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hints for crystal drawing

by margaret reeks

Teens » General

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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER XIV. TBICLINIC SYSTEMâCONSTRUCTION OF AXESâSIMPLE FORMSâCONBINATIONS. Plates XXXVII. To XXXIX. In the triclinic system there are three axes, all unequal and intersecting at oblique angles. They are lettered a, b, c. The angle between the c or vertical axis and the a axis is measured from the + end of one to the + end of the other, it is lettered ft. The angle between the c and b axes is measured from the + end of one to the + end of the other; it is lettered a. The angle between the a and b axes is measured from the + end of the one to the + end of the other and is lettered y. Plate XXXVII. shows the construction of the triclinic axes. It is dependent on the O plan and profiles of the axes. The a axis is in the same direction as in the isometric system but has this difference, the + end in the triclinic system is either depressed or elevated a certain number of degrees, so that the axis is no longer at right angles to the c or vertical axis, its position is similar to the a axis of the monoclinic system. The b axis is neither at right angles to the c norTEICLINIC SYSTEM 121 to the a axis, therefore its position is quite altered from that of the isometric 2 axis. We will assume the lengths given on the plate for the three axes, making as usual the b axis = unity. The angles are also assumed /3 = 140, a = 75, y = 135. We start by finding the direction of the a axis in the usual manner, then find its plan by a profile (see Fig. 2) exactly as for the monoclinic a axis. Having thus obtained the plan of its semi-length, we mark it on the 0 plan as shown at a, x. We next find the plan of the semi b axis, in the same way by a profile, shown at Fig. 3. Having then the plans of the semi a and b axes, it would at first appear quite easy to construct ...

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...MONOCLINIC SYSTEM - - . - . - 109 Fig. 1. Construction of Axes, assumed Axial Ratio 65 : 1 : -55 Fig. 2. Profile of a and c Axes- Fig. 3. Unit Prism {110}; Base {001} Fig. 4. Orthopinacoid {100} ; Clinopinacoid {010} ; Basal Pinacoid {001}. XXXIV. MONOCLINIC SYSTEM - - - - - - - 111 Fig. 1. Pyramid {111} Fig. 2. Pyramid {111} Fig 3. Combination of Pyramids Fig. 4. Gypsum. Axial Ratio -7 : 1 : -4; = 80 42'. Unit Prism {110} ; Unit Pyramid {111} ; Clinopinacoid {OlOj Fig. 5. Sphene. Axial Ratio -75:1: -85; = 60 17'. Unit Pyramid {111} ; Unit Prism {110}. XXXV. MONOCLINIC SYSTEM - 113 Fig. 1. Clinodomes {Oil} Fig. 2. Orthodomes {101} Fig. 3. Amphibole. Axial Ratio 55 : 1 : -29 ; ft 73 58'. Unit Prism {110}; Clinopinacoid {010}; Clinodome {Oil} Fig. 4. Epidote. Axial Ratio 1-58 : 1 : 1-8 ; 64 37'. Orthopinacoid {100} ; Orthodome {110} ; Basal Pinacoid {001} ; Unit Pyramids {111} {111} {111} {111}. XXXVI. MONOCLINIC SYSTEM - - 117 Orthoclase. Axial Ratio -66 : 1 : -56 ; Angle = 63 57'. Unit...