PERHAMITE, A RARE PHOSPHATE FROM MAINE

 

Although I enjoy collecting minerals, especially phosphates, in the pegmatites of the Black Hills of South Dakota I often wish the rocks of Maine were a wee bit closer to my home in Colorado! Since the journey is more than a day trip I am left with purchasing specimens found at rock and mineral shows, a good alternative. Interestingly, to an ole softrocker like me who spent most of his professional career looking for fossils in Mesozoic and Cenozoic outcrops, rocks of Maine generally lack exposures of these ages except for scattered small igneous intrusions and dikes of Mesozoic age. Maine rocks are Precambrian and Paleozoic (mostly Cambrian through Devonian) in age with a covering of unconsolidated sediments left behind by Pleistocene glaciers. The latter seems to support a heavy growth of trees and undergrowth that often inhibits the discovery of the underlying bedrock. However, there are many “stone” quarries around the state that have allowed geologists and rockhounds to collect a bonanza of interesting minerals and gems.  Geology.com noted that the first commercial gemstone mine in the U.S. dates back to 1821 when gem tourmaline was collected near Paris in a pegmatite on a hill known as Mt. Mica.  That mining has continued to the present. Most Maine collecting localities are in pegmatites scattered across the state (especially in the counties of Oxford, Androscoggin, and Sagadahoc ---AKA Maine Pegmatite Belt) that offer rockhounds and geologists opportunities in both commercial mines, such as Poland Mining Camps, and in abandoned quarries.  In addition, there is the Maine Mineral and Gem Museum in Bethel containing stunning displays, and also housing the Mineralogy, Petrology, and Pegmatology Research Group. The latter entity “focuses on exploring the pegmatites of Maine and their associated minerals.”

One of the most studied pegmatites in Maine is the Emmons Pegmatite found on Uncle Tom Mountain in Oxford County where beryl and other minerals were found in the early 1900s and first mined for feldspar in the 1930s; however, that venture was rather short-lived due to the long distance of the mine from a processing plant.  Early mineral collecting enthusiasts then started extracting beryl and ultimately collected about 5000 carats of gem morganite (pink beryl); however, specimen mining did not really take off until the late 1900s.; ---see the complete collecting and mineral story in Falster and others (Rocks and Minerals, v.94, no.6).

One of the interesting minerals from the Emmons that popped up in rummaging through my collection (from a purchase several years ago) was the phosphate perhamite [Ca₃Al_7.7Si₃P₄O_23.5(OH)_14.1-8H₂0], a specimen originally collected by Micromounter Hall of Fame member Gene Bearss.  The mineral is a rare calcium aluminum silico-phosphate that usually is found as isolated small spherules in late-stage pegmatite vugs. The Bearss specimen (photo below) shows the common morphology--a spherical aggregate of subvitreous hexagonal plates. Falster and others noted that perhamite from the Emmons pegmatite ranks among the finest examples of this species. Width of the spherule is ~1.0 mm.