Abstract: Amphiboles of the tremolite-ferro-actinolite series from the Bergslagen district in central Sweden have between 0 and 46% of their M1–M3 sites occupied by Mn. These amphiboles are divided into two groups; (1) a group characterised by Mn (M1–M3) > (Mg or Fe) (M1–M3), and (2) a group with much lower contents of Mn (M1–M3), showing the following chemical characteristics: (a) Mn (M1–M3) never exceeds Fe (M1–M3); (b) in Mg-rich members of the group [Mg (M1–M3) > (Mn or Fe) (M1–M3)], the highest amounts of Mn (M1–M3) occur in amphiboles with about 2.20 Mg (M1–M3) per formula unit (pfu); in Fe-rich members of the group Fe [(M1–M3) > (Mg or Mn) (M1–M3)] high amounts of Mn (M1–M3) [> 0.4 Mn (M1-M3)] pfu only occur in amphiboles with >3.40 Fe (M1–M3) pfu; (d) the lowest amounts of Mn (M1–M3) occur in amphiboles with Mg (M1–M3) = Fe (M1–M3). Mn (M4) increases with increasing amounts of Mn (M1–M3), indicating that solid solution towards the tirodite-dannemorite series is enhanced by incorporation of Mn (M1–M3).