peculiar way in which they are made: instead of being built once and forever, like we do with any metal beam, they are constantly demolished and reconstructed by the body cells (osteoblasts and osteoclasts), so that, effectively, your bones today are not the same bones you had one year ago.

This means that fatigue like we know in metallic alloys is less of a concern, as any microscopic damage is wiped out by this process. And by using this process we also see how the bones can self repair! And you thought that painting the Tour Eiffel or the Bay Bridge is San Francisco was a hell of a job!

You ask which other metal can be used to replace Calcium. In principle, any other element in the group 2 of periodic table, which contains

• Beryllium,
• Magnesium,
• Strontium,
• Barium,

could show similar chemical properties.

However calcium is the fifth most abundant element in the Earth's crust, the other being

• oxygen 46%,
• silicon 28%,
• aluminum 8.2%,
• iron 5.6%,
• calcium 4.2%,
• sodium 2.5%,
• magnesium 2.4%,
• potassium 2.0%,
• titanium 0.61%

Keep in mind that calcium is also widely used in our body:

The $$Ca^{2+}$$ ion acts as an electrolyte and is vital to the health of the muscular, circulatory, and digestive systems; is indispensable to the building of bone; and supports synthesis and function of blood cells. For example, it regulates the contraction of muscles, nerve conduction, and the clotting of blood. As a result, intra- and extracellular calcium levels are tightly regulated by the body. Calcium can play this role because the $$Ca^{2+}$$ ion forms stable coordination complexes with many organic compounds, especially proteins; it also forms compounds with a wide range of solubilities, enabling the formation of the skeleton.

Therefore, your closest replacement might be magnesium, based on its electronic configuration and abundance in Earth crust. But keep in mind that if you replace calcium with magnesium you also need to redesign all the system using calcium in our bodies!

• If using magnesium, you may need to redesign the whole crystal structure of bone in Cax(PO4)x(NO3)x as MaPO4 is not as versatile – Sonvar May 3 at 7:48

Silicon, as silica (SiO2).

Behold: the skeleton of the glass sponge.

https://www.atlasobscura.com/articles/sponge-record-climate-change-data

Glass sponges in the class Hexactinellida are animals commonly found in the deep ocean. Their tissues contain glass-like structural particles, called spicules, that are made of silica (hence their name). Some species of glass sponges produce extremely large spicules that fuse together in beautiful patterns to form a “glass house”—a complex skeleton that often remains intact even after the sponge itself dies.

https://oceanservice.noaa.gov/facts/glass-sponge.html

Silica is widely available on earth and is used by some animals to build skeletons / shells in the way most animals use calcium. The glass sponges are one example; radiolarian protists are another. As with "woven bone" it is not the mineral itself but the structure in which is laid down that confers the strength.

The glass sponges must be able to repair their silica skeletons. These animals get big, and very old.

• This is great, nice find! – Rob Apr 30 at 18:11

Found something that may work. Chitin, (C8H13O5N)n. If you can make an endoskeleton of it, its not based on Calcium.

• OP is asking about element. Chitin doesn't look like an element. – L.Dutch Apr 30 at 5:50
• Bone isn't just an element. its a mixture, as mentioned above, but chitin is a mixture that does not contain Ca. – Sonvar Apr 30 at 21:33

Why not carbon?

Carbon can form diamonds, carbon nanotubes stronger than steel and graphene sheets flexible as plastic but tougher than bulletproof glass.

A carbon based bone structure could use properties from all three forms to have bones that are virtually unbreakable.

Considered we are already a carbon based lifeform, carbon seems the obvious choice.

• This was actually used in James Cameron's Avatar movie. The aliens have carbon fiber skeletons. – Renan May 1 at 3:02

This isn't quite a direct answer as it's not replacing calcium in bones, but there are a few options for stronger biological materials.

• Limpet teeth use fibres of goethite which form 'the strongest' biological material known to man. Unfortunately, from my search I haven't yet found what constitutes 'strongest' in this instance. They're compared to spider silk which implies tensile strength, but I could well be reading into journalistic licence. For a substance to make good bones it needs good compressive and reasonable tensile strength. They're also described as 'hard', but hard substances can still have poor tensile or compressive strength. However, a benefit
• Abalone shells use a composite structure of calcium carbonate crystals and rubber-like flexible proteins. This gives them phenomenal shock resistance, and also reasonable compressive and tensile strength. This may be a very good option.
• Scaly-foot snails use shells that have a 3-layered composite structure with an outer layer that includes iron sulphides, a flexible mid-layer, and a calcified inner layer. This makes them very hard, very resistant to crushing forces and very good at dissipating shock.