Summary: The cell parameters of 35 members of the arfvedsonite-eckermannite series and catophorites (17 from the literature, 18 newly determined) are presented and an attempt made to relate them to chemical composition. In general, all parameters (including β, obtuse) decrease from arfvedsonite to eckermannite, but for different reasons: a, b, and β (very slightly) as the Mg: Fe²⁺ ratio and F increase; c as Fe³⁺ decreases. The apparent decrease in a and b with high Li is coincidental, due to its strong correlation with F, which varies inversely with (OH). In addition, b is increased by high Ca and slightly reduced by high A1.

Also, the empirical and calculated curves relating MgO and total iron as Fe₂O₃ (weight %) with the Mg ratio, 100 Mg/(Mg + Fe²⁺ + Fe³⁺ + Mn), for all amphibole groups are given, as a rapid method of obtaining an estimate of the Mg ratio.

The full lists of possible hkl values for an arfvedsonite and an eckermannite are enumerated up to 45° 2θ, Cu-Kα, to illustrate the difficulty in indexing X-ray photographs and diffractometer charts of this group.