Delaware mineralogical society

A Delaware 501(c)(3) non-profit earth-science educational organization

 

Mineral Hill, Elwyn, PA

Minerals

Mineral Hill, Elwyn, Pa
Field Trip Report
Delaware Mineralogical Society
Sunday, Nov. 22, 2014
 T. Pankratz

 

On Sunday, Nov 22 about 20 members of the Delaware Mineralogical Society and The Pennsylvania Friends of Mineralogy met and collected minerals at a construction site on Mineral Hill, near Elwyn, Pa.  

 Mary Ann Levan learned of this site from a friend who stopped to collect garnets (there aren’t any) and set about trying to get mineralogical and geologic information about it.   She found it surprisingly hard to find any hard information, especially for a place that so many people recognize by name. Through contact with Sandy Schenck (Delaware Geologic Survey), Hal Bosbyshell ( West Chester University) and Roger Mitchell (Delaware County Museum, Media Pa), she  learned that the museum “has the original plates” from the book by Samuel Gordon (The Mineralogy of Pennsylvania: Academy of Natural Sciences of Philadelphia Special Publication no. 1), and was able to purchase a copy.  This book is organized by county and township.  Within a township, it lists farms, mines and quarries and the minerals found there.  Mineral Hill is in Middletown Township, Delaware County.  A few relevant entries include:

Mineral Hill: exposures along the road from Media to Blackhorse; just west of Ridley Creek; about a mile northeast of Elwyn.  Wissahickon gneiss: sillimanite, cyanite, staurolite.

Crump’s Quarry, on the grounds of the Pennsylvania Training School for the Feeble-Minded.  Pyroxenite and metaperidotite: enstatite, actinolite, tremolite, hornblende, serpentine, talc, chlorite, vermiculite, deweylite, chromite, magnesite, ferruginous quartz.
 

Small  abandoned feldspar quarry and exposures about 1/8 mile north of Crump’s quarry. Pegmatite and metaperidotite.  Orthoclase, microcline- amazonstone, albite- moonstone, oligoclase- sunstone, almandite, vermiculite, columbite

Additionally, Bill Stephens found:

The Mineral Hill (Crump Serpentine mine at MH ) was well known for albite feldspar and oligoclase feldspar, semiprecious gem varieties known as moonstone and sunstone, respectively.  This is sodic plagioclase commonly found in association with recrystallized commonly serpentinized gabbroic rocks and dunites.

A mine north of Mineral Hill Crump's serpentine quarry was worked for feldspar including gemstones.  Some research into the old maps, maybe the second Pa Geol Survey reports may reveal its exact location, but suffice it to say the rocks, however weathered, were consistent with the ultramafic suite mapped across the Mineral Hill area and that includes Crumps Serpentine Quarry and the old feldspar mine.

 The museum has a collection of local minerals, as well as other natural history objects.  Some of the birds were collected by one of North America’s foremost ornithologists (Cassin).  This area was very active scientifically in the early history of the United States.   Today, you can go to the Delaware County Museum of Science to see a great collection of minerals from the Mineral Hill area, and also to attend a monthly science talk.

The construction project covers about 30 acres of the upper part of Mineral Hill on the north side of Pa 1 (Baltimore Pike) about a mile west of Media, Pa.   The construction project is primarily in the excavation phase though some roads, retaining walls and condominiums have been completed. Permission was obtained to visit and collect at the site.  Natural Lands Trust recently acquired the lower part of Mineral Hill towards Ridley Creek and created a development plan which lays out trails and public use amenities. 

The site contains an incredible mix of soil and rock types.  The two main rock types are serpentine and gneiss, with some pegmatite veins .The serpentine and gneiss appear to be interlayered, both vertically and horizontally.  It appears the serpentine was intruded by a granitic magma before  the Taconic Orogeny, and subducted and metamorphed during the orogeny.  Later erosion and uplift created fractures allowing pegmatite formation in veins.   Futher erosion brought it to the surface where today we have weathering gradation (hydration/oxidation) from surface mud, through saprolite, to hard rock.  Finally construction excavation has mixed and scattered everything and covered rocks with mud.   Thankfully, many of the rocks on mounds and along the silt fence have been washed fairly clean by rains.  Many of the exposed rocks, both gneisses and schists, are so weathered as to be easily broken by hand.  Many of the minerals found were weathered beyond suitability for collection and display.

There is a pile of 3’-5’ boulders that were excavated by blasting, though the construction company crushed much of the rock creating a pile of 4-5” stones and another of 1” gravel.   Most of this rock is gneiss.   The upper half of the site appears to be a mix of serpentines and gneiss.  There are many chlorite schist veins on the lower half creating green swaths where exposed by grading.  Though hilly, most of the site is graded smooth.  But there are a lot of different kinds of rocks up to 3’ on several hillsides, mounds and particularly along an erosion fence on the east and south sides.  I’ve never seen such a mix of different rock colors, compositions, textures in any one place before.   Many of the rocks had sections of slickenside. 

This site is clearly a rock collector's delight.  Not so much so for mineral collectors.

Here’s a summary of the minerals and rocks that were collected:

Clinochlore:   The abundant chlorite schist seems to be mainly clinochlore, though perhaps some talc is mixed in.   Virtually all of the chlorite schist is deep green, fine and easily crumbled by hand, even that still unexcavated.   One chlorite schist vein was found that contained clinochlore ‘books’ up to ½” thick with faces to 2”.    This clinochlore is a stunning metallic green.

Phlogopite:  Small chips of copper-colored phlogopite were scattered here and there about the site.  Ken Casey had the good fortune to find a 1/4” X 1” book that is as bright as a new copper penny.

The metallic green clinochlore and the metallic copper phlogopite were the most striking specimens found.

Vermiculite:  Occasional books of metallic-coppery vermiculite were found.  The largest was about 1”.

Anthophyllite:  Three types of anthophyllite were found

1.   Light green and pseudomorphing into talc

2.   White, asbestosform. 

3.   Red, possibly due to iron.

Actinolite:  Many actinolite specimens were found.   Some were deep green, others apple green and some weathered clear/white.  Occasionally green and clear/white in the same specimen.  Most of the crystals are 1/8 to 3/8”, occasionally to 1”.

Magnesite:  veins of white magnesite were abundant.  A few clumps were also found.

Deweylite:  One interesting piece of deweylite slickenside.   Much more is probably there.

Quartz:  Four different forms of quartz were found, though three all are likely the same having formed from excess silica produced by the weathering of the serpentine:

1.   The most abundant are rusty-red (hematite?) rocks and boulders that range from finger-sized to 3’ boulders, with highly irregular (rough) surfaces.  When broken the cores appears to be jasper.  They are particularly abundant along the erosion fence.   I broke a fist-sized piece from a boulder, washed it with a pressure washer, and treated it with Iron Out.  Viewed under a microscope the surface was completely coated with small quartz crystals.

2.  A few fist-sized pieces have a white and green coating.  They have the same rough texture though the quartz crystals are more weathered.

3.  A few 1 ½” specimens  were  found with  1/8” to ¼” clusters of quartz crystals.  The crystal clusters appear to radiate from a single point, possibly a magnetite crystal (verbal communication from Roger Mitchell).

4.  Massive grey quartz in pegmatite veins

Feldspar:  Three types of feldspar were found:

1.   White, massive albite in pegmatite veins and gneiss

2.  Pink, perthitic , microcline.   Some near-perfect cleavage rhombs (one nearly 5 lbs) were found in the creek just below the construction site. 

3.  Amazonite was also found in the creek just below the site, some mixed with microcline perthite, and some with albite.

4. Bill Stephens collected a large block of feldspar which contains megacrysts over 2" in diameter that may be both moonstone and perhaps sunstone, but the final determination as to whether they are gem grade or not remains to be proven.  The block came from a spoil pile, but an in-place exposure was found on the west side of the site.  A gentleman from another club worked this location intensely and recovered some nice feldspar megacrysts.

Granitic gneiss:   There were a lot of interesting gneiss rocks that predominated in the lighter (white) feldspar and quartz veins with small, distinct layers of black biotite (heavily collected by some).   There is also a lot of highly weathered gneiss with red bands, presumably an iron mineral.   And also the piles of gneiss that was blasted and crushed.

Chromite:   There are a few red rocks along the silt fence (with a black weathered exterior) that contained small octahedral crystals (up to 1mm) of chromite.   Even when crushed to dust, the red rock remains weakly attracted to a rare earth magnet.   There is a vein of this rock on the shoulder across the street and slightly uphill from the site.

Magnetite:  Ken Casey found a nice hand-sized specimen of a green serpentine with veins of magnetite.

Rocks and minerals from this site will be shown in the DMS Club Field Trip display case at the upcoming March (2015) Show.   Following that, these minerals will be on display at the Delaware Museum of Natural History for about 4 months.