(a) What do you understand by isomorphism and polymorphism? Discuss monotropy by citing example of diamond and graphite. (20…

(a)

Isomorphism denotes minerals with different chemical compositions but the same crystal structure, usually because ions of similar size and charge substitute in the lattice. Calcite (CaCO3) and rhodochrosite (MnCO3) are both trigonal carbonates and illustrate this structural similarity despite different cations; where substitution is continuous an isomorphous series forms, as in forsterite–fayalite olivine or the albite–anorthite plagioclase series. Polymorphism denotes the same chemical composition occurring in different crystal structures, as in diamond and graphite, both pure carbon, or in quartz, cristobalite and tridymite. The distinction is practical: isomorphism controls substitution and solid solution, while polymorphism controls stability, density and reactivity. Polymorphic transformations are enantiotropic when each form has its own stability field and the change is reversible at a transition point (α–β quartz at 573 °C at 1 atm), and monotropic when, over the whole temperature range at a given pressure, only one form is stable, so the other is always metastable and can change only one way, irreversibly, into the stable form. In a ΔG–T diagram, the Gibbs free energy line for graphite lies below that of diamond at ambient conditions, so graphite is the stable monotropic phase; at atmospheric pressure this is true at all temperatures, so diamond→graphite is monotropic. Diamond becomes stable only at high pressure (above about 4.5–5 GPa at mantle temperatures, i.e. depths over about 150 km), which is why natural diamonds come from kimberlites and lamproites, such as the Majhgawan (Panna) pipe in Madhya Pradesh. Diamond has sp3-bonded tetrahedral carbon in a three-dimensional covalent network, giving high hardness and a density of about 3.51 g/cm3. Graphite has sp2-bonded carbon arranged in layered sheets held by weak van der Waals forces, with a density of about 2.26 g/cm3. Although diamond is metastable at surface conditions, the kinetic barrier to breaking and reorganizing its strong covalent bonds is so high that conversion to graphite is negligible over geological time. Thus diamond-graphite illustrates monotropy: structure, bonding and density differ, but thermodynamics favours graphite while kinetics preserve diamond.

(b)

Safe disposal of radioactive waste in geological repositories requires deep, stable host rock with low permeability and low groundwater flow, such as granite, salt or clay, in a tectonically quiet setting with no active faulting, seismic hazards, or exploitable resources like mining, geothermal or water extraction. The site must maintain long-term groundwater chemistry that limits radionuclide mobility, with controlled redox conditions, low temperature, and limited microbial or organic activity. Residual oxygen trapped at closure is soon consumed by canister corrosion, minerals and microbes, restoring reducing conditions; however, radiolysis of water at the fuel surface can create a local oxidizing front that raises uranium and other radionuclide solubility, which is why reducing, slow-flowing groundwater and a sorbing buffer are essential. Heat from the waste must also be kept below limits (about 100 °C at the bentonite) to preserve buffer properties. A KBS-3-style diagram (the Swedish–Finnish concept used at Onkalo, Finland) would show, from centre outward, spent fuel in a copper canister with cast-iron insert, bentonite buffer, tunnel backfill, host rock at about 400–500 m depth, and the geosphere and surface environment beyond. Engineered barriers include the waste matrix, canister, buffer and backfill; natural barriers include host rock and the surrounding geosphere. Together they form multiple barriers that retard migration, maintain reducing conditions, and protect the biosphere and hydrosphere over thousands to millions of years.

``svg <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 446" width="640" height="446"> <title>Deep geological repository and multiple barriers</title> <defs><marker id="ar" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path d="M0,0 L10,5 L0,10 z" fill="#374151"/></marker></defs> <style>text{font-family:Arial,Helvetica,sans-serif;font-size:13px;fill:#1f2937}.t{font-size:15px;font-weight:bold}.b{font-weight:bold}.s{font-size:12px;fill:#4b5563}.c{text-anchor:middle}.e{text-anchor:end}.i{font-style:italic}.ar{stroke:#374151;stroke-width:1.2;fill:none;marker-end:url(#ar);marker-start:url(#ar)}</style> <rect width="640" height="446" fill="#fff"/> <text x="320" y="20" class="t c">Deep geological repository and the multiple-barrier concept (schematic)</text> <text x="20" y="46" class="b">Repository cross-section</text> <rect x="20" y="60" width="300" height="34" fill="#dcecd7"/><text x="30" y="81" class="s">biosphere / hydrosphere, surface facilities</text> <rect x="20" y="94" width="300" height="236" fill="#e5e7eb"/> <text x="30" y="118" class="s">host rock: granite / salt / clay</text><text x="30" y="132" class="s">(low permeability, stable, no active faults)</text> <rect x="110" y="60" width="18" height="190" fill="#fff" stroke="#374151" stroke-width="1.2"/><text x="134" y="170" class="s">access shaft</text> <rect x="60" y="250" width="220" height="26" fill="#fff" stroke="#374151" stroke-width="1.2"/><text x="70" y="267" class="s">deposition tunnel (backfilled at closure)</text> <rect x="81" y="276" width="18" height="40" fill="#c7a46a" stroke="#7c5a1e" stroke-width="1"/><rect x="85" y="282" width="10" height="28" fill="#b45309"/> <rect x="131" y="276" width="18" height="40" fill="#c7a46a" stroke="#7c5a1e" stroke-width="1"/><rect x="135" y="282" width="10" height="28" fill="#b45309"/> <rect x="181" y="276" width="18" height="40" fill="#c7a46a" stroke="#7c5a1e" stroke-width="1"/><rect x="185" y="282" width="10" height="28" fill="#b45309"/> <rect x="231" y="276" width="18" height="40" fill="#c7a46a" stroke="#7c5a1e" stroke-width="1"/><rect x="235" y="282" width="10" height="28" fill="#b45309"/> <text x="30" y="332" class="s" style="fill:#7c5a1e">bentonite buffer</text><text x="150" y="332" class="s" style="fill:#b45309">copper canisters with spent fuel</text> <path d="M305,100 L305,244" class="ar"/><text x="300" y="226" class="e s">≈ 400–500 m</text> <circle cx="470" cy="205" r="140" fill="#dcecd7" stroke="#2e7d32" stroke-width="1.4"/> <circle cx="470" cy="205" r="118" fill="#e5e7eb" stroke="#6b7280" stroke-width="1.4"/> <circle cx="470" cy="205" r="96" fill="#f3e8d2" stroke="#8a6d3b" stroke-width="1.4"/> <circle cx="470" cy="205" r="74" fill="#c7a46a" stroke="#7c5a1e" stroke-width="1.4"/> <circle cx="470" cy="205" r="52" fill="#f9c99a" stroke="#b45309" stroke-width="1.4"/> <circle cx="470" cy="205" r="28" fill="#fde68a" stroke="#92400e" stroke-width="1.4"/> <text x="470" y="209" class="c s b">fuel</text> <text x="340" y="46" class="b">Multiple barriers, inside to outside</text> <rect x="340" y="342" width="10" height="10" fill="#fde68a" stroke="#92400e"/><text x="356" y="352" class="s">1 spent fuel / vitrified glass matrix</text> <rect x="340" y="357" width="10" height="10" fill="#f9c99a" stroke="#b45309"/><text x="356" y="367" class="s">2 copper canister with cast-iron insert</text> <rect x="340" y="372" width="10" height="10" fill="#c7a46a" stroke="#7c5a1e"/><text x="356" y="382" class="s">3 bentonite buffer: swells, seals, sorbs</text> <rect x="340" y="387" width="10" height="10" fill="#f3e8d2" stroke="#8a6d3b"/><text x="356" y="397" class="s">4 tunnel backfill</text> <rect x="340" y="402" width="10" height="10" fill="#e5e7eb" stroke="#6b7280"/><text x="356" y="412" class="s">5 host rock (natural barrier)</text> <rect x="340" y="417" width="10" height="10" fill="#dcecd7" stroke="#2e7d32"/><text x="356" y="427" class="s">6 geosphere: slow, reducing groundwater</text> <text x="20" y="352" class="s b">Engineered barriers: 1–4</text><text x="20" y="367" class="s b">Natural barriers: 5–6</text> <text x="20" y="392" class="s">Each barrier delays release so that</text><text x="20" y="407" class="s">radionuclides decay before reaching</text><text x="20" y="422" class="s">water and biosphere (KBS-3, Onkalo).</text> </svg> `` India follows a closed fuel cycle: high-level waste from reprocessing is vitrified in borosilicate glass at Trombay, Tarapur and Kalpakkam and held in interim surveillance storage, while granitic terrains are being studied for an eventual deep geological repository, which would require a long-term safety case, institutional control and regulatory oversight by the Atomic Energy Regulatory Board (AERB).

(c)

Disseminated precious-metal deposits (Carlin-type, epithermal and porphyry Au–Cu) are mined in bulk by open pit and processed by heap or tank cyanide leaching, so their waste is huge in volume, usually low-grade, sulfide-rich and finely dispersed, giving high reactive surface area. Major environmental considerations include acid mine drainage from pyrite, arsenopyrite and other sulfides; cyanide residues from CIL/CIP or amalgamation mercury from older processing; tailings dam stability, seepage, erosion and dust; mobilization of heavy metals such as As, Sb, Hg, Pb, Cu and Zn under acidic or reducing conditions; and revegetation challenges caused by low fertility, metal toxicity and salinity. Treatment must therefore combine AMD neutralization, sulfide precipitation, passive treatment, cyanide destruction by advanced oxidation or biological methods, mercury removal, and robust tailings storage facility design with liners, seepage collection, covers and monitoring. Heap-leach pads need impermeable liners, leak detection and rinsing of spent heaps, and the International Cyanide Management Code sets handling standards; tailings dams should follow the Global Industry Standard on Tailings Management, given failures such as Brumadinho (2019). In India, Kolar and Hutti are lode/orogenic gold systems rather than typical disseminated deposits, but their waste illustrates cyanide, arsenic and mercury controls. For disseminated precious-metal waste, analogous concerns apply to porphyry Cu-Au and PGM-bearing ultramafic settings, such as PGE occurrences in the Nuasahi and Sittampundi ultramafic complexes. In India, the Environment (Protection) Act and environmental clearance conditions, together with progressive and final mine closure plans with financial assurance under the Mineral Conservation and Development Rules, 2017, require characterisation of waste (acid–base accounting), water-quality monitoring and post-closure stewardship.