Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 199

mineralchemistry and relationships 195
cent of the analyses and in the remaining five per cent, Ti, Cr, Mn and P
were determined. These five per cent are plagioclases only.
Except for the decrease of Ca in plagioclases and K in alkali feldspars in
the evolution of these minerals towards albitic compositions, these elements
show some systematic behaviour, which will be briefly described.
I he K20 concentrations in the plagioclases are mostly low, but increase
'vith decreasing An content, which seems to be a general rule (see Heier,
1962). In the most An rich plagioclases K20 is much less than 1 wt. per
cent, but below An50 higher concentrations may occur and below An38 most
plagioclases contain over 1 per cent KzO, steadily increasing towards the
anorthoclase concentrations. There is a clear tendency for K in the late
crystallized plagioclases of each sample (i.e. groundmass grains and pheno-
cryst margins) to be higher than in the early crystallized ones (i.e. pheno-
1ABLE 31 Representative analyses of feldspars
30-GI 27-Pc 80-Pc 80-Pc 36-Pe 20-Pc 166-Gr87-Pc 87-Pc 20-Pe80-Gr66-Gr 87-Pc 12-Mp
Sio2 45.13 45.64 46.71 46.88 46.51 47.51 47.05 48.14 48.49 49.88 50.91 50.48 52.23 53.61
Ah03 34.10 33.50 34.29 33.65 34.20 32.21 32.64 31.54 31.36 30.75 31.06 30.42 29.23 30.03
SrO 0.00 0.09 0.03 0.08 0.00 0.10 0.00 0.08 0.21 0.24 0.08 0.19 0.22 0.00
BaO 0.00 0.00 0.02 0.00 0.08 0.00 0.00 0.00 0.00 0.08 0.00 0.00 0.00 0.01
FcO 0.56 0.61 0.45 0.57 0.21 0.60 0.78 0.53 0.57 0.64 0.75 0.76 0.49 0.00
MgO 0.07 0.06 0.05 0.06 0.02 0.11 0.00 0.11 0.06 0.15 0.13 0.21 0.07 0.00
CaO 17.92 18.41 16.48 16.68 16.66 16.45 16.17 15.83 15.54 14.85 12.98 13.57 13.01 11.70
Na20 1.11 1.43 1.48 1.77 1.88 2.03 2.42 2.67 2.98 3.17 3.26 3.84 4.29 4.56
k2° 0.09 0.13 0.17 0.18 0.16 0.24 0.11 0.35 0.29 0.46 0.44 0.44 0.43 0.24
Total 98.98 99.88 99.68 99.86 99.73 99.25 99.15 99.25 99.51 100.22 99.61 99.91 99.97 100.15
Number of cations on basis of 32 oxygens
Si 8.423 8.474 8.605 8.643 8.578 8.817 8.743 8.932 8.978 9.156 9.312 9.266 9.534 9.669
A1 7.501 7.330 7.445 7.312 7.435 7.045 7.149 6.897 6.843 6.652 6.696 6.581 6.309 6.383
Sr — 0.010 0.003 0.009 — 0.010 — 0.009 0.022 0.025 0.009 0.019 0.023 —
Ba — — 0.001 — 0.006 — — — — 0.006 — — — 0.001
Fe 0.087 0.095 0.069 0.088 0.032 0.093 0.121 0.081 0.088 0.098 0.114 0.115 0.075 —
Mg 0.019 0.017 0.013 0.017 0.006 0.030 — 0.030 0.017 0.041 0.035 0.057 0.018 —
Ca 3.583 3.663 3.252 3.295 3.293 3.271 3.219 3.147 3.083 2.921 2.543 2.668 2.543 2.260
Na 0.402 0.515 0.527 0.631 0.673 0.729 0.871 0.961 1.070 1.127 1.156 1.365 1.518 1.593
K 0.020 0.029 0.039 0.042 0.038 0.056 0.027 0.082 0.069 0.108 0.101 0.101 0.099 0.054
Mol. per cent
An 89.4 87.0 85.1 83.0 82.2 80.6 78.2 75.1 73.0 70.3 66.8 64.5 61.1 57.9
Ab 10.0 12.2 13.9 15.9 16.8 18.0 21.2 22.9 25.4 27.1 30.5 33.0 36.5 40.7
Or 0.6 0.8 1.0 1.1 1.0 1.4 0.6 2.0 1.6 2.6 2.7 2.5 2.4 1.4
Numbers above analyses refer to rock sample. Pc=phenocryst core, Pe=phenocr.
edge, Mp=microphenocr., Gl=glomerophyric microphenocr., Gr=groundmass
grain, Gb=gabbroic xenolith, Hm=hydrous mineral xenolith, Sy=syenitic xenolith,
It=inclusion in titanomagnetite. 0.00=not detected.
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