(a) State the characteristic features of cosmic abundance of elements. What are the bases of estimation of cosmic abundance of…

(a) Cosmic abundance of elements

Features: hydrogen and helium make up about 98% of the mass of the universe, and all other elements about 2%. Abundance falls steeply with atomic number up to about Z = 45 and then stays low and nearly flat. Elements of even atomic number are more abundant than their odd neighbours (Oddo–Harkins rule), because paired nucleons are more stable. There is a peak at iron (Z = 26), the most stable nuclei, reflecting stellar silicon burning. Lithium, beryllium and boron are very scarce, since they are destroyed in stellar interiors and are not made in the main fusion chain. Heavy elements beyond iron come from neutron capture (s- and r-processes), giving small peaks near magic numbers.

Bases of estimation: (i) solar and stellar spectra, from absorption lines of the photosphere; (ii) chemical analysis of meteorites, chiefly CI carbonaceous chondrites such as Orgueil, whose non-volatile composition matches the solar photosphere and so represents the primitive solar nebula; (iii) solar wind samples, as from the Genesis mission; and (iv) cosmic-ray composition and interstellar medium observations, corrected for spallation. Nucleosynthesis theory is used to test the pattern.

(b) Chemical bonds in minerals

Ionic bonds arise from electrostatic attraction between oppositely charged ions: halite (NaCl), fluorite (CaF₂); such minerals are brittle with moderate hardness. Covalent bonds share electron pairs: diamond is very hard and insoluble. Metallic bonds hold ion cores in a delocalised electron cloud: native gold and copper are malleable and conduct electricity. Van der Waals bonds are weak attractions between neutral layers or molecules: between graphite sheets and talc layers, giving perfect cleavage and softness. Hydrogen bonds link layers in minerals such as brucite and gypsum and give ice its open structure. Most minerals have mixed bonds: Si–O in silicates is roughly half ionic and half covalent, and in sphalerite (ZnS) bonding is largely covalent.

(c) Hydrocarbons in the western oil fields

Mumbai Offshore Basin: a rifted passive-margin shelf basin on the west coast. Precambrian basement is overlain by Paleocene–Eocene syn-rift clastics, then Oligocene–Miocene shelf carbonates and clastics. Source rocks are Paleocene–Eocene shales (Panna Formation and equivalents). The main reservoir of the Mumbai High is the Miocene L-III limestone, with fractured basement also producing. Mumbai High is a large faulted anticline, trending roughly north-south, sealed by Miocene shale; oil is trapped in structural closures, fault blocks and stratigraphic pinch-outs.

Cambay Basin: a narrow, north-south intracratonic rift in Gujarat, filled with up to several kilometres of Tertiary sediment. The Eocene Cambay Shale is the source rock, and reservoirs are Eocene–Oligocene sandstones, as at Ankleshwar and Kalol. Traps are tilted fault blocks in a horst-and-graben pattern, roll-over anticlines and fault closures.

Kutch–Saurashtra is a related rift basin with thick Mesozoic sediments, still little produced.