These specimens from the Siglo XX Mine present defined, brilliant, translucent and light orange colour monazite-(Ce) crystals. They are accompanied by hyaline quartz and black-brown cassiterite on matrix. Some are twinned. Monazite is found in small quantities in this mine.
Monazite gets its name from the Greek word μουάζω "monazein" meaning "to be alone," alluding to its isolated crystals and their rarity when they were first found. Monazite is generally found in granitic pegmatites, but these crystals are found in hydrothermal veins with tin and the absence of thorium (generally an accompanying element of monazite).

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The Siglo XX mine is one of the most impressive Bolivian mining localities. It produces some amazing specimens of common species like quartz, few of the with the Japan-Law twin which are are well known in the mineral world. In this sample we can enjoy a good sized Japan-twin quartz crystals, with transparence and luster. This nice twin is disposed on a druse of hyaline quartz crystals.
To give a unique touch to the specimen a group of cassiterite crystals with good brilliance and deep black color, very rich in faces, interpenetrating between them and with some quartz crystals. A superb specimen.

Group of Sigloite crystals pseudomorph after Paravauxite, partially covering the rocky matrix. Paravauxite and sigloite differ chemically by the presence of a single water molecule. Sigloite is white to yellowish. The Siglo XX Mine is the type locality for Sigloite species.