**4. Conclusions**

Magnetite crystalized in hypersaline 2–5M brine at elevated temperatures 60–80°C and pH 6.5–10. At high alkalinity, their initial formation stage of hexagonal plates was preserved. At lower alkalinity, their morphology was unhedral cubes. Additional Si to the brines caused smaller crystals.

Lepidocrocite and goethite were formed at lower temperatures (40°C). High alkalinity effected crystal morphologies causing multi-domain structure in goethite and in lepidocrocite and twinning formation.

Additional Si hindered crystal growth.
