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What is its structure? I found an image of $\ce{B6O11}$ unit, but it looks incorrect:

unit The top left corner one.

  1. The oxygen can make max 2 bonds but atom $\mathrm{O8}$ has three!
  2. There are six oxygen with only one bond, that is the unit has $-6$ net charge, but $\ce{2Ca^2+} = +4$ positive charge.
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1) Oxygen CAN form 3 or 4 bonds. The most common example is $\ce{H3O+}$ ion This form of bonding is critical to understand basic nature of ammonia solutions, so for exact description of it look that part of the book.

2)Boron also can form more than 3 bonds if at least one its partner provides an electron pair for bonding, not one electron.

As a general rule I prefer to stay away from the chemistry of borates and alumina-silicates as it is ruled largely by ionic interactions and non-intuitive hydration/dehydration.

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In structural diagrams of polymeric salts 'unit' often refers to some recurring fragment... however, it is not nessesarily has same stochiometry than formal anion. In alumosilicates the anionic frame consists of $\ce{AlO4-}$ and $\ce{SiO4}$ tetrahedral units. However, they are usually connected by shared vertexes, so the exact formula of formal anion is different. Borates are a bit more complex system, since they may include both $\ce{BO3}$ and $\ce{BO4-}$ units, and oxygen is often connected to hydrogen.

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I looked into open cristallographic database. Unsurprisingly, it does NOT feature a compound $\ce{Ca2B6O11}$. However, salts are often written in 'dehydrated' form, so I looked for any borate with $\ce{Ca:B} = 1:3$ stochiometry. Several with rather complex crystal structure were found, so I left only two: one with rather simple structure of typical 'hidrated' borate anion and one with rather complex continous 'dehydrated' boron-oxygen layers. Note: the structures are refined from X-Ray , so they does not reveal hydrogen positions. Isolated oxygens of water molecules and cations are omitted.

$\ce{B3O8}$ unit from mineral Inyoite, $\ce{CaB3O3(OH)5·4(H2O)}$

B3O8

$\ce{[B3O6]_{\infty}}$ layer from mineral Fabianite $\ce{CaB3O5(OH)}$

(B3O6)_\inf

and its main building block

enter image description here

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  • $\begingroup$ The crystal structure of $\ce{Ca2B6O11}$ has actually been deposited to Inorganic Crystal Structure Database in 1980 and modified in 1987. The reference number is #23032. $\endgroup$ – andselisk Aug 12 '17 at 19:21
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$\ce{Ca2B6O11}$ is an anhydrous form of several main boron minerals with well-established both compositions and structures, such as:

Water depletion results in a rather noticeable structure rearrangement.

Crystal structure of $\ce{Ca2B6O11}$ [1] is actually deposited in ICSD database (#23032).
Unit cell: $8.68(1)$, $8.56(1)$, $10.78(2)$, $90.00$, $90.00$, $110.0(1)$; space group $P1\,1\, 21/b$ monoclinic, $Z = 4$:

$\color{#804000}{\Large\bullet}~\ce{B}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#3DFF00}{\Large\bullet}~\ce{Ca}$.

For clarity coordination polyhedra are only shown for $\ce{[BO4]}$.

enter image description here

enter image description here

Cross-linked structure features trimeric linear assemblies of $\ce{[BO4]}$ units connected via vertices into the 3D motif with $\ce{Ca}$ and $\ce{[BO3]}$. In order to answer your question regarding atom bonding, here are the environmental listings for all crystallographically non-equivalent atoms. for the quick reference, maximum number of nearest neighbous of oxygen is $4$, e.g. for $\mathrm{O2}$:

enter image description here

Boron

Forms two types of coordination polyhedra with oxygen: planar trigonal (C.N. = 3) and tetrahedral (C.N. = 4).

    B1   SYMM  O4    O3    O2    O8    B6    B5    B3
O4  1.42 I     -     -     -     -     -     -     - 
O3  1.46 I     107.9 -     -     -     -     -     - 
O2  1.48 I     113.1 110.4 -     -     -     -     - 
O8  1.56 I     108.8 107.7 108.8 -     -     -     - 
B6  2.46 2_765 33.3  120.2 79.9  124.8 -     -     - 
B5  2.57 2_655 95.4  89.5  136.5 28.1  123.8 -     - 
B3  2.64 I     86.3  125.6 27.0  116.8 53.4  142.5 - 

     B2   SYMM  O5    O9    O10   O11   B3    B4    B5    B4
O5   1.44 I     -     -     -     -     -     -     -     - 
O9   1.45 I     110.5 -     -     -     -     -     -     - 
O10  1.46 I     107.9 115.2 -     -     -     -     -     - 
O11  1.64 I     111.8 109.4 101.9 -     -     -     -     - 
B3   2.49 I     98.6  32.7  145.9 87.3  -     -     -     - 
B4   2.57 4_556 119.4 86.3  29.0  116.2 118.0 -     -     - 
B5   2.60 I     116.1 82.1  122.7 28.6  58.9  123.9 -     - 
B4   2.69 I     21.7  131.1 90.3  104.5 119.5 109.1 119.7 - 

    B3   SYMM  O7    O6    O2    O9    B6    B2    B5    B1
O7  1.44 I     -     -     -     -     -     -     -     - 
O6  1.46 I     113.5 -     -     -     -     -     -     - 
O2  1.48 I     111.1 106.5 -     -     -     -     -     - 
O9  1.49 I     108.3 109.9 107.5 -     -     -     -     - 
B6  2.30 2_765 26.2  116.5 85.9  125.4 -     -     -     - 
B2  2.49 I     95.9  90.0  138.2 31.5  121.1 -     -     - 
B5  2.51 I     114.2 27.3  126.0 84.7  129.8 62.8  -     - 
B1  2.64 I     85.3  111.3 27.1  126.0 59.5  156.2 137.8 - 

     B4   SYMM  O5    O10   O3    O1   B2    B6    B2
O5   1.46 I     -     -     -     -    -     -     - 
O10  1.47 4_566 108.7 -     -     -    -     -     - 
O3   1.52 3_766 113.6 105.1 -     -    -     -     - 
O1   1.53 I     109.6 115.5 104.4 -    -     -     - 
B2   2.57 4_566 98.7  28.9  132.6 95.6 -     -     - 
B6   2.68 I     134.2 91.5  99.2  27.9 79.3  -     - 
B2   2.69 I     21.5  126.3 113.8 89.4 108.8 116.0 - 

     B5   SYMM  O6    O11   O8    B3    B1    B2
O6   1.38 I     -     -     -     -     -     - 
O11  1.40 I     120.8 -     -     -     -     - 
O8   1.41 2_654 118.2 121.0 -     -     -     - 
B3   2.51 I     29.0  92.0  147.0 -     -     - 
B1   2.57 2_654 86.9  152.1 31.6  115.4 -     - 
B2   2.60 I     87.3  34.0  154.2 58.3  169.5 - 

    B6   SYMM  O7    O4    O1    B3    B1    O2    B4
O7  1.19 2_764 -     -     -     -     -     -     - 
O4  1.50 2_764 130.4 -     -     -     -     -     - 
O1  1.51 I     117.5 109.4 -     -     -     -     - 
B3  2.30 2_764 32.5  98.1  148.3 -     -     -     - 
B1  2.46 2_764 99.2  31.4  136.5 67.1  -     -     - 
O2  2.64 2_764 65.7  64.9  157.7 33.9  33.5  -     - 
B4  2.68 I     132.9 85.2  28.4  148.8 108.7 132.2 - 

Oxygen

     O1   SYMM  B6    B4    O7    O3    O5    O4    Ca1   O10   Ca2
B6   1.51 I     -     -     -     -     -     -     -     -     -  
B4   1.53 I     123.7 -     -     -     -     -     -     -     -  
O7   2.31 2_764 27.1  137.8 -     -     -     -     -     -     -  
O3   2.42 3_766 111.8 37.6  138.6 -     -     -     -     -     -  
O5   2.44 I     152.3 34.2  145.8 61.7  -     -     -     -     -  
O4   2.45 2_764 35.2  93.0  61.6  77.0  126.9 -     -     -     -  
Ca1  2.47 2_764 88.3  115.4 61.7  152.3 91.5  119.2 -     -     -  
O10  2.54 4_566 96.2  31.5  106.3 57.2  57.0  73.4  103.5 -     -  
Ca2  2.59 3_766 117.7 89.5  127.8 62.3  84.4  105.3 125.9 118.1 -  

     O2   SYMM  B3    B1    O6    O9    O7    O3    O4   O8   Ca2   B6
B3   1.48 I     -     -     -     -     -     -     -    -    -     - 
B1   1.48 I     125.8 -     -     -     -     -     -    -    -     - 
O6   2.35 I     36.5  126.4 -     -     -     -     -    -    -     - 
O9   2.40 I     36.5  145.7 61.2  -     -     -     -    -    -     - 
O7   2.41 I     34.0  93.3  61.3  59.4  -     -     -    -    -     - 
O3   2.41 I     140.8 34.4  160.5 130.5 108.7 -     -    -    -     - 
O4   2.42 I     93.5  32.7  104.2 116.9 60.7  57.5  -    -    -     - 
O8   2.48 I     130.9 36.7  106.6 167.0 111.8 59.8  59.4 -    -     - 
Ca2  2.51 2_654 94.7  91.6  58.7  115.0 107.4 114.3 99.6 56.4 -     - 
B6   2.64 2_765 60.1  66.6  80.2  84.4  26.7  85.4  34.0 89.1 107.1 - 

     O3   SYMM  B1    B4    O4    O10   O2    O1    Ca2   O8    O5    Ca2  O6
B1   1.46 I     -     -     -     -     -     -     -     -     -     -    - 
B4   1.52 3_766 132.5 -     -     -     -     -     -     -     -     -    - 
O4   2.33 I     35.5  112.8 -     -     -     -     -     -     -     -    - 
O10  2.38 2_755 131.8 36.7  138.2 -     -     -     -     -     -     -    - 
O2   2.41 I     35.1  108.5 61.4  96.8  -     -     -     -     -     -    - 
O1   2.42 3_766 146.5 37.9  111.4 64.0  143.6 -     -     -     -     -    - 
Ca2  2.43 4_557 117.1 93.1  151.4 60.3  99.9  96.4  -     -     -     -    - 
O8   2.44 I     37.6  169.6 61.2  141.9 61.4  150.5 91.3  -     -     -    - 
O5   2.49 3_766 100.7 32.4  82.8  58.4  84.0  59.7  118.6 138.3 -     -    - 
Ca2  2.59 I     90.6  89.6  58.0  124.5 119.2 62.1  137.7 93.5  83.3  -    - 
O6   2.68 2_655 81.6  133.1 74.8  144.6 114.6 95.3  96.8  55.5  137.2 53.8 - 

     O4   SYMM  B1    B6    O3    Ca2   O2    O8    O7   O1
B1   1.42 I     -     -     -     -     -     -     -    - 
B6   1.50 2_765 115.3 -     -     -     -     -     -    - 
O3   2.33 I     36.6  126.6 -     -     -     -     -    - 
Ca2  2.40 I     99.7  129.5 66.6  -     -     -     -    - 
O2   2.42 I     34.3  81.1  61.1  127.3 -     -     -    - 
O8   2.43 I     37.5  131.2 61.7  98.9  61.4  -     -    - 
O7   2.44 I     93.7  21.7  110.7 142.7 59.5  112.5 -    - 
O1   2.45 2_765 145.1 35.4  132.0 96.4  113.6 163.1 56.4 - 

     O5   SYMM  B2    B4    O10   O9    O10   Ca1   O1   O3    O11
B2   1.44 I     -     -     -     -     -     -     -    -     -  
B4   1.46 I     136.8 -     -     -     -     -     -    -     -  
O10  2.35 I     36.4  105.2 -     -     -     -     -    -     -  
O9   2.37 I     34.8  166.9 62.7  -     -     -     -    -     -  
O10  2.38 4_566 158.5 35.9  140.3 156.8 -     -     -    -     -  
Ca1  2.44 I     133.1 90.1  151.3 98.8  61.3  -     -    -     -  
O1   2.44 I     102.1 36.2  78.0  136.8 63.6  124.3 -    -     -  
O3   2.49 3_766 129.8 34.1  93.8  136.7 58.4  85.4  58.6 -     -  
O11  2.55 I     36.6  118.2 58.8  61.4  122.5 134.0 83.7 138.4 -  

     O6   SYMM  B5    B3    O2    Ca2   O8    O9    O11   O7    O3
B5   1.38 I     -     -     -     -     -     -     -     -     - 
B3   1.46 I     123.8 -     -     -     -     -     -     -     - 
O2   2.35 I     145.7 37.0  -     -     -     -     -     -     - 
Ca2  2.39 2_654 127.4 100.3 63.8  -     -     -     -     -     - 
O8   2.39 2_654 31.2  154.7 153.9 100.1 -     -     -     -     - 
O9   2.42 I     90.6  35.5  60.3  118.8 119.9 -     -     -     - 
O11  2.42 I     29.8  94.1  122.5 149.7 61.0  62.8  -     -     - 
O7   2.43 I     121.8 33.1  60.5  110.8 144.6 58.9  95.8  -     - 
O3   2.68 2_654 86.9  147.7 121.3 61.2  57.1  176.4 115.2 124.7 - 

     O7   SYMM  B6    B3    O1    O9    O2    O6    O4    Ca1
B6   1.19 2_765 -     -     -     -     -     -     -     -  
B3   1.44 I     121.3 -     -     -     -     -     -     -  
O1   2.31 2_765 35.3  154.4 -     -     -     -     -     -  
O9   2.38 I     141.0 36.5  146.5 -     -     -     -     -  
O2   2.41 I     87.6  34.9  119.5 60.0  -     -     -     -  
O6   2.43 I     122.4 33.5  151.7 60.3  58.2  -     -     -  
O4   2.44 I     27.8  93.6  62.0  116.8 59.9  101.4 -     -  
Ca1  2.46 I     97.1  134.5 62.4  98.1  144.3 138.3 120.2 -  

     O8   SYMM  B5    B1    Ca2   O6    O4    O3    O11   O2
B5   1.41 2_655 -     -     -     -     -     -     -     - 
B1   1.56 I     120.3 -     -     -     -     -     -     - 
Ca2  2.35 2_654 143.4 95.5  -     -     -     -     -     - 
O6   2.39 2_655 30.6  90.0  173.3 -     -     -     -     - 
O4   2.43 I     102.5 33.7  103.9 78.6  -     -     -     - 
O3   2.44 I     96.4  34.7  119.2 67.4  57.1  -     -     - 
O11  2.44 2_655 29.5  149.4 114.0 60.1  124.5 123.9 -     - 
O2   2.48 I     154.1 34.6  62.5  123.5 59.2  58.8  176.0 - 

     O9   SYMM  B2    B3    Ca1   O5    O7    O2    O6    O10  O11
B2   1.45 I     -     -     -     -     -     -     -     -    -  
B3   1.49 I     115.9 -     -     -     -     -     -     -    -  
Ca1  2.32 3_766 139.1 102.6 -     -     -     -     -     -    -  
O5   2.37 I     34.7  102.3 126.0 -     -     -     -     -    -  
O7   2.38 I     100.6 35.2  103.2 73.9  -     -     -     -    -  
O2   2.40 I     150.8 36.0  69.9  134.4 60.6  -     -     -    -  
O6   2.42 I     93.3  34.6  127.3 98.9  60.8  58.5  -     -    -  
O10  2.46 I     32.6  146.5 110.9 58.2  131.3 164.9 116.1 -    -  
O11  2.52 I     37.9  89.4  161.9 62.8  94.4  116.8 58.7  58.0 -  

     O10  SYMM  B2    B4    O5    O3    O5    O11   Ca2   O9    Ca1   O1
B2   1.46 I     -     -     -     -     -     -     -     -     -     - 
B4   1.47 4_556 122.1 -     -     -     -     -     -     -     -     - 
O5   2.35 I     35.7  132.1 -     -     -     -     -     -     -     - 
O3   2.38 2_754 157.3 38.2  139.7 -     -     -     -     -     -     - 
O5   2.38 4_556 107.1 35.4  138.0 63.2  -     -     -     -     -     - 
O11  2.41 I     41.7  135.0 64.8  155.3 101.3 -     -     -     -     - 
Ca2  2.41 3_766 126.0 95.1  90.4  60.9  124.0 129.1 -     -     -     - 
O9   2.46 I     32.2  90.1  59.1  127.1 79.1  62.3  141.2 -     -     - 
Ca1  2.46 4_556 105.9 89.0  131.7 87.6  60.6  67.8  113.0 105.5 -     - 
O1   2.54 4_556 98.6  33.0  99.2  58.7  59.4  132.1 93.6  70.8  119.4 - 

     O11  SYMM  B5    B2    O10   O6    O8    Ca1   O9   O5
B5   1.40 I     -     -     -     -     -     -     -    - 
B2   1.64 I     117.5 -     -     -     -     -     -    - 
O10  2.41 I     140.8 36.4  -     -     -     -     -    - 
O6   2.42 I     29.3  88.5  117.7 -     -     -     -    - 
O8   2.44 2_654 29.5  146.5 153.1 58.9  -     -     -    - 
Ca1  2.50 2_764 117.3 114.1 101.9 133.7 95.7  -     -    - 
O9   2.52 I     86.0  32.8  59.7  58.6  114.0 144.1 -    - 
O5   2.55 I     120.9 31.6  56.4  94.0  145.2 88.3  55.7 - 

Calcium

Features two heavily distorted oxo-environments with C.N. = 6 and C.N. = 8.

     Ca1  SYMM  O9    O5    O7    O10   O1   O11
O9   2.32 3_766 -     -     -     -     -    -  
O5   2.44 I     106.0 -     -     -     -    -  
O7   2.46 I     85.8  71.4  -     -     -    -  
O10  2.46 4_566 143.9 58.1  114.1 -     -    -  
O1   2.47 2_765 74.3  127.2 55.9  141.8 -    -  
O11  2.50 2_765 87.2  147.8 140.0 93.9  84.3 -  

     Ca2  SYMM  O8    O6    O4    O10   O3    O2    O1   O3
O8   2.35 2_655 -     -     -     -     -     -     -    - 
O6   2.39 2_655 109.6 -     -     -     -     -     -    - 
O4   2.40 I     170.9 79.3  -     -     -     -     -    - 
O10  2.41 3_766 94.9  152.3 76.8  -     -     -     -    - 
O3   2.43 4_567 94.9  128.5 77.7  58.8  -     -     -    - 
O2   2.51 2_655 61.1  57.5  125.3 150.1 102.9 -     -    - 
O1   2.59 3_766 77.3  98.4  103.5 74.1  131.5 113.7 -    - 
O3   2.59 I     129.2 64.9  55.4  89.8  129.3 118.9 55.6 - 

Bibliography

  1. Zarakina, N. V.; Brovkin, A. A. Kristallografiya 1976, 21, 502–506. (In Russian)
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The oxygen can make max 2 bonds but $\ce{O8}$ has three!

You will have better idea if you see 2D structure instead of 3D

enter image description here

You can clearly see no $\ce{O}$ atom have more then 3 bonds!

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