It’s not siderite, but this mineral, belonging to the space group R·3·bar·c, forms a series with rhodochrosite and has perfect rhombo·hedral cleavage in three directions. A fibrous, effluorescent variety of this mineral is known as lublinite or sometimes ‘moon milk’. A common example of paramorphy is the interconversion of aragonite and this (-5[1])mineral. Iceland (*) spar is a transparent variety of this uniaxial mineral (10[1])used in the construction of Nicol prisms. The ordinary refractive index of this material exceeds its extraordinary index by about 0.16, giving rise to ‘double refraction’. This mineral is harder than gypsum (10[1])but softer than fluorite, and it is the primary component of limestone. For 10 points, name this mineral that defines 3 on the Mohs hardness scale, composed of calcium carbonate. (10[1])■END■