A series of Articles printed in "Gem and Mineral Journal" newsletter of The Gem & Mineral Society of Lynchburg, VA Inc. - website

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    This is the first segment in a series of articles written by club member/ geologist Dave Woolley. It has been recently revised from previous editions. I hope you will enjoy the learning experience, and I also wish to extend sincere thanks to Dave Woolley for his continued support of our club. - Natalie Darling, Editor
   
A mineral is a naturally occurring, non-organic crystal composed of an element,
or of elements that are chemically bound in defined ratios.

Examples: gold [Au], carbon as graphite [C] or as diamond [C],

    halite (table salt) [NaCl], calcite [CaC03], quartz [SiO2],

    biotite (iron mica) [K2 (Mg, Fe)6-4 (Fe, Al, Ti)0-2 {Si6-5 Al2-3O20}(OH, F)4].

        Each mineral species has unique physical and chemical characteristics that are dependent on the

type of elements present, such as metallic or non-metallic,

and the weak and strong bonds that hold these atoms together in molecules.

        A crystal is a solid containing a geometrically arranged, repetitive pattern

of molecules. Repetition permits physical characteristics such as the shape

plus it controls hardnesses, breakage, and cleavages (splitting) that vary with orientation. The

elements and bonds control chemical reactivity and solubility.

A rock fragment is a particle composed of one or more minerals. Some rock species consist of only one
mineral such as dunite composed of olivine, others like granite
have more. The physical and chemical nature of a rock species is dependent on the character of its mineral
or minerals, and the contacts or matrixes between crystals.

Rock types:

Igneous Rock - Rock crystallized from magma, underground hot melted rock,

or from lava, erupted magma frozen on the earth’s surface.

Rock species examples: granite, diabase, and syenite from magma;

volcanic glass, pumice, and rhyolite, from lava.

Sedimentary Rock - Rock comprised of cemented sediments.

Rock species examples: sandstone, limestone, and shale.

Metamorphic Rock - Any previously existing rock that has been altered

by changes in temperature and/or pressure - usually increases.

Rock species examples: Quartz sandstone > quartzite. Limestone > marble.

Shale > slate > phyllite > schist > gneiss.

Granite > granite gneiss. Basalt > greenstone.

Bedrock is coherent fragmented rock found at or near the earth’s surface.

Rainwater penetrates soil and rock depending on porosity, permeability, and discontinuities.
Water flows vertically through pores in the earth by gravity towards the elevation of sea level.
Water may be blocked by impermeable rock or saturate the pores in slowly permeating earthmaterials,
rising up to elevations above the level of the oceans. This buildup of height forces
water to flow within the earth downhill towards the nearest ocean. The upper surface of this
pore-saturated down-slope flowing water within the earth is called the water table. The water
table lowers to sea level unless replenished by rain or melting ice. Any body of pore-saturated
water within the earth is called an aquifer. Gravity pushes water into the sea from rivers,
unconfined aquifers, and confined aquifers. The water table is located on an unconfined
aquifer.

A perched aquifer is an aquifer that is located on an impermeable barrier that occurs above
the water table. Examples: the clay of hardpan can create a shallow barrier retaining water;
certain clay-sediment layers, shales, weathered diabase, and weathered volcanic ash. The upper
surface of water retained on a perched aquifer is called a perched water table. A perched
aquifer is often recognized by noticing wet sampled soil or the depth at which wet drilling tools
are withdrawn, above later-measured water standing in a boring. At some locations water leaks
from the edge of a perched aquifer: water travels vertically from one perched water table to
another, to the water table. Water may become trapped between two impermeable layers,
encapsulated as a confined aquifer. The top elevation of water in a well, sealed to a confined
aquifer, is a hydraulic head, as opposed to elevations of perched water tables and the water
table, any of which can be lower or higher. The elevation of water tables and hydraulic heads are
time and location specific, usually measured in wells. Some places water travels in erratic
directions along discontinuities, downwards by gravity, and horizontally or upwards by a
hydraulic head: joints, faults, and cavities, plus fractured quartz veins in residual soil, act as highspeed
conduits. Discontinuities can be ideal sources for wells due to the access of larger
quantities of faster moving water. Adjacent earth material and discontinuities may lack water
depending on whether they contain or access an aquifer.

Note: Water is attracted by capillary action to a thin zone above any water table. Capillary attractionalso can hold some water in the pores of unsaturated earth.

An unconfined aquifer may intercept the earth’s surface as a spring.

A confined aquifer may produce artesian flow at the surface or from a wellhead. A discontinuity in

rock can become a small confined aquifer.

The water table typically intercepts streams, creeks, and rivers, although it may sometimes be just

below the bed of an intermittent or wet-weather creek,

and a losing-creek. The water table is located above a creek’s elevation

at a gaining-creek. Gaining-creeks speed some of an aquifer’s water to the ocean.

Combined with carbon dioxide from the atmosphere, rainwater forms weak carbonic acid [H2C03].
Plant acids and the break down of pyrite which yields sulphric acid contribute: acid solutions percolate
through soil into rock causing chemical weathering.

Chemical weathering Water flows over impermeable rock, through permeable rock, and along discontinuities in rock where minerals deteriorate by chemical weathering: particular minerals are attacked by the addition or removal of elements by hydration, oxidation, and/or dissolution. Chemical weathering progressively weakens, creating zones of weathered rock, decomposed rock, and finally, residual soil. Calcite is the first common mineral to succumb to chemical weathering: the feldspars then the micas are the last. Quartz resists weathering. Certain dunites, hornblendites, sandstones, and other rock species rapidly change from rock to sand as the matrix or contacts between crystals deteriorate. Certain shales weather to expanding clay within hours or days of disruption or exposure; pyritic rock and pyritic marine-soils release sulfuric acid. Acid released from one mineral attacks others. Cavities form in limestone, dolostone, and marble, plus rocks containing water soluble minerals such as halite, creating karst topography. Weathered rock is rock that has undergone slight chemical weathering, not just surface staining. Decomposed rock is rock that has undergone considerable chemical change. Residual soil is the material remaining as acids sequentially reduce susceptible minerals.

  Percolating water internally leaches clay particles downward from the surface “A” soil horizon to filter-out in the “B” horizon as hardpan, while colloids and dissolved minerals continue to move towards the oceans. The “C” soil horizon contains weathering rock fragments. The “A” horizon is often rich in organic material, while all horizons in humid regions typically have organic activity: roots often extend tens of feet deep into discontinuities within rock, searching for water and mineral nutrients.

Physical weathering is jointing and faulting, processes of fracturing that reduces rock to
fragments. Particle-impacts powered by gravity and water freezing also fracture.
Soil is a mixture of rock fragments (see page one) and cavities variably filled with air and
water, sometimes containing organic material. Particle sizes range from “fine-grained” colloid,
clay, and silt, through “coarse-grained” sand, gravel, cobble, and boulder. Soils are derived from the
weathering of rock - residual soil, or from the accumulation of sediments - sedimentary soil.
Susceptible colloid and clay-sized particles tend to weather rapidly to clay-minerals due to the high
surface area of the particles.

Note: A clay-sized rock fragment may not be composed of a clay-mineral. Particles adsorb water

onto their surfaces. Certain clay-minerals absorb water within their crystal structures making

those soils expand or swell when wet, and contract or shrink when dry: these minerals are

high plasticity clay, sometimes called “fat clay” by Engineers. Clay-minerals plus clay-sized

rock fragments that do not absorb much moisture are low plasticity clay, sometimes called

“lean clay”. Soils with clays are called cohesive: clays tend to bond particles together. Soils

lacking clays are non-cohesive: internal friction poorly holds these soils together.

A sediment is a physically and/or chemically weathered rock fragment that has been
eroded, transported and deposited by gravity through the agents of air, water, and ice, such
as aeolian – wind, fluvial – river, lacustrine – lake, marine – ocean, and glacial sediments – ice
deposited. Colluvial sediments are particles transported down a slope by gravity. Sediments may
occur in a range of sizes, be sourced or sorted to predominately one size, or be a mixture of sizes
reflecting multiple origins.

Five clay-mineral groups are recognized: kandite, illite, smectite, palygorskite, and vermiculite. The
first three groups each have increasing water absorption and swelling properties. Residual soils
derived from a one-mineral rock species tend to be composed of one clay-mineral; multiple-mineral
rock species may yield a mix of clays. One rock species may weather to different clay minerals
depending on variables
such as acids, dissolved-oxygen content of water, and the leaching of selected elements.

High aluminum content minerals weather to yield the kandite clay-minerals. Kaolinite of the
kandite group is the finely crystallized residue that forms as a consequence of acidic chemical
weathering of aluminum silicate minerals like the feldspars and alkali mica in “granitic” rock such as
granite, granodiorite, and granite gneiss. Kaolinite’s non-shrinking characteristic is useful in making
ceramics. Kaolinite was named after Kao-ling or “High Hill”, a mountain in China where kaolinite
was first mined, then shipped to England for the manufacturing of fine porcelain “china”. Kaolinite
has been mined from the large decomposed feldspar crystals of many Virginia anorthosites and
granite-pegmatites. Kandite-rich materials tend to be physically stable.

The illites are the dominant clays of sediments, mudstone, shale, and limestone formed by the
weathering of fine grained feldspar and mica sediments, generally in alkaline (limestone forming) marine
environments, or the recrystallization of colloid sediments, for example: glauconite. The weathering of
potassium aluminum silicate sediments such as the orthoclase feldspars and the alkali-mica, muscovite,
yields the illites. The clay-mineral illite was named by the Illinois Geologic Survey in honor of their
state. Illites are found in the expanding shales of Valley and Ridge Province.
Glauconite is found in “greensands” and other sediments of the Coastal Plain Province.

The smectites are widely found, often mixed with illites; they also form from sediments in alkaline
conditions, plus from the acidic weathering of “basaltic” rock like basalt, gabbro, and diabase. High
magnesium and calcium silicate minerals lacking potassium, like the pyroxenes, yield the smectites.
Poor drainage is necessary for the smectites:
if magnesium is leached, kaolinite is formed instead. Smectite is from the Greek word
“smektikos” meaning to cleanse, a reference to its soapy feeling. Montmorillonite of the smectite group
typically forms by the weathering of poorly drained volcanic ash and certain diabase. Montmorillonite,
the chief mineral of the volcanic clay, bentonite, is the most absorptive and expansive clay known.
Montmorillonite was named after Montmorillon, a town in west-central France. Illite and especially
smectite-rich materials tend to be problematic for construction. Watch especially for volcanic ash beds
in the limestones of the Valley and Ridge, rare metamorphosed ash beds in the Catoctin formation of
the Blue Ridge, and poorly drained diabase dikes in the Piedmont Provinces.

The palygorskite group of minerals is rare. The mineral palygorskite is sometimes found within
breccia and along mylonite of faulted dolostone. Palygorskite was named after Palygorskaya, Russia.
Palygorskite is found in the Lone Jack Quarry near Glasgow, VA.

Vermiculite forms primarily from the weathering of biotite, a common black-colored iron-bearing
mica; at the contact of granitic and basaltic rocks; plus from the chlorite and hornblende of basaltic
volcanic material in marine environments. Vermiculare is the Latin word meaning “to breed worms”:
vermiculite particles expand when heated looking like worms; particles act like compressed springs.
Vermiculite is responsible for “elastic” clay, silt, and sand because clay minerals may occur in larger-thanclay-
sized-particles. Piedmont and Blue Ridge Province footings to be placed in excavations within
vermiculite-rich soil derived from gneiss and schist containing biotite, find bearing-capacities diminishing
as vermiculite particles bound upwards.

stable

stable