I found an old bottle labeled "soda asbestos" in the lab. It is a granular material with a grain size of about 0.5 mm. A newer label put on top says "sodium hydroxide".
It is brown, and when dissolved in water makes a brown solution with some residual brown grains at the bottom. The solution has a very high pH as one would expect from sodium hydroxide.
I saw an MSDS online that says it does not contain asbestos, despite being called "soda asbestos".
So what is it? Impure sodium hydroxide?
EDIT
Ok, so let's go on an adventure.
Here's the bottle:
This is what it looks like inside. Has a plug that fell inside I guess? You can clearly see the beige material.
Is the red stuff soluble? Put in two bottles, one stirred, the other isn't. If after a few hours the left settles or the right dissolves, then we have an answer.
Final result - it is definitely insoluble.
Very high pH as one would expect from a "sodium hydroxide".
To identify the red stuff, I poured the solution out after it settled, and then waited for it to dry. I scraped some of the red residue to carbon tape and took it to our SEM. This is how it looks:
Overall chemical composition is dominated by sodium and iron, with traces of other things like silicon, calcium, and aluminium (I assume all as oxides):
You can find an occasional needle in there - I wanted to make sure it is not asbestos. A quick EDS map shows that it is not a magnesium silicate (proper asbestos) but rather some kind of aluminium-rich silicate.
I guess this mostly answers my initial question of what is "soda asbestos": it is sodium hydroxide with iron oxide impurities (that is, "rust") and some other minor non-hazardous solids.
Then a revised question would be why is it called "asbestos" if there is no asbestos in it, and why does it have iron impurities? Do they serve a purpose, or is it just a cheap version of sodium hydroxide for applications where the impurities don't matter?