Chapter 5: DIVERSIFICATION
This diversification chapter can be alternatively titled ‘miscellaneous’. Some stones are paired or grouped into individual figures for contrast and comparison even though each could have been easily placed into one of the four preceding chapters.
The floating ice photo was taken in Grand Canyon National Park. Although ice comes and goes in short time, it meets the traditional definition of minerals as natural inorganic solids with definitive crystalline structures. I like the banding in the ice-water mix as if it were a sequence of episodic diffusion, freezing, or thawing. It also resembles Lisegang rings – in this case, ice rings in homogenous water.
This naturally sculptured limestone comes from the vicinity of a contact metamorphic zone known for yielding colorful marble. As viewed from the true perspective that the rear half of the stone was buried in the ground before my retrieval of it off an abandoned mining road, this suiseki-viable stone is eye-catching for two contrasting components: black-coated ridges (veins) stand against the background of pale yellowish brown sinks (depressions). The burial line is marked by faint, disjointed, powdery-like white patches of caliche coating, which augments as the third but surficial component. Also, another patch of distinctive, grainy (~ 1 mm) caliche splatters and hides beneath the stone bottom to constitute the fourth visible component.
Before this chunk of limestone was split from its parental formation, it was fracutred and the cracks were refilled to form what are now the dark ridges: three prominent parallel ridges and two on the left at an oblique angle to the former set. The ridge veins extend to the rear but their relief becomes more subdued with waning darkness, and the hue of the ‘naked’ ridges merges with that of the background limestone sinks. Apparently, the dark material was eroded and stripped from the rear face.
So, what is the dark material? Lichen is here inferred. It is dull, not shining as in desert varnish or pavement; it is a symbiotic colony of algae and fungi. Using the sun light to synthesize nutrients, algae thrive only on the surface of rock. After the rock was turned over with the rear face down, lichen died out for lack of sun light exposure even if it did grow at one time on the rear face. Another reason for the inference, lichen appeared on the entire exposed right face of the right-most ridge vein but its darkness fades backward (downward) to the caliche line; it diminishes below the ground surface.
It is uncertain whether lichen had ever covered the entire sun-light exposed front face in the past. A big dark blotch between two ridges at the the lower-right quarter seems to favor such coverage. The thin cover, however, has been breached as evidenced by a cutout window of the dark material to reveal the underlying limestone. Why would a veneer of lichen stick with the ridges and act like a standout protector in resisting erosion or chemical dissolution? Plausibly, despite the similarirty in hue between the sinks and the naked ridges, the ridge material bears a slightly different ingradients that favor lichen growth.
The sinks or depressions, which appear on both the front and rear faces (actually the top and bottom surfaces), are relics of dissolution of limestone by bicarbonic acid as originated from dissolving carbon dioxide in water from the air or through soils. One cannot help but be amazed that water or moisture has done, despite its scarcity in the desert, the wonder of sculpturing in two centuries since the rock was displaced by mining activity and re-buried by natural slumping.
Being closed basins, the sinks or depressions have no outlet. Where did the dissolved carbonate go? A tiny fraction has redeposited as sporadic, fine-grained (< 0.5 mm across) brown caliche around the sink peripheries. The majority has been blown away by wind to expose the framework limestone that is crisscrossed by numerous, barely visible, protruding veinlets. This supposition does not imply that the wind has excavated the sinks because the wind cannot ablate the underbelly of a rock. Before its latest half-burial, the rock could have stood on the ground, like the present display attitude. This scenario can explain the loss of the dissolved material and the occurrence of the coarse-grained caliche at the bottom face as well as the undercutting of the lower rims (‘south rims’) of the lichen cover around sinks.
These two crystalline calcite specimens were collected by Dr. Douglas M. Morton in 1958 from veins in the contact metamorphic zone around the Crestmore Quarry. The site is renowned for yielding the largest number of different mineral species at a given locality in the world.
The white stone (height, 7 cm) is a marble containing angular, translucent quartz grains. Originally it must have been metamorphosed from a limestone containing small amounts of sand. It is a broken piece as shaped in a desert wash.
Sitting on top of the frame is a piece of dark, brownish red chert (height 7 cm). The stone makes a crispy, metallic clanking sound when it is gently tapped against a hard surface. Stippled with dents or pits and conchoidal fractures, the piece as a whole is well-polished and -varnished to give a lustrous sheen. When I found it, it was one among thousands of similar fragments in a desert wash. A few appear in rod, oval, or other odd shapes and share a common color of dark reddish brown. (See Figure 5-5 for additional chert specimens.)
The last of the trio is a chunk of petrified wood (height 7 cm), as inferred from its linear, fiber-like structure. The white coating doesn’t appear to be indigenous to the petrified wood. It is likely siliceous in composition, and thus it may represent a hard coating (lithified ash?) associated with the sedimentary matrix in which the petrified wood resided. Alternatively, it could be a fibrous piece of chert.
Figure 5-3a: Folded structures (left, 9x7x10 cm; right, 9x8x5 cm; from San Bernardino County, CA).
Folded structures in the two pieces of lava (left and middle) preserve the flow patterns at their semi-liquid state before the magma solidified at the ground surface. The folds in the gneiss (right) represent solid-state flow or deformation deep in the Earth’s crust but the temperature was not so high as to have melted the rock.
On the left is a rusty piece of lava in-filled by a vein of white quartz (as fluid before solidification), which subsequently has been offset by a series of small faults. The layers in the quartzite on the right were offset after metamorphism had transformed the original sand and clay layers into quartzite and green chlorite-rich layers (or epidote?), respectively. The gap along the fault was then filled with silica-rich fluid which turned (at temperatures less than 150o C) into a vein of chalcedony (cryptocrystalline quartz) as is visually distinct from the quartzite.
The layers near the fault in the quartzite piece display some drag folding as opposed to the clear, crispy cut in the lava piece, suggesting the former was faulted under ductile condition at depth (temperatures above 300o C) while the latter at brittle condition near the ground surface.
Figure 5-4a: Selenite (left, 15x18x9 cm, purchased) and gypsum (23x20x6 cm, San Bernardino County, CA; gifted by Mr. Joe Thompson).
Selenite is an opaque variety of gypsum (hydrated calcium sulfate) that has incorporated fine sand during its growth. An attached small crystal is the so-called ‘desert rose’.
Hanksite is famed for its hexagonal crystal with bi-pyramidal ends. It is composed of sodium-potassium sulfate-carbonate -chloride – an evaporite (salt) from dry lake for its unusual bonding of sulfate, carbonate, and chloride in one natural compound. The crystals can be pitted by moisture in the air if exposed, and dissolved in water. The solute can recrystallize as platy or columnar, clear trona (hydrous sodium carbonate-bicarbonate, as per my rudimentary experiment at home), which may dehydrate at temperature greater than ~30 C to form other crystals, some with white powdery coating. The inclusion in the crystal is clay, around which and in which hanksite grows. Dehydration in the air can cause the crystal to become powdery. It can be preserved in mineral oil or with periodic clear (paint) coating. Hanksite has been claimed by some religious faith to possess power of spiritual healing.
All chert pieces come from the same desert wash, including those with ‘faux oolitic’ outlook (protruding nipples). The five pieces without nipples show features of depositional relics in a dry wash of former pot of water in the desert, like the mud flakes in a drying mud pot. (See Figure 5-2 for one additional chert specimen.)
It is not clear whether the nipples on the chert represent external growth or extrusion from the interior. But unlike the chert, the oolitc limestone has a fine, radiated, fibrous internal structure. Because the interior of the oolitic limestone is porous (as revealed by a broken piece, not shown here), it is light. The chert with nipple is compact and dense. See also Figures 2-9.
A fossil is the relics of animal or plant that has been preserved or transformed naturally in the geologic past. It can be body or body parts, excrements, imprints, and activity tracks but usually excludes any species that is still alive today.
The stand is a piece of naturally carved and polished chalcedony (amorphous silica), or agate if one prefers to name it so. The gray piece is a spirifer fossil; it is knife-scratchable calcareous mud. Fossils often appear as carbonate (calcite or aragonite), pyrite (in anoxia burial sites with abundant iron and sulfur), or amorphous silica (e.g., petrified wood, coprolite in Figures 5-8, & -9). The two clam fossils are unusual for their high extent of chalcedony replacement.
On the left, the cut face is parallel to the bedding plane while the panel on the right and its mirror-image counterpart is cut across the bedding. The contrasting patterns manifest differences between diffusivity or dispersivity along and across the bedding.
Guess what the interior will look like if the exterior is not chipped off partially? The broken cobble on the right (from a San Diego beach) exhibits a dark reaction rim (so-called weathering rind) around a piece of aplite, of which the broken face is also tainted yellowish by geochemical reaction with seawater.
The stone on the left (from a valley east of the Sierra Nevada Ranges, CA) reveals that a core of dark, fine grained diorite is enclosed by a thin white layer of very fine-grained material. Both parts are very well eroded and rounded. Is the exterior a secondary accumulation around an eroded and rounded core near the ground surface? Or, the core was actually engulfed as an inclusion in a rising magma; and a chunk of the hosting rock, plus the inclusion, has been greatly eroded to its current form. The choice hinged on what the white cover is, sedimentary, igneous, or a weathered product.
Sometimes the Liesegang ringing is a near-surface phenomenon only. It diffuses little into the interior, as exemplified in this homogeneous, cut-face of solidified volcanic ash. Such homogeneous, fine-grained texture is an attribute for stone carving material.
It is common to find pebble or cobble of oval or ellipsoidal shape in the beach. But it is rare to see a circular disk (actually a spheroid of high oblateness) like the one depicted here. Several material conditions must be met to have such shape: To begin, the stone is platy-equant. It should be competent and free of fissure so as not to crumble under wave action. It must also be homogeneous and isotropic to prevent preferential erosion in certain direction such as to form ellipsoidal cobbles.
Water-rock chemical interaction alters its color to an overall reddish background (orthoclase and plagioclase) dotted with grayish, translucent quartz and dark minerals (mica and hornblende).
The two light-colored spheroids resemble the one in Figure 5-7a but the two, bearing hardly any visible quartz crystal grains, are monzonite.
The dark one is basalt, which bears a few phenocrysts — minute plagioclase and quartz crystals — in the overall dark groundmass (matrix). Also it has tiny cavities, apparently evacuated by falling-loss of phenocrysts.
Those oblate-spheroidal disks are seasonally observable at the beach in winter when the sand is washed offshore to bare the rocky coast.
This piece of petrified wood has been naturally polished, varnished, and likely stained as well. Its wooden structure is very well preserved. The surface relief reflects differential resistance to natural abrasion in the desert.
Other pieces of my petrified wood collection also exhibit relief caused by differential abrasion but they are not naturally polished or varnished at all. An artificial cut face of petrified wood, however, can be polished. And such polished petrified wood is fairly available in rock gift shops.
Almost all of my petrified wood specimens are not naturally polished. On account of my limited inventory of petrified wood, why does the nature rarely polish the petrified wood?
The tree-ring features of these petrified woods are very well preserved. Even though quartz and petrified wood are predominantly composed of silica (silicon dioxide), their physical states are different. Quartz is crystalline whereas petrified wood is amorphous (non-crystalline or crypto-crystalline). Accordingly they respond differently to wind-abrasion polishing. Another obvious example for different physical states and responses, sandstone mainly as an aggregate of consolidated or cemented grains of quartz will not be well polished either because the small, individual grains cannot respond coherently to the natural or artificial abrasion agents.
Presumably these two are silicified excrement of unknown dinosaur species. Each coprolite is more than 65 million years old.
The reddish in the white chalcedony is indicative of intrusion (diffusion) by iron-bearing fluid. The fluid solidified and later became magnetic. Like petrified wood, the cut-faces of these coprolites are polishable.
Figure 5-10a: Petrified palm (left, 25x9x19 cm; right, 23x4x13, from Louisiana; purchased).
The dark portion in the left specimen is dotted with visible palm pores. The white in the right specimen is chalcedony (some with vugs or cavities), and the yellow (and light brown) is calcite.
Figure 5-10b: Petrified fern (10x24x6 cm; source unknown). Front and rear views.
This piece is unusual in the sense that the fiber is slightly pliable as if it were not fully petrified. See the center piece of Figure 5-9 for its bottom face.
This breccia comes from a landslide- prone rocky coast in southern California. It consists mainly of angular mud clasts (gray or brown fragments), cemented or partly replaced by a network of crisscrossing siliceous matrix or veinlets. Some fragments dropped out to allow drusy quartz crystals (tiny protruding quartz) to grow in the vacated sites, the largest cluster of which is about 3 cm across. On the other hand and possibly, some vacated cavities (druses) could be the original water pockets entrapped in the landslide debris. The black spots in the picture are empty holes.
After the piece fell or detached from its parental rock to the beach area, some mud flakes are recessed by erosion from the more erosion-restive siliceous matrix. And quite a few flakes endure further cracking and subsequent fissure-filling by calcareous cement, as inferred from its effervescence in weak acid.
The pink purple lepidolite is lithium-bearing mica It can host gem-quality tourmaline.
Silica in nature can appear crystalline or amorphous (non-crystalline, or crypto-crystalline). There are numerous varieties because silica is among the most common compounds on the Earth and it can crystalize or solidify under various geologic conditions.
Among the common crystalline varieties is crystal clear quartz, black smoky quartz, yellow citrine, or purple amethyst, with colors reflecting different trace amounts of metallic elements. The amorphous can be collectively named chalcedony but differences in color, texture, shape, constituent, and occurrence lead to various geological or trade names. Some prefer reserving chalcedony for those with fibrous texture and definitely opal for those having hydrous component. To the unaided eye, the naming without geologic context can be a challenging mental exercise.
Agate is a banded and curved chalcedony. It was formed by successive lining of a former gas or liquid chamber, typically inside a lava flow. Depending on material supplies and evolving conditions of deposition, the chamber can be fully occupied or partially filled. The latter allows crystals to grow inward as exemplified in geodes (Figures 1-11, -15, and -18).
Relative to their hosting rocks, agates are more resistive to erosion and weathering. Consequently, they may survive as solitary nodules amid river deposits. Those nodules do not grow within the deposits, as might happen for chert forming.
Chert is sedimentary, transformed from debris of silica rich organism like radiolarian or diatom (Figure 4-7), or silica rich mud cracks through evaporation (Figure 5-2 and -5). It can also appear as post-depositional nodules in sedimentary rocks (as replaced later by silica-bearing fluid). Similar replacement is exemplified by petrification or silicification of some buried plants and animals. A chert in chalk or carbonate rock is usually named flint (Figure 2-13b).
Figure 5-13a and b
Jasper is an opaque chalcedony, typically red-colored due to the presence of ferric iron. Its color, however, can range from yellow through red to black, depending on concentration and type of trace elements in the specimen.
Unlike cavity-filling precipitation of agate in volcanic rocks and also unlike deposition of or replacement by chert in sedimentary rocks, jasper is a silica replacement product associated with volcanic rocks. Each specimen’s exterior may be so irrgular as to render it being ‘useless’ but some are appealing for suiseki (the art of stone appreciation in Japanese). However, their saw-cut faces may reveal beautiful color banding. Jasper is fairly polishable for jewelry making although impurity often stands in the way. Once cut and polished, a small piece of jasper may not be visibly distinguishable from agate or chert. Often nucleated from micro-organism, chert can be identified through thin section under microscopy.
The dark brown piece in the upper middle of Figure 5-13b is dense, homogeneous, and highly responsive to neodymium magnet; and it has conchoidal fracture. It is fairly polishable but its identity as jasper is yet to be confirmed.
Top, intensively folded gneiss (migmatite); middle left, sandstone; middle right, ‘fossil-false’ chalcedony; and lower three, migmatite. The latter include white-mineral dominated leucosome (left, 6 cm wide), dark-mineral dominated melanosome (right), and an intermediate mesosome (middle). Look at the interlocking and fuzzy boundaries between the light and dark minerals as produced by partial melting and recrystallization.
The chalcedony flake is fully covered with reddish brown varnish and ‘impact-like craters’. Those crater-like depressions have resulted from erosion and dissolution of its weaker, impure components.
This specimen has two distinctive pyrite crystals. The big-grained ones are more or less cubical; their surfaces are marked by three orthogonal sets of conspicuous striations. Other pyrite crystals, populated in the rear and bottom, are brighter and are about one-fifth of the large crystals in grain size. Also abundant in the rear are clusters of small quartz crystals.
Ulexite is an evaporite commonly associated with borate, hanksite, etc. It has a simple chemistry — a hydrated sodium calcium borate hydroxide. It possesses a fine fiber structure that allows light to transmit along its fibers like an optical fiber so as to earn a nick name: TV Rock. The image of an object can be seen through on the other side of a good quality ulexite (not this one though).
Actinolite is a member of the amphibole mineral group – a hydrated calcium, magnesium, and ferrous silicate. Nephrite (soft jade) is one group member. Typically actinolite occurs in schist as slender or acicular green crystals. Its fibrous variety is one type of asbestos. Believe it or not, actinolite powder has long been used in traditional Chinese medicine as an aphrodisiac stimulant for men. Notwithstanding the claim, beware that a silicate eater is on his perilous fast tract to reincarnation. See also Figure 5-19a.
Figure 5-16b & c: Schist (21x14x5 cm; from western San Bernardino County, CA).
The actinolite mentioned above comes from Pelona Schist (a formation name) dominated Blue Ridge. The two pictures were taken at slightly different viewing angles of the same specimen. The blueness in the picture on the left is accentuated by placing the specimen over an artificial lawn. Although the blue picture conforms to the naming of Blue Ridge, the specimen is not blue schist for its absence of glaucophane as a major constituent mineral. The cause of turning bright blue in the picture from the natural dark gray blue and pale green is unknown. (Neither artificial filtering nor lighting was imposed. See also the unaltered color of actinolite pictures in Figures 5-16a and 5-19a for comparison.) The color contrast between the two pictures demonstrates a potential pitfall of using picture alone for rock or mineral identification.
Each piece was rounded, cobble-like before it was cut to reveal its orbicular structure. The dark- and light-colored minerals are interposed in radiating stripes and concentric bands. Because of the overwhelmingly dark appearance, the rocks are more akin to diorite than granite. The dark likely consists of hornblende and biotite while the light is plagioclase with little amount of quartz.
How does the orbicular structure come about? There is no definitely answer. It must be a complicate history of episodic mineral precipitation and segregation as relative mineral concentrations in the fluid and corresponding freezing (melting) points evolve through step changes.
The spheroidal inclusions (orbicular) appear in aggregate in the host rock. Also, the radiating stripes and concentric bands are visible but not as well developed as in the three specimens depicted in Figure 5-17a.
When metamorphism progresses further, part of gneiss is partially melted to form the so-called migmatite – a mixture of igneous and metamorphic rocks. The light grey curvy seams are quartz, which has a lower melting point relative to the mafic minerals and accordingly wiggles like a fluid.
In the appreciation of stone arts (suiseki), usually only one cut per rock specimen is permitted for display stability. As practiced in Japan, carving is not honored. The practice in other cultures may allow cutting, carving, smoothing, and staining. The piece here is at its natural state, free standing on the bank of a creek, near a pit for ruby exploration in the past.
Among countless gravels in a bay beach, this spotted, tan cobble is eye-catching for its bearing of a pale white dike. One saw-cut reveals the tan is only a veneer over a dark green rock dotted with discrete anhederal crystals of either lighter or darker color. The rock is not porphyritic; it is an altered peridotite. The groundmass-like mineral is serpentine as hydrated and altered from olivine; and the phenocryst-like minerals are relatively more weathering-resistive pyroxene. The dike is pyroxenite of which the pyroxene is bigger than the equivalent crystals imbeded in the peridotite. (See Figure 3-1 for a perdotite without visible pyroxene.)
A tan trace of alteration has resulted from weathering interaction with sea water along a recently developed fracture. Also, there are three sets of tiny, parallel veinlets which are darker than the serpentine and which mark more intensive alteration along micro fissures before interaction with sea water has occurred.
The green actinolite crystals occur in acicular, columnar, or radiated forms. The brown or whitish minerals are feldspar. Also, see Figure 5-16 about actinolite.
This piece of rock and the two below come from different contact metamorphic zones.
Talc with a calcite core (Right, 12x4x15 cm; from San Bernardino County, CA) and accessory chalcopyrite (barely visible at lower left corner). The finger-nail scratchable is greenish gray talc.
The piece on the left also has a white calcite core as enclosed by dark green, flaky chlorite group minerals (or actinolite). Both metamorphic rocks came from a dump site of mining for talc or asbestos in eastern San Bernardino County.
The asterisk-like phenocrysts in this slice of volcanic rock are eye-catching. The rock is porphyritic with brownish red phenocrysts embedded in the dark or black aphanitic groundmass. These phenocrysts occur in two forms: isolated individual crystals and asterisk clusters of crystals; but interestingly both belong to the same type of mineral. All crystals developed while the lava was still flowing and accordingly the isolated crystal grains aligned in general with the flow direction (along the long dimension of this slice). Each asterisk cluster rarely grows radially outward from a center because the asterisk arms are not orderly arranged and quite a few arms are interposed by hairline-wide dark groundmass. I believe the clusters represent localized congestions of crystals in a slowly moving lava, like log or twig jamming in a receding stream.
The rock bears no visible quartz. Its groundmass is opaque to the unaided eye. One conspicuous light grey streak is likely crack filling of unknown secondary mineral. The phenocrysts, somewhat altered to generate reddish hue, could be either alkaline feldspar or feldspathoid (silica-deficient feldspar). If feldspar, the rock is porphyritic trachyte-andesite; otherwise it is an uncommon porphyritic phonolite. My 3 ½-year old grandson, Heru, has a different idea — a spider-crab rock. Stare at it to reach the illusion: ‘crabs’ float above a dark floor.
This rock gives an impression of false porphyritic volcanic rock. It bears phenocryst-like crystals but the abundance of andesite rock fragments precludes its being porphyritic. A volcano spewed ash and rock fragments into the air and the airborne mixture settled to the ground ultimately. Then, the ash was consolidated, hardened with the rock fragments. The specimen is a piece of pyroclasts (tuff for its fine grain texture) – i.e., orignated from fire (volcano) and amassed mechanically (falloff from the air). The powder-like ash is prone to chemical interaction with water. Alterations of ash to chlorite result in greenish-yellow tone.
A conventional wisdom says a nodule of marl (calcareous mud) dehydrates and its interior contracts and cracks. Then, a mineral-bearing fluid infiltrates into the desiccated, polygonal mud cracks and eventually precipitates to generate various distributary arms of mineral deposits. In this specimen, the yellowish is calcite and the brown lining the cracks is likely aragonite — a reaction rim between the marl and calcite.
Something is worth noting. 1) All cracks taper to the peripheral of the nodules and the nodule does not crumple from interior cracking such that it retains its exterior integrity. Why? 2) There is no visible fluid entry or exit path way. How does the fluid come and go? (Unrecognized path way?) 3) The fluid brings the deposits all the way to the tips of cracks without any obstruction so as to leave no gap or few unoccupied pockets (in this specimen). 4) The invading fluid is unsaturated at the entrance, how much fluid is needed to pass through to pack the cracks without leaving a trace of successive deposition? Hundreds or thousands of the crack volume? Anyway, a septarium is the sedimentary equivalent of a volcanic agate with one notable exception: the septarian are not banded, apart from aragonite rims.
Many rock-shop visitors are often enthralled by the uncut agate-like nodules. Some will bid on the prospect of a beautiful piece of agate when a nodule is cut open. However, few can predict the outcome because the grandeur depends on the geode quality as well as the location and orientation of the cut face. Here is an example of lucky cut, which was followed by another parallel cut at 3.5 cm apart from the middle. The two cut faces show 2D views of a 3D geode. Fascinating is the agate banding and geometry!
A geode has one chamber encrusted with chalcedony. The source material of agate can be silica-rich lava that is traped during chamber formation, or hydrothermal fluid that infiltrates later into an empty chamber. Regardless of contemporary or not, when the fluid temperature drops to freezing point, a thin band of deposit in-plates the chamber. The extraction of silica by deposition changes the chemical composition and lowers the freezing point of the residual fluid. When the next freezing point is reached, deposition resumes. The process repeats at go-and-stop mode to yield multi-bands until the fluid is exhausted. At the last stage of evolution, drusy quartz may develop if free space is available for crystal growth. The color or shade of banding reflects slight variations in chemical ingradients and perhaps amounts of microscopic gas bubbles. My scenario is analogous to the ice banding depicted in the cover picture of this chapter except that the water freezing point does not evolve with time.
Hello! My name is Chalcedony – a crypto-crystalline mineral capped with white opal. I have a translucent, reddish brown body without definitive crystal form (amorphous). My exterior is roundly knotty but harmoniously smooth overall. You can touch me and get away with a nice cool feeling because my skin is very well polished and varnished to provide good thermal contact, and my body conducts heat away from your finger tips efficiently. I am going to tell you how I came into being. Here is the journey to what I am.
Many million years ago, there were magma activities under what is now Mojave Desert, California. Some magma extruded onto the ground surface as lava during volcanic eruptions. As heat escaped into the space, magma and lava cooled down and subsequently crystallization (solidification) occurred to form plutonic and volcanic rocks, respectively. Although rock-forming silicates settled out of the original fluid magma system, some fluids remained. Those residual fluids and the ground water that flowed near the magma mingled to form hydrothermal fluids — the hot, primordial soups out of which I was born and fed to grow.
The hydrothermal fluids contain various dissolved minerals, including silica (silicon dioxide) which is surplus or leftover from forming silicate minerals. As the temperature declined after heat dissipation, the fluids became supersaturated with silica (that is, the fluids have dissolved more silica than what is normally dissolvable) and were unstable. The instability was breached later by a slight disturbance like, for example, ground shaking due to earthquakes. Silica nucleated immediately as solid around some rocks. That was the conception of me. Henceforth, I grew unceasingly to become a knotty chalcedony until a stable saturation of silica in the solution is re-established for the new but lowered temperature setting.
Meanwhile as the hydrothermal fluids continued to cool, new coating was added to the exterior of my chalcedony body. Such processes repeated, leading to a successive zoning of chalcedony, which is slightly visible in my body structure. Deposition of chalcedony stopped at lower temperature, say around 100 to 200 degrees celcius. In its stead, silica incorporates water molecules to form white opal, which glazed over the lightly tanned chalcedony core.
Unlike sedimentary layering, chalcedony is not extensively deposited. It occurs in lenticular bodies, patches here and there. Each body may spread a few feet across but attains only a couple of inches in thickness.
We resided within a host of igneous rocks, which were rising along with a large-scale regional tectonic uplift. Accompanied with the uplifting, the overburden rocks above our host were being eroded away at the ground surface. Consequently, the overburden pressure on us was reduced. Decompression led chalcedony bodies to expand and shatter. Meanwhile as we rose, our body temperature dropped too. Cooling resulted in contraction, which in turn induced further splintering. In short, expansion and contraction cracking slackened my connection with the rest of the chalcedony bodies. Eventually, having been exhumed by uplift-erosion, I saw the sun light in the desert. A new life began.
Not long after the exposure, I tumbled into the desert floor as a small isolated piece of chalcedony. Being broken loose from a relatively massive chalcedony body, I started with a jagged outlook. Dusty wind deburred my rough exterior and reshaped me as well. Occasionally flash floods rolled me around down slope. As a result, my exterior was smoothed and polished but my opal coat was partially torn, thinned, and stripped to result in a cap only. Additionally wind sprayed a thin film of air-suspended mineral matter over me, including the opal overcoat. I was glossed with a desert varnish. Ultimately, an art piece was created – a translucent, reddish tan mound (chalcedony) with white snow cap (opal).
I did not lie on the ground surface long enough to have my underbelly stained in reddish hue, as happened to innumerable desert residents. My grace was salvaged by a catastrophic flash flood. It buried me vertically a couple of feet deep under the desert wash. Hibernating underground, I have escaped from further onslught or denudation at the desert floor. Inevitably the debris above me was slowly washed away by infrequent flash floods and my top emerged again. One morning, my tiny outcrop reflected a light beam into the eyes of one passerby. She alerted her husband. He, a 75-year old man, carefully dug me out. “What a beauty!” I was greeted.
After washing me clean, the man found my opal cap was locally cracked. Yes, my thin opal coating had gone through dehydration cracking when I was last buried under the hot, dry desert. The cracks, up to two centimeters long and ranging from hairline to pencil-lead wide, expose the underlying chalcedony and the whole stone becomes a little more enchanting (e.g., imagine the leftmost part of the cap as a beguiled, virtual animal head with eyes, ears, nose, and mouth). Some cracks exhibit thicker white rims, apparently resulted from wrap-up arching during dehydration (like dry mud flakes). The dehydration cracking hints that the white coating is indeed opal.
Immediately after seeing me, Kai, one grandson of the couple, wanted to adopt me as a pet rock. The grandpa replied to his kindergartener, “You are too young to take care of a pet”. Who knows? I may RIP in a museum.
Hi! We two are chalcedony too – genetic cousins to the chalcedony with opal cap. We are like twins with very dissimilar outlook. We and our cousin grew up in similar environment but all ended distinctively. Unlike our cousin who was once buried by debris flow to preserve its well polished and varnished exterior, we stayed on the desert floor after losing contact with our ancestral chalcedony buddies. We were relentlessly bombarded by particles empowered by wind and sometimes by flash floods.
We are fairly resistant to weathering because chalcedony is essentially silica, which is chemically inert on the Earth’s surface. However, physical attack by water did us in. Our appearances seem to favor a theme that we resided by river or seashore where moving water, plus biological activities, erodes, corrodes, and perforates rocks. That is a misconception because we indeed come from desert where surface water is notoriously scarce. If so, how and what did water do to us?
When we were tumbling around in the desert wash, microfissures incurred. Despite its scarcity, moisture does condense on rocks in cold night. Water in puny amount can infiltrate the fissures, for example, through capillary suction and the infiltrating water may freeze during winter or cold night. Transforming into ice, it expands so as to exert stress around the fissures and to eventually widen and lengthen the fissures at nearly imperceptible pace. After the ice thaws, the enlarged fissures suck in more water, readying for greater expansion later. The repeating freeze-thaw cycles over long time weaken our body. Litle by little the deteriorated spots on our skin are plucked away by wind-sand blasting. Thus, our exterior was unevenly excavated and our underside was slightly stained brownish red as we rested on the desert soil, which bears rusty ferric iron oxide.
The stone on the right of the display is engraved in accordance with the chalcedony’s depositional structure; it retains some opal but shows no dehydration crack. On display, it sits like how it sat in the desert floor. Its bottom is stained.
For the stone on the left, its rear surface actually faces the ground for gaining brownish red stain. The chalcedony is accompanied by feldspar (pale red in color), suggesting it comes from the margin of an ancestral chalcedony body. The feldspar is dotted with numerous pinhole indents, some of which are darkened. The indents likely mark the incipient carving. The dark ones are indicative of where lichen (symbiotic algae and fungi) has anchored to breed. Really? Could they signal alternatively the presence of manganese dioxide too?
In spite of highly varied texture on the exterior, both stones possess some polished and varnished patches of chalcedony. Are those patches relics of the past? Or, do those represent the contending forces of excarvation versus polishing/varnishing at present?
In summary, we are nature’s sculptures – small wonders in the vast, magnificent desert!
Now comes a distant relative – one piece of chalcedony from one desolate corner in an intermittent creek near an abandoned mine at the margin of desert. Its lineage is a little murky. Its latest life story is clear, however. After being knocked off its parental chalcedony body, it has been crafted and polished by brute force of sporadic running water, which is loaded with fine, suspended particles. Figuratively the resulted landscape is painted with broad brushes, in contrast to the intricacy of an equivalent sculpture shaped meticulously with the aid via cyclic freezing/thawing of the scarcely available desert water/moisture. It casts a distinctive, natural charm by forsaking any makeup of the telltale desert varnish. In order to beautify the stone further, regrettably, some jutting at its exterior has been rounded off such that you may suspect whether this short introduction is a fake story.
This concludes my Wandering in Rock Country. It is not perfect but all things must converge to an end. And the end sparks a new beginning; and the beginning and end with something in between go around and around.