[Chemical composition] The number of Fe 3+ in the crystal structure of the attapulgite can be greater than 1, which is called iron- rich attapulgite. The attapulgite and the scutellite Ca 2 AlAl 2 [Si 2 O 7 ][SiO 4 ]O(OH) form a complete homogeneous image series.
[Crystal structure] monoclinic system; 
a 0 = 0.888 to 0.898 nm, b 0 = 0.561 to 0.566 nm, c 0 = 1.15 to 1.030 nm; β = 115 ° 25' to 115 ° 24'; Z = 2. The crystal structure of epidote is: the [AlO 5 (OH)] octahedron in the structure is connected in a co-edge manner to a chain extending along the b-axis, and the chain is further entangled with the [FeO 6 ] octahedron The chain is connected by an isolated tetrahedron [SiO 4 ] and a double tetrahedron [Si2O 7 ], and a large gap between the chains is filled with Ca 2+ , which is an irregular eight-coordinate polyhedron. The difference between the crystal structure of the scutellite and the attapulgite is that the octahedral space occupied by Fe 3+ is completely replaced by Al 3+ .
Figure G-19 The crystal of the green sapphire (cited from Pan Zhaoyu et al., 1993)
Parallel double sided: a{100}, c{001}, l{101}, r{102}, e{101}; slanted square column: m{110}, o{111}, n{111}
Figure G-20 epidote crystal is short column, column mask vertical
[Form] The crystal is often columnar and extends parallel to the b-axis (Figure G-19). The crystal faces on the parallel b-axis ribbon have distinct streaks (Figure G-20). Can be (100) into a polycrystalline twin. The reason why the attapulgite often appears to extend the direction parallel to the b-axis, {100} the more developed plate-like crystals and the octahedral chain extending parallel to the b-axis in the structure and the parallel {100} chain layer formed by it. In addition, it is often a columnar, radial, or cluster-like aggregate.
[Physical properties] gray, yellow, yellow-green, green-brown, or near-black, the color becomes deeper with the increase of Fe 3+ content, and the identities of the same amount of Mn are replaced by a small amount of Mn; Gloss; transparent. Cleavage {001} is complete. Hardness 6. The relative density is 3.38 to 3.49 (larger as the Fe content increases).
[genesis and occurrence] The formation of epidote is related to the hydrothermal action (mainly equivalent to the mid-temperature hydrothermal stage), which mainly forms the green curtain petrochemical, that is, the original magmatic rock, metamorphic rock and sedimentary rock are formed by the replacement of the hot liquid. A kind of surrounding rock alteration. In the power associated with the broken back metamorphism, Ca 2+ can be formed epidote precipitated from the group minerals feldspar, amphibole and pyroxene in. It is also widely developed in the green schist facies in regional metamorphic rocks. In addition, epidote is also a common mineral for the metamorphism of basic magmatic rocks.
[identification features] columnar crystal form, obvious crystal facet stripes, a set of complete cleavage of parallel {001}, characteristic yellow-green color can be distinguished from similar olivine and hornblende.
[Main use] has mineralogical and petrological significance.
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