What are these tiny holes?

[Aga Tentakulus]

Presumably on spectrum XE4 the electron beam hit a grain of some iron oxide mineral, that often contains titanium.  But titanium would not be present in green vitriol, the "iron" ingredient of iron-gall ink.

Titanium in the ink.
People know this, they just don’t always remember it.
Water, colouring agent, gum arabic, writing sand. A brilliant combination.
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Titanium in the sand.
Almost certainly rutile. Found everywhere.
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Dispersed by glaciers and rivers.
Nagelfluh
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