Photomicrographs of Feldspar

This photo shows a large plagioclase grain which is easily identified by the twinning in the photograph with polars crossed. The grain shows a combination of two types of twins which are probably Carlsbad(simple twin) and albite(multiple twinning). The cloudiness seen in PPL is caused by patchy alteration of the feldspar. The highly birefringent, fine-grained alteration product is probably sericite, a mica.(*)

This photo shows a pebble-sized fragment composed almost entirely of microcline. Microcline can be identified easily by the cross-hatched twinning which it invariably shows. Although the microcline shows little alteration, feldspar grains in the upper left, including multiple-twinned plagioclase, are brownish colored as a result of alteration. In contrast, the quartz in the upper right is relatively clear and unaltered.(*)

Grains showing perthitic intergrowths, comprising blebs or lamellae of sodium-ruch feldspar in potassium-rich feldspar, are not uncommon in sediments. This photo shows a very coarse sand-sized fragment of perthite. Most of the other sediment grains are quartz and the matrix contains highly birefringent mineral grains too small to identify at the magnification shown.(*)

Complex twinning in a plagioclase feldspar grain. Exact types of twinning are best determined on a universal stage, but albite, carlsbad, and pericline twins are probably present here. Twin types can sometimes indicate source area. Pink tint grains resulted from staining for plagioclase(Ordovician Newtown Gneiss, Connecticut).(**)

Plagioclase feldspars (unstained) in a volcanic sandstone. Note euhedral crystal outlines, well-defined crystal zonning (growth-composition lines) and the albite twinning. All these features, taken together, are indicative of volcanic plagioclase(Tertiary Horse Spring Fm., Nevada)(**)

Microcline feldspar with typical microcline grid twinning. Although such twinning is characteristic of most triclinic alkali feldspars, it is most commonly shown by microcline. Some small inclusions of plagioclase with albite twinning are present here(Ordovician Newtown Gneiss, Connecticut).(**)

Yellow grain in center is a microcline feldspar with spindle twinning-the irregular lamenar twins can often be used to distinguish mircocline. Yellow color is a stain for K-spar. The brown, elongate grain derectly to the left of the mircocline is biotite(Precambrian Hitchcock Lake Mbr. of Waterbury gneiss, Connecticut).(**)

A plagioclase feldspar largely replaced by calcite. To accurately determine composition of sandstones it is often necessary to recognize feldspars in very advanced stages of destruction. Calcite replacement is one very common form of diagenetic alteration(Pennsylvanian-Permian Sangre de Cristo Fm., New Mexico).(**)