Periclase

Periclase (MgO) is a material with good properties that offers
slow but present degradability when spilled into nature.
It degrades faster than quartz.

Advantages:

• High crystal structure symmetry - simple cubic rock salt structure
• degradability
• Magnesium is a very common element
• Magnesium and its compounds are highly nontoxic environment and health friendly

Disadvantages:

• not the highest material strenght but decent
• degradability
• maybe questionable passivatability?

Basic properties:

• Crystal structure: simple cubic – like rock salt NaCl
• Hardness: Mohs 6
• Density: 3.58g/ccm
• Melting point: 2800 C°
• Water solubility: barely soluble but still measurably soluble => not suitable for externally water exposed nanomachinery
  (wiki-TODO: find quantitative numbers)

Related

• Base materials with high potential

MgF₂ (different crystal structure) is also a magnesium based compound that is quite a bit more but still limitedly water soluble.
But fluorine is not terribly abundant and a health risk in too high concentrations.

External links

• As the mineral periclase
• As the chemical substance magnesium oxide

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• MgF₂ Magnesium fluoride mineral sellaite – Mohs 5.0 to 5.5 – 3.15g/ccm – very slightly soluble in water (0.13g/liter) – rutile structure