Whats the impurity?

I bought some Iron powder, which was advertized as "Iron mineral powder" by a someone that wan´t sure what was selling. On asking if it was grinded Iron, she replied that it was not, that it was "mineral" iron. As it was really cheap (200grs for 1/2 usd - Argentinean exchange rate), bought it.
I need fine Iron powder for making FeS. (and was tired from filing a block of iron...)

Its a black magnetic powder. On dissolving in HCl (5M aprox) a yellow solution is obtained. (first test tube)
For testing if Iron, on adding NaOH/KOH a black precipitate is formed. On adding Ammonium oxalate an orange precipitate is formed.
I think its Iron.

But also a non dissolved leftover was found - did not weight it, but its small compared to the Iron powder. (lets say 1 - 2%) Lets call it "unknown".
The unknown was decanted, HCl removed by suction, and more HCl was added. It was boiled for 2 min. Did not dissolved. (I do not have concentrated HCl, only 5-6M (20%) muriatic acid. The solution was almost clear - faint yellow. This was done to remove leftover Iron.

The unknown is not magnetic (tested with a neodymium magnet). It was washed, decanted, and heated to remove leftover water. (small picture on the right) Its a silverish powder.

Then 2cm3 HCl (20%) was added + 2cm3 of HNO3 (60%) and boiled for 2 minutes. Did not dissolved.(the solution became opaque yellow - second test tube.

Acids were removed by suction, washed by decanting 2 or 3 times with water. Water removed by heat.

Then 1 cm3 of concentrated (almost clear) H2SO4 was added. Boiled for 2 minutes. It did not dissolved. Solution became grayish opaque (3rd test tube)

Acid were removed by suction, washed by decanting 2 or 3 times with water. Water removed by heat.

NaOH pellets were added, melted and boiled for 2 minutes. (as this is my first time with boiling NaOH, and know that boiling alkaly eats glass, did not push more time)

the 4th test tube shows the unknown not dissolved in hardened NaOH.

So:
1) Do I really have Iron Powder? (my understanding is yes)
2) Its possible to determine what the unknown should be?
3) What other test could I perform?
4) Maybe do the same experiment but boil it longer?

also:
3) How do you work with concentrated acids solutions to find if anything is dissolved?
a) conc. H2SO4
b) aqua reggia
(diluting 1/10 to work with not so concentrated acids? But also as these quantities are small, maybe I could not detect anything...)

Thanks

Quote: Originally posted by j_sum1

Oxalic acid will precipitate iron as iron(II) oxalate dihydrate. Roasting tgis will give you finely divided iron powder. So fine it is pyrophoric. I d9n't know if that is helpful for you. But it is fun.

Quote: Originally posted by Rainwater

"mineral iron" sounds like magnetite. if you leave it out in open air, it will turn into red hematite. if so the remaining insoluble stuff would be quartz. [Edited on 3-12-2023 by Rainwater]

Quote: Originally posted by RU_KLO

I think Rainwater is correct, after seeing pictures of magnetite its the same color (black). so its not iron but iron oxide. Regretfully I cannot use it for FeS making. ( this confirms why when heating with sulfur did not produce FeS). So Im waiting if I can transfrom it to iron with the oxalate route.