Many quartz crystals, especially larger crystals, will exhibit some type of distortion of the basic crystal form. Although there are several types of distortion that occur at the locality, the most common occurs when the crystal is stretched along the y-axis. This creates a broader and flatter crystal with an unusual termination. The two prism faces parallel to the y-axis are much longer than the remaining prism faces. Also, the rhombohedral faces above and below these two prism faces are equally stretched along the y-axis.
The distortion is most noticeable at the termination of the crystal where an apex is formed by the two stretched rhombohedral faces instead of a 3 or 6 sided pinnacle. This apex is allowed to form, because the angle of the rhombohedral faces in relation to the prism faces remains constant and does not decrease to accommodate the expansion. On quartz from the Monroeville locality, this type of distortion can be very slight to moderate.
However, rarely is the crystal longest along the y-axis. The amount of distortion is best measured by the length of the apex termination. The photo to the left displays a 3rd generation crystal with this type of distortion. The y-axis distortion on this crystal is larger than average for the locality. Crystals from both the 2nd and 3rd generations of quartz will show this distortion.
One of the more unique crystal forms of quartz is the pseudo-cubical variety. Pseudo cubical crystals are created by severe rhombohedral distortion and disproportion. One rhombohedron is proportionally very large in comparison to the other. Also, the prism faces are generally very small, incomplete or non existent as illustrated in fig IV to the lower left. This form has been observed only in 3rd generation crystals. Although large examples of these occur at several worldwide localities, most notably high temperature alpine deposits in Austria, pseudo cubic crystals at the Huron River are generally less than 3 mm in size.
A typical example of this form is displayed in the photograph to the right. Quartz crystals often show varying amounts of this distortion and only those that are severely distorted appear cubic. Pseudo-cubic crystals are usually observed among regular diamond like 3rd generation crystals and are of similar color and clarity.
Other Quartz Modifications
Rarely, quartz crystals from this locality will show additional modifications to the basic form. The most common of these modifications is the trigonal bipyramidal “s” face. This face occurs at the intersection of two prism faces and two rhombohedral faces. An example of this modification is displayed in the photo to the right. The face modification has only been observed on crystals from the 2nd generation of quartz.
Crude second order prisms have also been observed on 2nd generation quartz crystals. These prism faces are typically very slight and occur in pairs, often incomplete, between two first order prism faces. A larger than average second order prism can be seen on the quartz crystal in the photo to the left.
Quartz var. Chalcedony
As mentioned previously, the first generation of quartz occurs as isolated growths of snow white botryoidal chalcedony. This generation of quartz is by far the most seldom generation encountered. Spotty sulfide minerals, especially pyrite, are often intergrown and dusted on the chalcedony , as is evident in the photo to the right.
This first generation chalcedony is typically coated with crusts or seams of drusy multigrowth second generation quartz. An example of the first two generations of quartz is displayed in the photo to the left. As is true of this specimen, much of the chalcedony at this locality can only be observed on the broken edges of specimens. Although the second generation quartz appears to radiate away from the chalcedony, chalcedony does not have a radiating structure.