Forsterita Grupo del Olivino | Fayalita-Forsterita
Forsterita is a mineral with formula (Mg,Fe)₂SiO₄, in the Silicatos (Nesosilicatos) group. This specimen comes from Tinguatón, Tinajo, Lanzarote, Islas Canarias, España and joined the Terrium collection in 2025.
Description
Olivine–pyroxene xenolith (peridotite) in a remnant of basaltic ejecta. A "foreign fragment" within the rock. A deep-seated piece torn off and entrained by ascending basaltic magma, then trapped within the basalt on solidification—a rock within another, not formed in situ.
About Forsterita
In the olivine group, the structural and chemical situation is straightforward: there is a continuous solid solution between two well-defined endmembers, forsterite Mg₂SiO₄ and fayalite Fe₂SiO₄. The Mg/Fe ratio varies, but the crystal structure does not. The fayalite–forsterite series, commonly referred to as "olivine", is a continuous solid solution between Fe₂SiO₄ and Mg₂SiO₄; intermediate names formerly used according to the Mg/(Mg+Fe) proportion are now obsolete, and only the two endmembers are distinguished, with the boundary at Mg/(Mg+Fe) = 50%: if magnesium predominates it is forsterite; if iron predominates, fayalite. Hence many specimens labelled simply as "olivine" are in fact forsterite with some iron, while peridot is the gem variety of forsterite. Tourmaline is also a group, but olivine is a line; tourmaline is a map.
About the locality
In Taganana, in the Anaga massif (Tenerife), basalts crop out that include ultramafic xenoliths with abundant olivine. In these greenish fragments the magnesian member of the fayalite–forsterite series is prominent, typical of peridotites carried by basaltic magma.
Technical data
- Catalogue No.
- 0387
- Composition
- (Mg,Fe)₂SiO₄
- Name
- Forsterita
- Variety
- Grupo del Olivino | Fayalita-Forsterita
- Group
- Silicatos (Nesosilicatos)
- Category
- Meritum Persé
- Matrix
- Basalto
- Associations
- piroxeno
- Mine
- Tinguatón
- District / Municipality
- Tinajo
- Province
- Lanzarote
- Region
- Islas Canarias
- Country
- España
- Size (cm)
- 8 x 5 x 4
- Weight
- 294 g
- Acquired
- 2025
- Etymology
- Named in honour of Adolarius Jacob Forster; within the olivine group it denotes the magnesium-rich end-member of the series with fayalite.
- Quality
- Buena
- Value trend
- Estable
Related specimens
- Abellaíta — España
- Abernathyita — España
- Abernathyita — España
- Abernathyita — España
- Aerenita — España
- Aerinita (fibrosa) — España
- Analcima — España
- Analcima — España