J. D. Dana’s Mineralogical Contributions. 85 
Chapman adds Crednerite ; but we have seen no measurements 
of the angles of that species that confirm this reference. 
Hydromagnesite is formed from the alteration of Brucite, and 
the change is one now in progress. 
7. Homeaomorphism of Gypsum and Heulandite. 
The similar pearly cleavage of Gypsum and Heulandite, two 
species monoclinic in crystallization, suggests the idea of an ap- 
proximate isomorphism: and this is sustained by a consideration 
of their crystals, although not obvious in the ordinary mode of 
viewing their forms 
Crystals of gypsum sometimes have a hexagonal outline with 
a bevelment of each of the six sides, by a pair of planes, either 
of which pair might be taken as planes of the fundamental prism. 
The interfacial angles of these pairs of planes or prisms are 138° 
28’, 143° 42’ and 111° 42’ (B. and M.) The first of the prisms 
here mentioned is also a cleavage prism, cleavage taking place, 
though with some difficulty, parallel to its faces. There is some 
reason, therefore for considering this the fundamental prism, al- 
though usually taken as planes of the fundamental octahedron. 
here is a second imperfect cleavage parallel to a plane trunca- 
ting the edge of the prism 111° 42’ (M: M of authors), a fact 
that evinces the prominent importance of the plane. 
we make the former cleavage 0 
directions, the lateral faces of the fun- id 1(2) l 
damental prism (J), the latter its basal 2(v) 2i(m) 
plane (O), the planes hitherto observ- 3i(e}}83(a) 3(x) 33(y)| 
ed (see Brooke and Miller) will be as |—~|—— ~ laa) 
shown in the annexed table, in which ilk) 
O is the base, and the columns contain |— SEER PEN Ps 
the different vertical zones of planes. 2 eee) 
The occurring octahedral planes are situated upon the acute basal 
edges and angles of the fundamental prism. The angle J: J= 138° 
28’, 2i : 21 (clinodiagonal planes 2P/ @:2P’ w)=111° 42’; and the 
faces meeting at 143° 42’, are planes of the fundamental octahe- 
dron (P or 1). The axes, 
W: bes Te Slco, 
and the angle C (the inclination of the vertical axis) =66° 14’. 
Now in Heulandite, J: J=136° 4’; 1: 1=146° 56’; and the 
axes 
© @tbre=105:1:2475. C=86° 26 
The dimensions and the angles here mentioned (excepting C) 
are therefore very similar in these two species, and the crystals 
are much alike in habit. There is a wide difference between the 
two in the inclination of the vertical axis which makes l 
discrepances in some of the angles; for example 2i: 2i in gyp- 
oe a 
rge 
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