Photomicrographs of Quartz

 This shows subrounded quartz grains which  are single crystals, taken with crossed  ploars(XPL). The matrix between the sand  grains contains opaque iron oxide and some  calcite. The latter shows high-order pink and  green interference colors. (*)

 The three rounded grains in the center are  made up of a number of quartz crystals in  different orientations and are thus composite  or polycrystalline quartz.The composite  nature of the grains is clear only in the view  taken with polars crossed. Note that the  boundaries between the crystals are sutured.  This is the characteristics of quartz from a  metamorphic source. The much finer sidiment  surrounding the composite quartz grains  contains monocrystalline quartz and brownish  clasts of fine- grained material which are  probably shale or slate fragments.(*)

 This shows a composite quartz grain viewed  under crossed polars, in which not only are  the crystal boundaries whthin the grain  sutured, but also the crystals are elongated in  a preferred direction. Such grains are called  sheared quartz or stretched metamorphic  quartz. In this type of quartz, individual  crystals normally show undulose extinction  as a result of stain.(*)

 The quartz grain in the center of the field of  view is made up of parts of two crystals. One,  comprising the upper left portion of the grain  is showing a mid-grey interference color,  whereas the rest of the grain comprises a  crystal with areas showing a slightly different  interference colors. The left- and right-hand  sides are in extinction and interference colors  become progressively paler towards the  center of the grain. Such a grain would show  sweeping extinction (undulose extinction)  when rotated.(**)

 Detail of volcanic quartz crystal. This grain  has straight extinction, a euhedral outline,  and a large 'negative crystal' or vacuole. The  vacuole has the same crystallographic  orientation as the complete quartz grain,  hence the term 'negative crystal'. This feature  is common but not ubliquitous in quartz of  volcanic origin(Pleistocene Yellow Group(tuff),  Wyoming).(**)

 A volcanic quartz grain with euhedral,  bipyramidal outline. Euhedral shape,  embayments, straight extinciton, and scarcity  of inclusions are all indicative of an extrusive  igneous source, but none, by itself, is  conclusive evidence(Pleistocene Yellow  Group(tuff), Wyoming).(**)

 A nondetrital quartz grain(in a nonsedimentary  'source' rock) showing rounded outline and  embayment. Thus, not all original grains are  angular, and embayment is not restricted to  volcanic quartz. Quartz crystal(photo center)  is surrounded by plagioclase feldspar  (Pleistocene Yellowstone Group(tuff),  Wyoming).(**)

 Large grain in center is a single-crystal,  slightly undulose quartz grain ('end phase' or  'igneous' quartz of Krynine, 1940 and 1946).  Grain extinguishes completely with between 1  and 5 degrees of stage rotation. Such  extinction behavior is best studied using a  universal stage but can be done with less  accuracy on a flat stage. Slightly undulose  quartz can be derived from most types of  source terrains(Upper Cambrian Gateburg  Fm., Pennsylvania).(**)

 A semicomposite quartz grain with slightly  undulose extinction. Grain consists of a  number of separate quartz crystals with very  closely aligned optic c-axes. Such grains are  common in hydrothermal veins but also occur  in many metamorphic and plutonic rock  types(Upper Triassic New Haven Arkose,  Connecticut).(**)

 Quartz grains with abundant needle-shaped  mineral inclusions. The inclusions in this case  are sillimanite, but actinolite, tremolite, rutile,  and other minerals can also be found as  needle-like inclusions in quartz. Detrital  quartz grains with sillimanite inclusions are  excellent evidence for a metamorphic source  area (Paleozoic andalusite schist, New  Hampshire).(**)