Refinement of the crystal structure of a synthetic non-stoichiometric Rb-feldspar

Abstract

The crystal structure of hydrothermally synthesized Rb-feldspar (monoclinic, space group C2/m, a = 8.839 (2) There are two geochemical implications from this refinement: (1) identification of both rubicline triclinic with (Al,Si) ordered distribution and synthetic monoclinic RbAlSi 3 O 8 with (Al,Si) disordered distribution implies that Rb cannot be one of factors disrupting the (Al,Si) ordered and disordered distributions in feldspars; and (2) natural and synthetic feldspars capable of accommodating the large cations tend to incorporate &Si 4 O 8 , excess Al and H 2 O components in their crystal structures.

Aluminum position in Rb-feldspar as determined by X-ray photoelectron spectroscopy

“…Chemical composition of synthetic Rb-feldspar is in good agreement with analyses of previous Rb-feldspars (Kyono and Kimata 2001). Analysis of Rb-feldspar sums to less than 100 wt% by about 3.7 wt% (Table 1): this deficiency is presumed to result from structural H 2 O because of the presence of a clear IR peak assigned to OH-stretching vibration (Kyono and Kimata 2001).…”

“…Analysis of Rb-feldspar sums to less than 100 wt% by about 3.7 wt% (Table 1): this deficiency is presumed to result from structural H 2 O because of the presence of a clear IR peak assigned to OH-stretching vibration (Kyono and Kimata 2001). The numbers of cations for each feldspar were calculated on the basis of eight oxygen atoms per formula unit (apfu): the sums of the values of the T-cations (Si + Al) were always over 4.000 apfu, whereas those of M-cations were consistently below 1.000 apfu (Table 1).…”

“…The synthesis procedure for Rb-feldspar is that of Kyono and Kimata (2001). Powder X-ray diffraction patterns were recorded using a Rigaku RAD-A instrument equipped with monochromated CuKa radiation in the range of 2q=2-70.…”

“…Since Schwantke (1909) proposed the formation of myrmekite, the problem of excess Si and Al in feldspars has defied convincing explanation (e.g., Bhatty et al 1970;Grundy and Ito 1974;Teertstra et al 1999;Kyono and Kimata 2001). On the other hand, several techniques for Al/Si configurations have been developed during the last 40 years (e.g., neutron diffraction, NMR, TEM, IR, and Raman spectroscopy; Smith and Brown 1988).…”

Porphyry indicator minerals and their mineral chemistry as vectoring and fertility tools

“…Although these results are promising, there remains more work to be done to confirm that this behaviour is systematically observed in mineralized systems and, indeed, what it represents. Experimental studies by Kyono and Kimata (2001) …”