oligoclase

A total of 7 primary minerals, that is, quartz, kaolinite, illite, K-feldspar, arsenopyrite, calcite, and magnesite, were chosen, and another 5 secondary minerals (chlorite, dolomite, goethite, oligoclase, and smectite-Ca) were expected to be precipitated (Table 2).

Their highly reflective surfaces produce optical effects such as aventurescence in oligoclase, often called sunstone (Gubelin and Koivula, 1986, pp.

The plagioclase has oligoclase nuclei and albitic rims.

These data indicate that the plagioclase has An values of 23.02-36.15 and that the granodiorite contains oligoclase (An = 23-28) and andesine (An = 30-36) plagioclase (Fig.

Plagioclase is cloudy to saussuritized in some rocks especially in the more calcic cores but where fresh it is of intermediate anorthite content (calcic oligoclase to andesine).

In addition, the mineralogical composition of the feldspar was determined from X-ray diffractogram (Figure 2(a)), and the following mineral phases were identified: oligoclase, quartz, illite, and calcite.

This is also supported by the presence of albite and oligoclase, oligoclase also indicating an intermediate volcanic source.

Subhedral grains of plagioclase (oligoclase) often show zoning caused by slight chemical variations.

It is composed of feldspar, usually potash feldspar and oligoclase and quartz, with a small amount of mica and minor accessory minerals, such as zircon, apatite, magnetite, ilmenite and sphene.

The melanophyres of the Ilfeld Basin, which come from the earlier or earliest of the eruptive episodes, are brown to black, with a fine-grained groundmass composed of oligoclase, augite, biotite, apatite and magnetite, with phenocrysts, most of which are tabular crystals of augite (Luedecke, 1896).

Day and his colleagues immediately recognized that these meteorites were unusual because of elevated contents of a light-colored feldspar mineral called oligoclase.