Minerals and their identification

Rocks are made of minerals, and many engineering properties of rock and soil trace back to the minerals present. Quartz makes sand hard and durable; mica makes a schist split easily; calcite dissolves slowly in acidic groundwater and creates caves; smectite clay swells when wet and shrinks when dry; pyrite oxidizes and expands, heaving floor slabs and attacking concrete. A civil engineer does not need to identify hundreds of minerals, but should recognize the dozen or so that make up most rocks and soils and know which ones signal trouble.

What a mineral is

A mineral is a naturally occurring, inorganic solid with a definite chemical composition, or a limited range of compositions, and an ordered internal arrangement of atoms called a crystal structure. Glass is not a mineral because its atoms are not ordered; ice in a glacier is technically a mineral; coal is not, because it is organic. Each mineral's composition and crystal structure determine its physical properties, which is why simple physical tests can identify minerals in the field.

About eight chemical elements make up nearly all of the Earth's crust by mass. Oxygen and silicon together account for about three quarters, followed by aluminum, iron, calcium, sodium, potassium and magnesium. It follows that the most common minerals are silicates, built from silicon–oxygen tetrahedra: a small silicon atom surrounded by four oxygen atoms. The way the tetrahedra link together, as isolated units, single or double chains, sheets or three-dimensional frameworks, controls much of a silicate's behavior.

Physical properties used for identification

Minerals are identified in hand specimen by a combination of properties. No single test is decisive; the art is to use several together.

• Hardness: resistance to scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond). The scale is relative, not linear: corundum at 9 is far more than one step harder than topaz at 8. In the field, a fingernail is about 2.5, a copper coin about 3.5, a steel knife blade or nail about 5.5, and window glass about 5.5. A mineral that scratches glass is harder than about 5.5; quartz scratches glass easily, calcite does not.
• Cleavage and fracture: cleavage is the tendency to break along smooth planes controlled by weak bonds in the crystal structure. Micas have one perfect cleavage and peel into thin sheets; feldspars have two cleavages at about 90°; calcite has three cleavages not at right angles, so it breaks into rhombs. Minerals without cleavage break by fracture, such as the curved, shell-like conchoidal fracture of quartz.
• Luster: the way the surface reflects light, described as metallic, glassy (vitreous), pearly, silky, greasy or earthy.
• Color and streak: color is the most obvious property but often the least reliable, because small impurities change it. Streak, the color of the powder made by rubbing the mineral on unglazed porcelain, is more consistent. Hematite may look black, silver or red, but its streak is always reddish brown.
• Specific gravity: the density relative to water. Most common silicates are about 2.6 to 3.0; metallic minerals such as pyrite (about 5.0) and galena (about 7.5) feel noticeably heavy.
• Reaction with dilute acid: calcite fizzes vigorously in cold dilute hydrochloric acid; dolomite fizzes only weakly, or only when powdered. This simple test distinguishes limestone from dolostone and from quartz-rich rocks in the field.
• Crystal form and other properties: some minerals grow in characteristic shapes, such as cubic pyrite or six-sided quartz prisms. Magnetite is magnetic; halite tastes salty; some clays become slippery and sticky when wet.

The rock-forming minerals

Roughly a dozen minerals make up the bulk of common rocks. Knowing them, and their engineering significance, covers most practical needs.

Clay minerals

Clay minerals deserve special attention because they dominate the behavior of fine-grained soils and weak rocks. They are sheet silicates formed mostly by the chemical weathering of feldspars, micas and volcanic glass. Individual particles are tiny plates, usually less than 2 micrometers across, with large surface areas and electrically charged surfaces that attract water. Three groups matter most.

• Kaolinite has strongly bonded layers, little ability to absorb water between them, and low plasticity. It is relatively stable and forms in warm, well-drained weathering environments.
• Illite has layers held by potassium ions. It is the most common clay mineral in many glacial and marine clays and has moderate plasticity.
• Smectite, including montmorillonite, has weakly bonded layers that let water enter between them. Smectite soils have very high plasticity, swell greatly when wetted and shrink and crack when dried, and have low residual shear strength. Bentonite, a smectite-rich clay formed from altered volcanic ash, is used for drilling mud and slurry walls precisely because it swells and seals.

The Cretaceous shales of the northern Great Plains, such as the Pierre Shale, contain abundant smectite. They are responsible for expansive soils that damage foundations and pavements across parts of Colorado, Wyoming, the Dakotas and Montana, and for slow landslides on shale slopes. Expansive clays are also widespread in Texas and other southern states, where they are among the most costly geologic hazards to light structures. In contrast, the glacial lake clays of the Spokane area are mainly finely ground rock flour with illite and are more prone to consolidation settlement than to swelling.

Minerals that cause engineering problems

A short list of minerals causes a disproportionate share of trouble.

• Sulfide minerals, especially pyrite and the more reactive pyrrhotite, oxidize when exposed to air and water. The reaction produces sulfuric acid and sulfate, and the new sulfate minerals take up more volume than the original. Pyritic shales placed as fill beneath floor slabs have heaved buildings, and pyrrhotite in concrete aggregate has caused serious deterioration of house foundations in eastern Canada and in Connecticut.
• Soluble minerals, such as halite, gypsum and anhydrite, and to a lesser extent calcite and dolomite, can dissolve and leave voids. Gypsum dissolves much faster than limestone and can create cavities within the life of a structure. Anhydrite takes up water to become gypsum, expanding as it does.
• Swelling clays, mainly smectites, cause heave and pressures large enough to lift light buildings.
• Reactive silica, including opal, chalcedony, chert and strained or microcrystalline quartz, reacts with the alkalis in cement paste in the alkali–silica reaction, producing an expansive gel that cracks concrete.
• Asbestiform minerals, such as chrysotile and some amphiboles, occur naturally in some serpentinites and other rocks. Excavating them can release fibers, so sites in these rocks need dust control and worker protection.
• Mica in large amounts makes soils hard to compact and gives low strength along foliation in schists.

Priya keeps a small identification key in Python for the field team. It does not replace experience, but it reminds a junior engineer which tests separate look-alike minerals.

# Simple field identification helper for common minerals
minerals = {
    "quartz":    {"H": 7.0, "SG": 2.65, "acid": "none",   "cleavage": "none",  "lustre": "glassy"},
    "feldspar":  {"H": 6.0, "SG": 2.6,  "acid": "none",   "cleavage": "two",   "lustre": "glassy"},
    "calcite":   {"H": 3.0, "SG": 2.71, "acid": "strong", "cleavage": "three", "lustre": "glassy"},
    "dolomite":  {"H": 3.5, "SG": 2.85, "acid": "weak",   "cleavage": "three", "lustre": "glassy"},
    "gypsum":    {"H": 2.0, "SG": 2.32, "acid": "none",   "cleavage": "one",   "lustre": "glassy"},
    "muscovite": {"H": 2.5, "SG": 2.8,  "acid": "none",   "cleavage": "one",   "lustre": "pearly"},
    "pyrite":    {"H": 6.3, "SG": 5.0,  "acid": "none",   "cleavage": "none",  "lustre": "metallic"},
}

def candidates(scratches_glass, acid, lustre, sg=None):
    out = []
    for name, p in minerals.items():
        if scratches_glass != (p["H"] > 5.5):
            continue
        if acid != p["acid"] or lustre != p["lustre"]:
            continue
        if sg is not None and abs(sg - p["SG"]) > 0.3:
            continue
        out.append(name)
    return out

print("Hard, glassy, no fizz:", candidates(True, "none", "glassy"))
print("Soft, glassy, strong fizz:", candidates(False, "strong", "glassy"))
print("Hard, metallic, SG 5.0:", candidates(True, "none", "metallic", sg=12.6 / (12.6 - 10.1)))

Hard, glassy, no fizz: ['quartz', 'feldspar']
Soft, glassy, strong fizz: ['calcite']
Hard, metallic, SG 5.0: ['pyrite']

Minerals in soils

Soils inherit their minerals from the rocks they came from, filtered by weathering and transport. Resistant minerals survive: quartz dominates most sands because it is hard, has no cleavage and barely reacts chemically, while feldspars and dark ferromagnesian minerals break down during long transport and weathering. Glacial soils are an exception, because glaciers grind rock mechanically without much chemical weathering. Glacial tills and rock flour therefore contain fresh feldspar, mica, carbonate and even some sulfide grains that would not survive in a deeply weathered tropical soil. This explains why many Canadian and northern US tills fizz in acid: they contain ground-up limestone and dolostone picked up by the ice.

The clay-size fraction of a soil, particles finer than about 2 micrometers, is not always made of clay minerals. Glacial rock flour can be clay-sized quartz and feldspar with little plasticity, while a small percentage of smectite can make a soil highly plastic. That is why engineers classify fine soils by their plasticity (the Atterberg limits) rather than by grain size alone, and why a geologist's description of the source rocks helps predict how a fine soil will behave.

Identifying minerals in practice

In the field and core shed, mineral identification uses a hand lens (10× magnification), a steel knife or nail, a streak plate, a small bottle of dilute hydrochloric acid and a magnet. The geologist looks first at the overall rock, then at individual grains, noting color, luster, cleavage and hardness, and testing with acid. Where identification matters for design, for example in assessing aggregate reactivity, sulfide content or clay mineralogy, laboratory methods are used. Thin sections viewed under a polarizing microscope reveal mineral grains and their textures; X-ray diffraction identifies clay minerals and other fine crystals; and chemical tests measure sulfur and sulfate contents.

Concrete aggregate sources in the United States are evaluated for alkali reactivity and other deleterious materials using ASTM test methods, including petrographic examination under ASTM C295 and the reactivity tests referenced by ASTM C1778. After the damage to foundations in north-central Connecticut, the state introduced requirements and guidance for testing aggregate for pyrrhotite. The geologist's mineral identification is the starting point for deciding which laboratory tests are needed.