Calcite is a common mineral composing such rocks as chalk and marble. It has the formulae CaCO3 and a relative hardness of 3. It effervesces vigorously with HCl. Clear specimens exhibit double refraction. Occurs as widespread sedimentary rock masses such as limestone. Crystalline metamorphosed limestones are called marbles. Argentine is a pearly lamellar variety; aphrite is foliated or chalklike; dogtooth spar, a form in acute rhombohedral or scalenohedral crystals; calc- sinter and calc-tufa are lose or porous varieties formed in caverns or wet grounds from calcareous deposits; agaric mineral is a soft, white friable variety of similar origin; stalaclite and stalagmite are varieties formed from the drillings in caverns. Iceland spar is a transparent variety, exhibiting the strong double refraction of the species, and hence is called doubly refracting spar.
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