Comparative Study on Petroleum-Bearing Quartz Crystals from Madagascar and Pakistan

Gem-quality petroleum-bearing quartz is a type of quartz containing large (visible to naked eye), coloured, fluorescent organic inclusions. It is rare in the market and mostly named after the origin, including petroleum-bearing quartz from Pakistan, petroleum-bearing quartz (produced in Madagascar) from Madao, petroleum-bearing quartz from Braziletc. The first two of which dominate the market, having similar appearance but the prices differ largely. Therefore, it is necessary to distinguish their origins. In this paper, 20 samples from Madao and Pakistan (10 samples for each) were collected and studied by conventional gemmological methods, digital microscope system and Raman spectroscopy. The results showed that they have differences in crystal habits and inclusion characteristics: The I/b (the length-width ratio of the rough crystal) of petroleum-bearing quartz (produced in Madagascar) from Madao is between 1.9-4.4, indicating that they belong to the short-column to long-column quartz, while the I/b of petroleum-bearing quartz from Pakistan is all below 1.5, belonging to the bipyramidal quartz. Petroleum-bearing quartz from Pakistan has a 40% probability of showing trigonal bipyramid face(s), while petroleum-bearing quartz from Madao lacks of it. There are four types of organic inclusions in petroleum-bearing quartz from Pakistan: Negative crystal-shaped three-phase inclusions, irregular three-phase inclusions, black solid-phase inclusions and dry inclusions with craquelure-like inwall. The liquid phase is light yellow to bright yellow and emits yellow-white to blue-white fluorescence under long-wave ultraviolet light. While five types of organic inclusions were shown in petroleum-bearing quartz from Madao: Negative crystal-shaped three-phase inclusions, irregular three-phase inclusions, gas-liquid two-phase inclusions, solid-liquid two-phase inclusions and black solid-phase inclusions. The colours of the liquid phase including colourless, croci and jacinth, colourless and croci ones emit yellow-white to blue-white fluorescence under long-wave ultraviolet light, while some of the jacinth ones emit red fluorescence. Black solid-phase inclusions in both samples have been identified as bituminous by Raman spectroscopy, characterized by a pair of peaks near 1 340, 1 600 cm^-1 and a pair of weak peaks centered at about 2 940, 3 200 cm^-1. However, other organic inclusions could not be identified due to the strong fluorescent interference. The inclusions in petroleum-bearing quartz from Pakistan are all protogenetic, while there are different stages of pseudosecondary inclusions distributed along the healing fractures in petroleum-bearing quartz from Madao. This phenomenon may indicates that petroleum-bearing quartz from Madao was formed by multiple stages of growth, while petroleum-bearing quartz from Pakistan crystallized in only one stage of hydrothermal event.