Monday, October 21, 2019

RIFT BASINS, THAUMASITE AND LAZARD CAHN



The last Post described two minerals that had Lazard Cahn (Honorary Life President of Colorado Springs Mineralogical Society) labels accompanying the specimens.  However, I have a third Cahn specimen that lacks a label other than what is written on the outside of a small box with lid: L. Cahn, Thaumasite (white), Berger’s Quarry, Patterson, N.J.  The inside of the lid states: Thaumasite, CHY, biotite.  I am unable to interpret CHY.  The "specimen" is a residue of several hundred fragments of various minerals as noted in the microphotographs near the end of the Post. This box was gifted to me from the family of Willard Wulff, a Charter Member of CSMS and a student of Lazard Cahn. 


Burger’s Quarry (not Berger's; AKA Upper New Street Quarry) is one of several quarries near Paterson, New Jersey, that have excavated and produced “Trap Rock” for decades. Geology.com defines trap rock “as a name used in the construction industry for any dark-colored igneous rock that is used to produce crushed stone.  Basalt, gabbro, diabase, and peridotite are the most common rock types referred to as trap rock.”



The traprock exposed at Burger’s Quarry are part of several igneous formations exposed in various Mesozoic rift basins that extend from Nova Scotia, Canada to North Carolina and even further north and south in the subsurface and to the east buried by sediments of the Continental Shelf.


The Eastern North American Rift Basins along the Atlantic coast associated with the early Mesozoic breakup of the supercontinent Pangaea.  Map courtesy of  Roy Schlische and the Structural Geology and Tectonics Group at Rutgers University.

These rift basins are associated with the breakup of the supercontinent Pangaea around the Late Triassic--Early Jurassic (either side of the 200 Ma absolute age boundary) that resulted in the formation of the proto-Atlantic Ocean as the continents pulled apart.  Today we look at the Mid-Atlantic Ridge and see these same rifting processes working to further separate North America from Europe and Africa.

View of supercontinent Pangaea.  Map courtesy of Institute of Geophysics at The University of Texas Austin.
Although the Atlantic Ocean represents “full” and continuing rifting, there were several basins in North America where rifting was aborted. These basins are structurally known as half grabens.  A graben is where two normal faults bound each side of the basin—as easily noted in the Basin and Range Physiographic Province.  In a half graben there is only a single bounding normal fault. One of the best-known rift basins in the eastern U.S. is the Newark Basin located in northeastern New Jersey (and the site of Burger’s Quarry). 


Location of Newark Basin outlined in white.  The green represents Triassic and Jurassic sedimentary rocks.  The red arc in the northeast are the Wachung Mountains.  Precambrian rocks bound the western edge of the Basin.  Beyond those are the early Paleozoic rocks associated with the late Paleozoic Appalachian Orogeny.  The yellow and orange are Cenozoic Coastal Plain sediments or sedimentary rocks.  Map is Public Domain and courtesy of the U.S. Geological Survey.
Early on, as the Basin was subsiding, massive unsorted sediments from the surrounding highlands were pouring into the lowland and later consolidated to form conglomerates, sandstones and mudrocks (shale etc.) known today as the Newark Supergroup. These sedimentary rocks are usually red or orange due to the oxidation of iron oxide minerals that were in the original sediments. There are also several lacustrine formations that represent large lake systems within the Basin.
The Palisades Sill (intrusive)is along the east side of the Newark Basin and extend north along the Hudson River.  The Watchung Mountains are exposed flood basalts (extrusive) cropping out along the eastern side of the Basin.  Top map courtesy of Columbia University.  The lower drawing of the Newark Basin half graben is Public Domain and courtesy of the U.S. Geological Survey. 
The sedimentary rocks of the Newark Supergroup are well known for their enclosed fossils including dinosaurs, plants, “fish” and various invertebrates.  Perhaps the most famous tracks are those of various dinosaurs discovered in “redbeds” of the Newark.   In fact, Eubrontes, a three toed dinosaur only known from their tracks, was designated the State Fossil of Connecticut in 1991.  In the same state, Dinosaur State Park protects hundreds of tracks discovered in 1966.
Tracks named Eubrontes.  Paleontologists believe that a three-toed dinosaur created these trackways in the rift basin sedimentary rocks.  Since dinosaurs fossils cannot be positively "connected" to the tracks these impressions are termed ichnofossils. Public Domain photo courtesy of Dinosaur State Park.
The tensions associated with the pull-apart basins allowed continued faulting and tilting contemporaneous with the eruption of flood basalts and the emplacement of subsurface dikes and sills due to decompression melting of rocks of the earth’s mantle as they travel upward along thermal plumes.

Sills are igneous rocks that are sheet-like and tabular in design and intrude older rocks parallel to bedding or foliations planes (concordant).  They are usually fed by dikes, igneous structures that intrude across existing bedding planes (discordant).  Magma forming sills cools at a slower rate since the structures are intrusive into preexisting rocks.  This allows for crystals to grow larger than those associated with extrusive lava flows, usually basalt.
Cartoon showing the formation of sills that are intruded parallel to bedding plains (Palisades Sill) while dikes (dykes) intrude across bedding planes and may flow on the surface (flood basalts such as the Watchung Mountains).  Courtesy of Differencebetween.net
The best-known igneous structure associated with the basins is the Palisades Sill that trends along the Hudson River for about 50 miles north of New York City. The layered rocks form massive cliffs along the River and are a well-known landform.  The Sill is composed of diorite, a magmatic rock with larger crystals than the extrusive basalt and was the first igneous rock to appear in the Basin, perhaps around 200 Ma in the earliest Jurassic. 

Still in the early Jurassic, volcanic rocks broke to the surface in the form of eruptions and flood basalts and formed the parent rock of the Watchung Mountains in the northeastern part of New Jersey.  Olson (1980) described these episodes as three separate flood basalts that may have filled the entire basin with each eruption.  After each major volcanic episode, the basin continued sinking and was again filled with sedimentary rocks with the end result being alternating layers of basalt and red sedimentary rocks.  With aborted rifting of the basin, deposition and volcanism ceased and erosion became dominant.
Highlands of the Watchung Mountains with Paterson, New Jersey in the far background. Public Domain photo.
Today the Watchung Mountains are three (plus some smaller remnants) parallel ridges of exposed basalt that are about 400-500 feet higher than the surrounding landscape.  The First, Second and Third Watchung (as the exposures are known) are major landforms and conservation groups fight to preserve their unique plant and animal life as well as the exposures of columnar jointed traprock.

Perhaps more than any other group of minerals found in the Watchung Mountains, the best known are the zeolites (microporous aluminosilicate minerals) and their companions.  One cannot attend a rock/ mineral show without noting Watchung specimens of analcime, stilbite, apopolite, chabazite, datolite, heulandite, stilbite and especially of pectolite (sodium calcium silicate) and prehnite (calcium aluminum silicate): both silicates often appear with zeolites.  It appears that at Burger’s Quarry the extrusive magma cooled in a lake and the basalt forming pillow lava.  Many of the fine zeolite specimens crystalized in the spaces between the pillows.

The Cahn specimen in my collection, thaumasite, looks like (in my opinion) a zeolite.  However, it is a sulfate [Ca3Si(OH)6(CO3)(SO4)-12H2O], but one that often appears with zeolites.  In the Watchungs thaumasite is formed when the basalt is heated by geothermal action.  It is usually colorless, but may be white, and forms prismatic hexagonal crystals (like apatite).  Thaumasite is quite soft (3.5 Mohs), leaves a white streak, and is translucent to transparent.  The clear, nice hexagonal crystals have a vitreous luster while the smaller fibrous crystals have a silky luster.  Thaumasite is not a common mineral and outside of the rift basin basalts is sometimes found with other calc-silicate minerals (as at Crestmore, California) or in hydrothermal copper deposits.


Photomicrographs of microcrystals of mostly transparent thaumasite mixed with biotite and others!  The longest crystal of thaumasite in the top photo (near the center) is ~ 1 mm in length.  All of the other thaumasite crystals are less than 1 mm in length.  The dark sheet mineral is evidently a  mica of the biotite family.  The greenish-brown fragments are unknown---perhaps smoky quartz sine many fragments show conchoidal fracture.   Notice the .5 mm crystal near the center of the lower photo "perched" on unknown fragments of another dark (although not shiny) mineral. Perhaps the crystal is titanite, or perhaps not.

I continue to search dusty drawers in small rock shops for additional Cahn specimens or mineral labels.  If any reader has a Cahn or Wulff label for thaumasite, please contact me.

REFERENCES CITED

Olsen, P. E., 1980, Triassic and Jurassic Formations of the Newark Basin in Manspeizer, W., ed., Field studies of New Jersey geology and guide to field trips: New York State Geological Association, 52nd Annual Meeting, Newark, New Jersey, Rutgers University.

Tuesday, October 15, 2019

PUCHERITE AND SERPENTINE: LAZARD CAHN


I cannot endure to waste anything so precious as autumnal sunshine by staying in the house.  Nathaniel Hawthorne


                          
Lazard Cahn 1865-1940  

It is always nice to locate minerals that are associated with Lazard Cahn, the Honorary President of the Colorado Springs Mineralogical Society (CSMS).  The Society can trace its origin to November 1936 when 13 individuals met for the purpose of organizing a local mineralogical society.  Lazard Cahn was elected as the Permanent Honorary President; hence, the designation of such on all CSMS publications continuing into the 21st century.  I note with interest that at the initial meeting the new members spent their time examining micromounts under binocular microscopes.  Evidently the new society was the outgrowth of interest by persons studying microscopic crystals (AKA micromounts) under the instruction of Mr. Cahn (twice per week at his office).  The Society was active early on and by 1939 mineral displays were exhibited throughout Colorado Springs by the Chamber of Commerce. So, when mineral dealer and CSMS member Austin Cockell offered me the two mineral specimens complete with Cahn labels, I was only two happy to snap them up.




In addition to being a Cahn specimen, pucherite [Bi(VO4)], belongs to the vanadate class of minerals that are related to the arsenates and phosphates. These vanadates contain the element vanadium (5+ oxidation state) plus oxygen (with a 2- charge) arranged in in a tetrahedron where four oxygen ions are at the corners and surround the central vanadium ion.  Each of these tetrahedra then has a charge of 3-. 

In pucherite, the positively charged bismuth (3+) is outside of the tetrahedron and neutralizes the vanadate ion. 

The phosphates [PO4 - - -] and the arsenates [AsO4- - -] ions are of similar size, have the same 3- oxidation state, and often replace and substitute for each other.  Of the three groups, the vanadates are by far the rarest with only carnotite (hydrated potassium uranyl vanadate) and vanadinite (lead chlorite vanadate) being recognizable and somewhat common minerals.

At first glance pucherite may “look like” its more common relative vanadinite. My specimen has the same reddish brown to yellowish brown color; however, the well-defined crystals with sharp angles do not have the hexagonal barrel shape of vanadinite but are tabular to equant and sometimes prismatic.  They have a vitreous to adamantine luster, are fairly soft with a hardness of 4 (Mohs), a distinctive yellow streak, a conchoidal fracture and are transparent to translucent. 


Photomicrographs of submillimeter crystals of pucherite.  Width FOV, from top down, 1.0 cm, 1.1 cm, 1.4 cm, 5 mm.
Cahn’s label describes the collecting locality as “Scheoberg Saxony Pucker mine.” Today we know the mineral was named for, and collected at, the Pucher Shaft, Wolfgang Maa├čen Mine field, Schneeberg, Erzgebirgskreis, Saxony, Germany.  MinDat.org noted the area is a polymetallic deposit (Ag-Bi-Co-Ni-U-bearing veins), was worked for silver and bismuth since the 15th century, later for cobalt and, in the 20th century, for uranium. Most of the veins are hydrothermally formed. The rocks in the field are early Paleozoic and late Precambrian metamorphosed igneous and sedimentary rocks that were intruded by late Paleozoic granitic rocks.  All of this tectonic activity was a geologic mountain-building event caused by the collision of Gondwana and Eurasia to form the supercontinent of Pangaea. As for pucherite, it is a rare alteration product of other bismuth minerals in the oxidized zone of hydrothermal ore deposits (MinDat.org). Pucherite (Orthorhombic Crystal System) is also trimorphous (same chemistry but different crystal structure) with clinobisvanite (Monoclinic Crystal System) and dreyerite (Tetragonal Crystal System).

A second Cahn specimen is labeled “Serpentine Big Timber Mont.”  This rock was a little more difficult to pluck out information since: 1) serpentine is not actually a true individual mineral; and 2) I could not find any “serpentine mineral” located/collected/mined from near Big Timber.  But, I will go with what I have. The Dictionary of Geology (2019) describes serpentine as a “family of silicate minerals rich in magnesium and water, derived from low-temperature alteration or metamorphism of the minerals in ultramafic rocks. Rocks made up of serpentine minerals are called serpentinite. Serpentine minerals are light to dark green, commonly varied in hue, and greasy looking; the mineral feels slippery.”  There are about 20 varieties of serpentine that are hydrous magnesium iron phyllosilicate ((Mg, Fe)3Si2O5(OH)4) minerals,  Many of these minerals are similar to each other and often are very difficult to distinguish between without the use of electronic gizmos.  Various serpentine minerals are mined for magnesium, for a variety of asbestos, and for decorative rock and carving stone (fake jade). 
Serpentine, chrysotile?, Big Timber, Montana.  Width of specimen 7.0 cm.
My best estimate is that the Cahn specimen is a variety called chrysotile [Mg(Si2O5)(OH)4], a serpentine type of asbestos.  There are several asbestos minerals (composed of thin fibers) that are usually classified as either Amphibole Asbestos or Serpentine Asbestos.  The latter includes chrysotile (low temperature) with white curly fibers while antigorite (high temperature) and lizardite (low temperature) have platy habits.  The Cahn specimen has bands of the white curly fibers “emplaced” in a soft (~3 Mohs), very dark green (almost black) mineral with a greasy luster and feel.  There are no visible crystals and the specimen has a massive habit with the white curly fibers emplaced in bands.  

Photomicrographs of above specimen showing "stripes" (each 1 mm or less) of white asbestos fibers. 
Serpentine minerals form by hydration and metamorphism of ultramafic rocks —those igneous and metamorphic rocks with high magnesium and iron content and low silica (such as peridotite, dunite, kimberlite, anorthosite and chromite)--in a process termed serpentization.  This action is usually associated with subduction zones along orogenic belts.

But, I have not solved the location of the Big Timber locality info. 


Lazard Cahn was born in 1865 and died in 1940 in Colorado Springs. His rock and mineral dealership was located in New York City from 1897-~1910 and in his Colorado Springs home at 6 North 8th St. and office 510-512 Exchange National Bank Building from 1908 until his death.   
 


                                     REFERENCES CITED


Dictionary of Geology, 2019, www.theodora.com/geology/glossarys

Cripple Creek mining leftovers.
Fall has always been my favorite season. The time when everything bursts with its last beauty, as if nature had been saving up all year for the grand finale.
             
Lauren DeStefano


ADDENDUM:  This is a great time of year , especially in area where one can experience yellow leaves and snow within a 24 hour period.  On October 9 I woke up knowing this was the day for magic. I grabbed a couple of bottles of water, jumped (well more likely boosted myself) in my pickup and headed up the mountains to my favorite small breakfast joint where I beefed up on a giant omelet and hot coffee.  It was then off to Cripple Creek and then back to the Springs via a back (and rough) road road that was built on the bed of the former Colorado Springs and Cripple Creek Railroad.  This winding railroad carried ores from the cripple Creek and Victor mines to the reduction mills in west Colorado Springs.  The road offers gorgeous views of the south flank of Pikes Peak.  The temperature was around 80 degrees.
The Cathedrals on the south flank of Pikes Peak.

The magic continued the next day as temps plummeted to the teens overnight and the win chill was 5 degrees at mid-morning coffee where snow was blowing in sideways from Wyoming.  I don't believe anything touched those flakes from Cheyenne to the Springs.

 
A remaining tunnel on the railroad roadbed.