Chapter 2: CARBONATE PLAY
Rising content of carbon dioxide (CO2) in the atmosphere by our increasing consumption of fossil fuels (coal, oil, natural gas) is believed to have triggered or accelerate global warming. In addition to its greenhouse effect, high CO2 content (along with sulfur dioxide released from coal crude oil) can contribute to acid rain and will eventually lead to acidification of sea water. Thus, reduction of CO2 gas emission is urgently needed. Some hope to sequester existing CO2 gas by bonding it forever as carbonates or carbon-bearing silicates in the underground.
Natural carbonate rocks hold more carbon than the combined amount of carbon in the atmosphere, biosphere, fossil fuels, soils, and sea water. Most of carbonate rocks are calcium carbonate in the form of mineral calcite in limestone and marble.
Generally limestone holds debris of dead organisms in the ocean, as cemented by calcium-carbonate bearing fluid. It can also precipitate from calcium-carbonate bearing solution. Calcium carbonate has a peculiar property that it is more soluble in cold than in warm water. Hence, most limestone accumulates in shallow, warm water on the continental shelf rather than in the vast, deep, cold ocean floor. When the limestone is subjected to high temperature and pressure through deep burial or by intrusion of magma, its fossils are destroyed and the limestone is metamorphosed into crystalline marble. Calcite can also grow out of hydrothermal solutions to form beautiful crystals, as seen frequently in museums. Dolomite is another common carbonate and its molecule bears equal number of calcium and magnesium atoms.
Karst terrain, as characterized by rugged but magnificent landscape coupled with numerous caverns and underground streams, has resulted from dissolution of limestone or marble by natural CO2-tinted groundwater. The dissolved calcium carbonate often re-deposits, following flow paths along fissures to caves, as stalactite (hanging down from cave roof) and stalagmite (protruding upward from cave floor) from fluid drips and drops over millions of years.
Limestone and marble are vulnerable to attack by bicarbonic acid-laden moisture in the air as attested by pit erosion in ancient marble monuments or tombstones. Both are soft and can be scratched with a knife. Limestone may bear visible fossils but marble is devoid of fossils. Depending on what impurities they carry and how they evolve, marble and limestone can have a wide variety of colors and patterns.
Despite scarcity of water in the desert, occasional storms can create flash floods that cause noticeable but short-term havoc. Imperceptible dissolution of limestone and marble can leave memorable imprints over millennium on the desert landscape as well as on the broken and detached rocks that scatter over the desert. The dissolved calcium carbonate in water (the solute) can be re-born as interstitial cement, encrustation over rocks, or caliche in soils.
This chapter is devoted to those naturally broken hand-specimens of marble, limestone, and concretion (or drip stone) from desert, hill slope, river bank, and beach. Some desert pieces are dying artfully by wind and water erosion while others are growing by chemical deposition to have pretty complexion. We’ll see examples of natural destruction and re-birth.
Mylonite is a rock that has resulted from earthquake or aseismic displacement along a fault zone. It connotes a mechanical origin, frequently with flow-like texture. It can have different sets of mineral compositions, depending on what are smashed during fault slip. It is identified with texture, not by mineral composition as commonly practiced for rock naming.
In this stone, the more competent or indurate components like sandstone crumbled brittlely into fragments with sharp edges. Some were metamorphosed into quartzite as well but not so severe as to completely obliterate their pre-slip sedimentary traits.
Meanwhile at high temperature raised by frictional heating during fault slip, the less competent component such as limestone liquefied and flowed to engulf more indurated solid fragments. In the wake, the ductile flow (flow in solid state) left numerous visible flow tracks, including brown streaks of limestone-entrapped powder of pulverized solid fragments. Thus during fault slip in this rock at high temperature and under intense shear stress, the brittle sandstone fragments appear briefly like floaters in the stream of ductile limestone.
This stone is a sequel to the mylonite depicted in Figure 2-1. However, the two stones appear different, especially in the flow structure, reflecting spatial variations in response to fault slip.
The mylonite was exposed by tectonic uplifts to break into individual stones, which endure further onslaught on the ground surface by wind abrasion and chemical attack by acidic water. (Natural water can be slightly acidic owing to dissolving carbon dioxide from the air or soil.) The limestone component is prone to abrasion and acid corrosion. Consequently, limestone recedes more, relative to the more resistant sandstone and shale. In the absence of water, however, desert limestone can remain fairly indurate as evidenced by the preservation of its sharp ledges and spikes. Despite their awesome rough exterior, the two stones in Figures 2-1 and -2 can be handled comfortably with bare hands.
Owing to compression, the two pieces of carbonate mylonite are relatively heavy as compared to ordinary limestone of similar size.
This tetrahedral shield-like marble is a master piece of meticulous carving by desert wind and water. It represents a destructive but very creative phase of natural forces.
The peanut-sized concretions, however, signal a constructive phase. They have resulted from evaporation of water that bears chemicals dissolved from rocks along various intermittent flow paths. The fine, delicate texture of concretions suggests each stone has taken a long, tumultuous journey to fruition. Lying flat on the ground surface (not standing upright), each grows likely around a tiny piece of core rock at minute rates over millennium after surviving deposition, erosion, and dissolution in endless cycles of drying and wetting seasons. Their exterior appears pristine, implying each piece has tumbled around to infuse with additional calcareous mud and likely including rehabilitation of any defacing, which had occurred along each stone’s journey.
Reflection: Construction follows destruction somewhere else, and vice versus in cycle. Your waste is someone’s gain.
Characterized by dipping, red-black beds or strips of various length and width, this stone bears a distinctive set of scratch-like, irregularly-spaced, horizontal grooves. Some grooves are partially filled with secondary mineral, suggesting their older age relative to the deposit-free grooves on the rear side of the stone. Perhaps the presence or absence of secondary deposit is related to sun light exposure as per moisture retention and evaporation. None of the grooves circumscribes completely, suggesting that they have resulted from etching and re-deposition of dissolved calcium carbonate at ground surface.
Another set of short etching develop at random orientation over all faces.
They are the products of natural carving by weak acidic water in the desert. To me, the right piece looks like a statue of an old local Earth God in my birth village in Taiwan.
The white patches on the left piece are secondary deposit of calcium carbonate but the white on the right piece is primary carbonate.
Figure 2-5: Staircase limestone (6x7x17 cm; source in California?) – viewed from different perspectives.
This ‘staircase’ stone is a piece of gray limestone, which seems like a welded stack of over-stepping trapezoid plates. ‘Stairs’ appear on all four lateral faces (left figure). The edge of steps was rounded off by mechanical erosion or dissolution by water. And viewing at different orientations, some stairs could be preferably called, instead, a series of rhythmic troughs and ridges, which evolve from the original layering. The trapezoidal corners remain sharp, typical in a dry limestone terrain. The stone is also crisscrossed by a network of subtle ridges. Small veins of erosion-resistant fills stand out amid this otherwise fairly homogeneous limestone.
Water is scarce in the desert. But, when it is available and indeed it is available once in a long while, water can do a wonderful job of landscaping. It works beautifully on limestone of homogeneous property as witnessed in this ‘staircase’ stone.
More commonly, dissolution and deposition of limestone can yield magnificent Karst landscape, which attracts millions of tourists yearly. It can also create sinkholes, posing hazards to urban area residents but creating scenic view in the wild. Slow dissolution by exposure to weak acid like vinegar is a nuisance to home owners who have marble kitchen counter tops. But, nature does its own way regardless what we categorize the process as beneficial or hazardous.
Figure 2-6: Vermiculated limestone (16x9x7 cm; from San Bernardino County). Life is tough, depending on what reality you have to face.
This ventifact of limestone has four contrasting sets of face expressions – a piece of desert rock enclosed with wind-abraded or sand-blasted facets. The front set (A, B, and C) exhibit vermiculation (corrugation) on three pre-existing facets. The rear set (H) lacks ventifact-like facet and is absent of ‘worm gnawing’ feature. Facets E and F are half-way between the first two sets. The bottom face (D) has the greatest roughness and it is only slightly vermiculated. The stone is also girdled with a conspicuous, three-mm wide black belt (vein).
Vermiculation on limestone occurs through extremely slow rate of dissolution by moisture in the desert. Dew or moisture can accumulate during cold night; but the water can also evaporate rapidly by wind blowing or sun light heating. The facets that retain the moisture will endure more vermiculation, as compared to facets that lose moisture rapidly. Hence, those vermiculated are under the shadow of sun light or in the lee side of a right-to-left breeze (yellow arrow) on the front face. Note that calcium carbonate is more soluble in the cold, shaded side than in the warm, sun-facing side.
This hypothesis leaves some questions unanswered. Are the transitional patches E and F being vermiculated? Or, are their vermiculation imprints being erased? Brown stain occurs because of chemical reactions with or deposition of staining agents. Is the brown stain spreading or shrinking?
Bottom face D was shielded from wind and sun light. Why is it pale and hardly vermiculated? Or, is it a relatively new face of breakage? Again, I have more questions than answers.
Figure 2-7: Marble with cap rock (11x9x6 cm; from San Bernardino County) – symbiosis between the weak and the strong.
This white marble is capped by a protective layer of mud, as hardened by siliceous fluid and tainted with brownish red ferric oxide. The ‘cap’ is a late encrustation over the marble near the ground surface, not coeval with the marble-making metamorphism.
The marble has been re-shaped by water erosion and chemical attack by bicarbonic acid in the water. Its underside (not shown here) exhibits markings of grating erosion along the subtle bedding traces. The grating in the underside is more pronounced because the moisture gets to stick there longer to react with the marble and the carbonate is more soluble in the cold underside.
The surface outside the bottom is also subjected to erosion by wind and water. For this reason, the head-wind face is better polished than the lee face which retains relatively more erosion or dissolution grating. The uneven or varied extent of polishing also reflects the exposure time since the cap disintegrated. No desert varnish appears here because the corrugated cap and the CO2-vulnerable marble cannot retain the varnish even if it was coated. If the stone were left unturned in its natural state, can one attempt statistically to infer the prevailing wind direction in the recent geological past from similar rocks in the field?
Erosion of the marble undercuts the cap rock and causes it to collapse around the cap’s peripheral edge rather than going through slow denudation.
Reflection: Like our protective police need the support of the protected public, the cap protector cannot stand alone when the protected is eroded away, as evidenced by some residual caps.
This bell-shaped monument of limestone is slugged by a log of quartz vein. Dissolution of limestone follows bedding planes and crisscrossing fissures. The quartz vein stands out because it is more resistant to erosion and weathering but it is more brittle as to possess more sharp-edged breakage when it tumbles in an intermittent creek, where this piece was found.
The chert sublayers stand out against the recessive limestone sublayers because of differential resistance to weathering and erosion, chemical or physical. Both have been altered by post-depositional silicification and layering distortion. Prior to natural alteration, the chert and limestone were probably biogenic deposits, produced respectively during cyclic warm and cold periods, with each period lasting hundreds to thousand years. The silicified limestone is not knife-scratchable and it is hardly responsive to weak acid but its scratch hardness is still softer than the chert, which is masked by the reddish dark coating.
On the saw-cut face, the chert turns white while the limestone retains its originally exposed brownish yellow hue. Both are machine polishable. As such, the rock can be made for decorative slab that is more resistive to scratch and acid dissolution than a ‘pure’ marble piece is.
The two pieces of limestone have impressive ‘oolitic surfaces’. Those mm-high rounded buds (typically 2 mm across) populate the entire exterior except the part covered by the rusty, siliceous crust. Most of the buds are slightly dimpled or pitted at their individual top centers. The buds look alike on all facets of the rocks; and it is hard to perceive a three dimensional image of the interior characteristics that yields the buds. (See Figure 5-5 for an oolitic limestone and 4-4 for sweat pimples on mudstone.)
The question is whether the buds have originated from biological (fossil) or non-biological related erosion. For finding a clue, the stone on the left above has been chipped off in the field. Its interior appears homogenous and nothing seems to be indicative of any fossil.
However, polishing on a cut-off face reveals circular or oval cross-sections of buds (Figure 2-9b). The subtle texture, center dots for example, is further exposed through acid etching (light part versus with un-etched dark part). The stones represent a colony of globular nodules, each of which grew spherically from a centered seeding grain. Alternatively and more likely, the buds originate biologically (i.e., fossils). Dissolution around individual buds has resulted in similar appearance on exterior surfaces.
Each of the two stones has a ‘cauliflower-like’ calcareous skin that grows over one core body of unknown rock type. During its formative life, each stone had tumbled over numerous times in flash-flood channels so as to situate itself for universal encrusting with calcium carbonate, which is omnipresent in the water as calcium bicarbonate and often appears as caliche over rocks near the ground surface. The ledges and the granular texture of skin suggest these features originate through dripping of residual fluid beneath the stone as the water is being evaporated. Evaporative precipitation on the upper surface tends to have a skin with subdued relief. In essence, the two are concretions, growing in an intermittent gully in the desert.
Those are calcareous drip stones from the flood plain of a small intermittent creek except the white piece from China. Unlike their cousins depicted in Figure 2-10a, they are very porous and typically do not start from the seeding of alien rock cores. They are essentially tufa — carbonate deposited from water on the ground surface at ordinary air temperature.
This artifact-like stone is an art piece created by carbonate-consuming organisms under glacial lakes in Quebec, Canada. During the spring and summer, the bacteria heighten their activities in the increasingly warming water and amass minute particles of calcite on the rock or in the mud, colonized as oval concretions (nodules). And during the winter, the bacterial become dormant and stop multiplication. Through many cycles of bacteria growth and hibernation, concretions of various shapes and sizes are formed. The go-and-stop processes appear to have fused together the concretions generated at different time periods. Essentially each new colony concretion develops around the old ones after a time elapsed between distinctive growth episodes to accumulate exotic aggregates and thus give rise to the American Indian’s tales of fairy stones.
In mineral composition, travertine is a piece of calcite/aragonite mixture. As calcium carbonate (precipitated out of hot water when exposed to lower carbon dioxide partial pressure in the air), it typically rims and floors hot springs pools. Also frequently it forms a series of cascaded terraces and pools downstream. At distance, this stone appears layered, like a piece of cake with white icing atop a lightly tanned body that overlies a white base. The tanning could simply be a thin coat, as resulted from activities of some organisms, over the white carbonate. Vertically the stone seems fibrous or column-twisted, a vestige of deposition in flowing warm water. Travertine, one type of limestone, is often used as tiles for wall decoration or flooring in building construction. Sometimes those tiles exhibit vug (small empty cavity) as resulted from re-dissolution of carbonate.
The leaf-like dendrite lies on the bedding plane of limestone (Left). It is not a plant fossil. Dendrite results from diffusion of manganese oxide along micro cracks on the bedding planes; however, the supply of manganese-bearing solution is fairly limited.
The diffusion can also run across the bedding, as shown on the stone to the right. The diffusion is asymmetric; each band of dendrite protrudes upward from a smooth base interface. The orientation of the protuberance indicates this piece of stone on the right is displayed upside down because gravity will aid downward diffusion. The dark brown color reflects oxidation of the original ferrous diffusion fluid. The two light brown strips do not have visible dendrite emanated from them, indicating the invading fluid was scarce, exhausted, and immobilized by deposition before it could diffuse away from the bedding planes.
The picturesque landscape on the limestone plates have resulted from diffusion and oxidation of invading ferrous solution. Apparently the diffusion of fluid was more lavish than what was available for the dendrite in Figure 2-12a. (The specimen presumably came from Italy).
It is noted that the stones in these two figures require cutting and polishing to clearly reveal the diffusion patterns.
The three largest calcite crystals exhibit their characteristic exterior rhombohedron and rhombohedral cleavages. If broken along the cleavage, the ending parts will resemble the original crystal shape. The transparent piece is the so-called Iceland spar. Chlorite impurity taints the otherwise white calcite on the middle plate (from Riverside County; gifted by Mr. Joey Ybarra). The other smaller crystals are broken pieces, some of which have been corner-eroded in the desert (from Imperial County).
The two stones in the back are chalk-coated over cores of black flint (samples probably from England). The one in the front, however, is a piece of opal (from New Mexico), which is cryptocrystalline, hydrated silica or quartz, not carbonate.
Figure 2-14a and b
Stones in Figure 2-14b have been partially polished, of which the middle one is 8 cm wide. The images on the right of the two figures represent the same stone. These eye-catching stones come from one small, idled exploratory mining dump in a California desert (San Bernardino County; purchased).
These translucent calcite crystals exhibit rhombohedral exterior and cleavage. All exterior bears erosional relics, revealing cleavage traces in particular. Some surfaces show signs of dissolution and re-deposition of calcite, likely acquired in the exposed dump site.
Most specimens are twinned, with easily recognizable pairs of reentrant and salient surfaces on opposing ends of each stone. Most hinge lines of the twins bisect symmetrically the obtuse angles of the exposed core surfaces and the lines are often traceable, even across the surfaces that have been impaired and obscured by erosion and re-deposition.
The core of each specimen is well demarcated by cleavage, providing strong contrasting hues between the black core and the white, translucent ambient. Unlike the Liesegang banding or dendritic texture in other stones (Figures 2-12, 3-8, 3-9 and 5-6a), the two contrasting parts do not have visible diffusional transition zone in between. However, it does have occasional, alternating dark and light banding within the black core.
The calcite crystallization nucleated with black-hueing agent (manganese, iron, or magnesium), due to higher crystallization temperature in the presence of the agent. As the temperature was dropping and after the agent was exhausted, the white calcite continued to grow outward. The blackening of the core represents chemical differentiation, not physical diffusion, during the crystallization of calcite. Could it reflect the evolving segregation between or crystallization temperature for two solid end members of a solution? If so, we might expect to see specks of pink rhodochrosite (manganese carbonate) or greenish siderite (iron carbonate). Neither is seen.
The three small rhombohedral calcite crystals (from Imperial County) are also clear but their surfaces have been etched unevenly by dissolution and erosion. The holding-base to the two sets of calcite crystals is blue calcite from Riverside, California.
About 600 million years ago at the onset of Cambrian period when our Earth had a biological explosion that commenced life evolution unseen in other planets of our solar system, trilobites synthesized clear calcite crystals and aligned those tiny crystals to form their composite eyes, in addition to using calcite to construct their protective exoskeleton. That unique design and manufacturing process had lasted for 300 million years until the end of Paleozoic Era when the trilobites became extinct. And started about 100 years ago, Iceland spar had also been used for gunnery sighting for decades.
The marble is partially encrusted by scattered lumps of reddish, secondary siliceous material. The lumps stand out because they are more resistive to chemical dissolution than the marble, of which the surface is wrinkled with a mesh of visible ruts. The ruts around the lumps are relatively deeper and more visible.
The left piece was in the light shadow and the shadowing resulted in the illusion of rusty brown hue.
The green variety of zoisite (aka anyolite, purchased) comes from Tanzania, while the two pieces of calcite are from Alaska and the agate is from Utah.
It is a manganese-bearing rose-pink zoisite, which is a metamorphic mineral (hydrous calcium-alumino silicate). The specimen comes from a desert in San Bernardino County.
Figures 2-17a, b, c, and d
Marl is a mixture of soft clay and carbonates (Figure 2-17a). It has often been said that mud cracks may shape like hexagons. I had not seen any impressive hexagonal cracks until 2011 when we visited Death Valley National Park (Figure 2-17b) with our two-year-old grandson Jay. The middle two pictures were taken from a high-elevation playa (dry lake) known locally as the Rock Racing Field. In the playa there are numerous boulders up to a few tens of centimeters across that have left behind visible migrating tracks but no sign of entry into the lake (Figure 2-17c). What is the driving force? The boulders are driven by wind to skate on the playa when it is ice-covered once every few years or decades. The dry mud is very competent such that normal walking, as permitted on a posted sign at the site, will not damage the tracks and mud cracks. I am very impressed by and appreciative of tourists who have kept the site at pristine condition. There is no park ranger guarding this mysterious treasure of nature. (My grandson did not touch the boulder, nor jump over it. The jumping picture was incidental, not staged.)
The object in Figure 2-17d appears like a perfect stone wall built by a master stone layer. In fact, it is a view of cracked granite in Mt. Rubidoux, Riverside, California.
Despite drastic contrast in band colors, all are calcium carbonate with red-hueing agents or impurity. The banding has resulted from chemical differentiation during the metamorphism when the limestone was transformed into marble, like the banding in gneiss. It is not a post-metamorphism diffusion product, as happens for Liesegang banding in Nevada wonder rocks or some sandstones (See Figures 3-8, 3-9 and 5-6a).
The green marble is ‘protected’ with slices of silicious layer in the back, which is relatively more resistant to weathering and erosion.