Mn-Cr garnet

Table 5.25 Parameters of the Kieflfer model for calculation of Cy and harmonic entropy for the 12 garnet end-members in the system (Mg, Fe, Ca, Mn)3(Al, Fe, Cr)2Si30i2. is the lower cutoff...

Steady-state photoluminescence spectra of natural garnet were connected with a Cr " broad emission band peaking at 880 nm (Tarashchan 1978). Two strong bands, one in the yellow, and the other in the red characterize CL spectra of natural hydrogrossular. The first band at 590 nm was ascribed to Mn " substituting for Ca ". The red one extends into the NIR and has narrow peaks at 689,701 and 717 nm and was not interpreted (Ponahlo 2000). 

Fig. 4.56. a-d Laser-induced steady-state luminescence spectra of garnet demonstrating Cr, Mn, and possibly Mn and V centers... 

Figure 29 X-ray maps of Mn, Ca, Fe/(Fe + Mg) ( FM ), Cr, and Sc in garnets from Fall Mountain, New Hampshire. White box indicates area of major element maps. Different generations of growth can be linked to prograde reaction history, to better understand mineralogical changes attending metamorphism and anatexis, and overall P-T evolution. Black line shows location of composition traverse plotted in Figure 30...

Garnet-structure ferrites. Garnet, ideally A3 B2[T04]3, is isometrie (Ia3d) with a straeture built of TO4 tetrahedra and BOe oetahedra linked by shared vertiees that form AOg dodeeahedra. The A-sites aeeommodate divalent (Ca, Mn, Fe) and trivalent (Y, REE) elements. The B-site may contain various di-, tri- (Fe, Al, Ga, Cr, Mn, In, Sc, Co), tetra- (Zr, Ti, Sn, Ru) and even pentavalent elements (Nb, Ta, Sb). The T-sites are filled with tetravalent (Si, Ge, Sn) but may be also occupied by trivalent (Al, Ga, Fe) and pentavalent (P, V, As) elements. Synthetic garnets form multicomponent solid solutions that provide opportunities for incorporating of HEW elements into the structure [149]. 

When the O/Si ratio is 4, the structural units are the isolated (Si04) tetra-hedra which cannot join to each other but are connected by the positive ions in the crystal structure. The resulting structure is termed an island silicate for which garnets (Mg, Fe , Mn,Ca)3(Cr,Al, Fe )2(Si04)3 and olivines... 

In the crystal structure of garnet, with the general formula A3B2C3O12 [A = Ca, Fe +, Mg, Mn"+, R B = A1, Cr, Fe +, Mn +, Ti, V -", Zr C = Si (including H4 instead of Si) Al, Ti], there are three types of sites which are filled with metal ions tetrahedrally coordinated sites filled with C cations, octahedrally coordinated sites... 

---