Helvite

the group name for a mineral series with incomplete isomorphism, including helvite Mn₄(BeSi0₄)₃S, danalite Fe₄(BeSi0₄)₃S, and genthelvite Zn₄(BeSi0₄)₃S. The helvites are related to beryllium silicates, and they have a framework structure similar to the structure of the minerals of the soda-lite group. The composition of the intermediary minerals in the helvite group varies according to their Mn, Fe, and Zn content. The physical properties vary, and the color ranges from reddish brown (helvite) to yellow and colorless (genthelvite). The density of helvite is 3,200 kg/m³ and of genthelvite, 3,700 kg/m³; their hardness on the mineralogical scale is 6.0 to 6.5. Helvite crystallizes in the cubic system. It is found in the form of tetrahedric crystals, irregular granules, and impregnations in pegmatites, quartz veins, skarn deposits, and greisenized alkaline syenites and granites, enriched with sulfide minerals. In these forms it is found in Mount Francisco (western Australia), Iron Mountain (New Mexico), and elsewhere. Helvite is a valuable ore for the extraction of beryllium.