Farneseite is a mineral from the cancrinite sodalite group with 14 layer stacking. It is a complex silicate mineral with formula . It was named after a location in Farnese, Lazio, Italy. It is a member of the cancrinite-sodalite group, approved in 2004 as a new mineral species. The group is characterized by the number of stacking layers making up each member, with farneseite being one of newest minerals in the group with a 14 layer stacking structure. It is a clear transparent mineral and has a hexagonal crystal system with crystal class of 6/m and space group of P63/m. The specimens discovered in Farnese were in a pyroclastic rock from the Làtera Cauldera region.
In the volcanic region of Latium, Italy, a few scientists found some crystals with hexagonal morphology while doing a study of the cancrinite-group minerals. These crystals were in a rock sample they had collected from a small village called Farnese in the Viterbo Province, north of Rome. They believed the substance to be sacrofanite, but after several diagnostic tests they observed the powder IR spectrum of the sample showed some differences from sacrofanite. They then performed a complete chemical and structural analysis which confirmed that the substance indeed was a new feldspathoid species related to the cancrinite-sodalite group. The cancrinite group are minerals with hexagonal and trigonal crystal systems with various anions and cations located in cages within the main frame. The crystal structures of the members of the group are characterized by the six membered rings of SiO4 and Al04 tetrahedra. The layers are lined up along the c axis in the ABCABABACBACAC… fashion.
Composition
Farneseite has an ideal chemical formula of [(Na,K)46Ca10]Σ=56(Si42Al42O168)(SO4)12•6H2O. An electron microprobe was used to obtain the micro-chemical composition of farneseite, which broke down the large chemical formula into smaller constituents of oxides that make up the mineral. The bulk of the mineral is composed of Si and Al oxides, both adding up to about 60% of weight. Other oxides of Ca, Na, K and S are also present and make up the difference. A tiny fraction of a percent weight is also due to the presence of F and Cl. Trace amounts of Fe, Ti, Cr, Mg, Mn and Ba were also detected (<0.1 Oxide %). H2O content was also calculated and FTIR spectroscopy confirmed only H2O molecules and no hydroxide ions in the sample.
Structure
A powdered sample of farneseite was used in an X-ray diffraction analysis to determine its structure. From the results obtained, it is evident that the space group of the mineral is P63/m. The unit cell dimensions in the a b c axis respectively are 12.8784 Å, 12.8784 Å and 37.0078 Å. The results also indicated a 14 layer stacking sequence of ABCABABACBACAC… where A, B and C indicate the positions of each layer of a six membered ring. The structure of farneseite agrees with the other minerals in the cancrinite-sodalite group which have layers of six-membered rings of tetrahedra, with each ring linked with three other rings on top and three at the bottom. This makes up a three-dimensional framework. The entire frame of the mineral is made up of Si and Al atoms. The other elements such as O, Ca, Na and K then fit along this ring in specific sites called cages. Within the group, there are various types of cages such as the cancrinite cages, the sodalite cages, the liottite cages, the losod cages among many. The cages have affinities to host certain types of anions and cations. The larger cages host large anions such as SO4, while the smaller cages host the relatively small anions such as Cl, F and H2O. The six membered ring then repeats 14 times, giving the mineral its special 14 layer stacking characteristic. This makes it unique. All minerals in the cancrinite-sodalite group have a similar framework, however, the number of layers of stacking are key in differentiating each mineral. The simplest minerals in the cancrinite-sodalite group have the basic two layer sequence, differing slightly in their Al/Si structure and placement of other anions and cations. These are cancrisilite, vishnevite, hydroxycancrinite, davyne, microsommite, pitiglianoite and quadridavyne.