silicates
Anorthite
Ca(Al2Si2O8)
- Crystal system
- triclinic
- Hardness
- 6–6.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- White to grey
- Where it forms
- In calcium-rich metamorphic rocks and in basic igneous rocks. Anorthite-rich plagioclase makes up most of the lunar highlands.
- Chiefly from
- Vesuvius, ItalyJapanthe Moon
Safety
No hazard from handling.
Where it comes from
Not commercially traded to any real extent.
Historical use
The calcium end of the plagioclase series. The pale highlands of the Moon are anorthosite, a rock made almost entirely of calcium-rich plagioclase, and the contrast between those highlands and the dark basalt maria is what makes the face visible from earth.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Anorthite has no crystal-trade presence at all, which is why it is here. The pattern of what gets sold is not a pattern of what is significant; it is a pattern of what is pretty.
By any other measure this mineral is a heavyweight. The bright areas of the Moon — the parts that make it look like a face — are anorthosite, and they are bright because calcium-rich plagioclase crystallised early in a molten lunar ocean and floated. When you look up at a full moon, most of what you can see is this.
Identity checked against IMA-CNMNC master list, June 2025.