Ferro-actinolite is the ferrous of the actinolite-tremolite continuous solid solution series of the double chain calcareous amphibole minerals. All the series members belong to the monoclinic crystal system.
The following formula comparison indicates the position of individual well-known members within the series: tremolite : ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2 actinolite : ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2 ferro-actinolite: ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22(OH)2
Some other substitute cations that may replace either Ca, Mg, or Fe include potassium (K), aluminium (Al), manganese (Mn), titanium (Ti), and chromium (Cr). A fluorine (F) anion may partially replace the hydroxyl (OH).